Monarch Watch

The Milkweeds of Canada (Asclepias spp.)

Status, Distribution, and Potential Impact from Noxious Weed Legislation

Author:
David J. White - R.R. #3 - Lanark, Ontario - KOG IKO
31 March 1996
Report prepared for the Canadian Wildlife Service, Ottawa

Note: All information contained in this article reflects the knowledge of the subject as of the original date of publication.

Table of Contents
ABSTRACT

ACKNOWLEDGMENTS

1.0 INTRODUCTION
1.1 Milkweeds and the Monarch Butterfly
1.2 Description of the Milkweed Genus
1.2 Biological and Economic Significance of Milkweeds

2.0 SPECIES ACCOUNTS
Asclepias exaltata (Poke Milkweed)
Asclepias hirtella (Milkweed)
Asclepias incarnata (Swamp Milkweed)
Asclepias lanuginosa (Milkweed)
Asclepias ovalifolia (Oval-leafed Milkweed)
Asclepias purpurascens (Purple Milkweed)
Asclepias quadrifolia (Four-leaved Milkweed)
Asclepias speciosa (Showy Milkweed)
Asclepias sullivantii (Sullivant's Milkweed)
Asclepias syriaca (Common Milkweed)
Asclepias tuberosa (Butterfly-weed)
Asclepias variegata (Variegated Milkweed)
Asclepias verticillata (Whorled Milkweed)
Asclepias viridiflora (Green Milkweed)

3.0 NOXIOUS WEED LEGISLATION
3.1 Weed Legislation Summary
3.2 Potential Impact from Weed Legislation

4.0 INFORMATION SOURCES
4.1 References
4.2 Collections Consulted

LIST OF TABLES

TABLE 1: Summary of Milkweed Provincial Distribution and Status
TABLE 2: Summary of Weed Legislation Affecting Milkweeds

LIST OF FIGURES

ACKNOWLEDGMENTS

I would like to express my appreciation to a number of people who helped with this study.

First I would like to thank Dr. Erich Haber, National Botanical Services, Ottawa for support and advice. Erich also conducted the library computer database searches and located some literature. Gerry Lee, Chief of Habitat Conservation, Canadian Wildlife Service, Hull, administered the contract and provided helpful contacts.

Information on weed act legislation was kindly provided by the following people: Tom Hodgson of Agriculture Canada, Ottawa; Walter Yarish of Alberta Agriculture, Edmonton; Dave Ralph of the Ministry of Agriculture, Fisheries, and Food, Kamloops, British Columbia; Jo Anne Buth of Manitoba Agriculture, Carman; Kevin McCully of the New Brunswick Department of Agriculture; Doug Doohan of the Nova Scotia Department of Agriculture, Truro; Sherry Ross of the Prince Edward Island Department of Agriculture, Fisheries, and Forestry, Charlottetown; Doug Billet of Saskatchewan Agriculture; Pierre Lavigne, Ministere de le'Agriculture, des Pecheries et de l'Alimentation, Sante-Foy, Quebec; and Howard Lang, Ontario Ministry of Agriculture, Food, and Rural Affairs, Guelph.

Mike Shchepanek, Canadian Museum of Nature (CAN), Ottawa, facilitated access to the library and herbarium at CAN. Cathy Keddy, Carleton Place, Ontario, provided background information on weed control legislation. Gisele Mitrow, Agriculture Canada (DAO), Ottawa, helped with access to the library and herbarium at DAO. Don Lafontaine, Agriculture Canada, Ottawa provided background information of the Monarch Butterfly.

Funding was provided by the North American Wetlands Conservation Council (Canada) and Ducks Unlimited.

Status, Distribution, and Potential Impact from Noxious Weed Legislation

Note: All information contained in this article reflects the knowledge of the subject as of the original date of publication.

ABSTRACT

The present report summarizes status and distributional information on the Milkweeds (Asclepias spp.) that occur in Canada and considers whether noxious weed legislation poses a threat to any of the species. Since the Monarch Butterfly (Danaus plexippus) requires Milkweed as a larval food, this insect could be put at risk due to Milkweed eradication programs.

Of the fourteen species of Milkweed that occur in Canada - all of which are native - Common Milkweed (Asclepias syriaca) is the most widespread and locally abundant. It occurs from southeast Saskatchewan to Prince Edward Island. Swamp Milkweed (Asclepias incarnata) is also fairly common and is found from Prince Edward Island to southeast Manitoba. Showy Milkweed (Asclepias speciosa) is a western species that occurs from southern Manitoba to southern British Columbia. Oval-leafed Milkweed (Asclepias ovalifolia) is also a western species that occurs from extreme northwestern Ontario to southern British Columbia. Green Milkweed (Asclepias viridiflora) is primarily a western species that is found from southern Ontario to southern British Columbia. Whorled Milkweed (Asclepias verticillata) occurs from southern Ontario to southern Saskatchewan. Poke Milkweed (Asclepias exaltata) and Butterfly-weed (Asclepias tuberosa) occur in southern Ontario and southwest Quebec. Milkweed (Asclepias lanuginosa) only occurs in southern Manitoba where it is rare. The following five species occur only in Ontario where they are considered rare: Milkweed (Asclepias hirtella), Purple Milkweed (Asclepias purpurascens), Four-leafed Milkweed (Asclepias quadrifolia), Sullivant's Milkweed (Asclepias sullivantii), and Variegated Milkweed (Asclepias variegata).

Common Milkweed (Asclepias syriaca), although native, has been able to successfully colonize areas that have been disturbed by agriculture or development and in many areas the plant spreads in a "weedy" manner. This weedy behavior has been the main reason that some species of Milkweed have been listed as noxious weeds: Common Milkweed (Asclepias syriaca) is listed in Nova Scotia, Quebec, Manitoba, and Saskatchewan; and Showy Milkweed (Asclepias speciosa) is listed in Manitoba. The Ontario weed act lists all species of Milkweed as noxious weeds, however, the primary concern is for Common Milkweed.

The Monarch Butterfly (Danaus plexippus) occurs across much of Canada as adults, however, its breeding range is mainly restricted to south and central Ontario, and southern Quebec. Although the Monarch larva can feed on a range of Milkweed species, it is largely dependent on Common Milkweed (Asclepias syriaca). From the point of view of the Monarch Butterfly, it is primarily the Ontario populations of Common Milkweed that are important to its breeding in Canada. The enforcing of noxious weed legislation in some provinces-like Nova Scotia where there is currently a very limited population of Common Milkweed-could impact peripheral colonies of Monarch Butterflies. In Ontario, where there is a very abundant and spreading population of Common Milkweed, it is unlikely that there are sufficient resources available to enforce the noxious weed act to the point where the Monarch Butterfly could be put at significant risk.

1.0 INTRODUCTION
The purpose of the present report is to summarize status and distributional information on the Milkweeds (Asclepias spp.) that occur in Canada and consider whether noxious weed legislation poses a threat to any of the species. This information is needed to determine whether the Monarch Butterfly (Danaus plexippus) that requires Milkweed as a larval food, could be put at risk due to Milkweed eradication programs.

All Milkweeds that occur in Canada are native species. Common Milkweed (Asclepias syriaca), although native, has been able to successfully colonize areas that have been disturbed by agriculture or development and in many areas the plant spreads in a "weedy" manner. The behavior of this species has been the main reason that "Milkweeds" have been listed in some noxious weed legislation. Since the Monarch Butterfly (Danaus plexippus) requires Milkweed as a larval food (Urquhart & Urquhart, 1979), this migrating insect could be at risk itself in Canada should Milkweeds be reduced or eliminated in some areas due to the enforcement of noxious weed legislation.

1.1 Milkweeds and the Monarch Butterfly

The Monarch Butterfly (Danaus plexippus) is a very unusual insect in that it spends the spring and summer breeding season in central and eastern North America but migrates to northern Mexico to survive the northern winter as dormant adults (Urquhart & Urquhart, 1979). During the breeding season, Monarchs depend on various species of Milkweed for larval food. In Canada, there are 14 species of Milkweed known, however, one species-Common Milkweed (Asclepias syriaca)-is by far the most important food source for the Monarch (Crolla & Lafontaine, 1996; Urquhart & Urquhart, 1979). In eastern North America, Malcolm et al.., (1989) report that the late summer breeding range of the Monarch coincides with the range of Asclepias syriaca - Milkweed they describe as "extremely abundant". Although adult Monarch Butterflies have been recorded in all Canadian provinces, their breeding is largely restricted to southern and central Ontario, and southern Quebec (Crolla & Lafontaine, 1996; Urquhart & Urquhart, 1979). Breeding records, according to Crolla & Lafontaine (1996), are as follows: British Columbia-rare; Alberta-rare; Saskatchewan-sparse; Manitoba-uncommon; Ontario-very common; Quebec-common; New Brunswick-rare; Nova Scotia-rare; Prince Edward Island-none; Newfoundland-none. The breeding area is also the center of abundance of Common Milkweed, the principle larval food plant (Crolla & Lafontaine, 1996; Urquhart & Urquhart, 1979). Although the Monarch can use various species of Milkweed as larval food and the adult butterfly occurs across much of Canada it is clear that the main food is Common Milkweed (Asclepias syriaca) and the main breeding range is in southern and central Ontario (Crolla & Lafontaine, 1996; Urquhart & Urquhart, 1979). Therefore, from the perspective of the Monarch Butterfly, it is the Ontario populations of Common Milkweed that most are important. For an up-to-date and thorough review of the status of the Monarch Butterfly in Canada, refer to Crolla & Lafontaine (1996).

1.2 Description of the Milkweed Genus

In North America, there are over 100 species of Milkweed that belong to the genus Asclepias (Woodson, 1954; Gleason, 1969). Although the distribution of many species overlaps, hybrids between species are rare (Kephart & Heiser, 1980). There are 14 species of Milkweed known in Canada-refer to Table 1. Milkweeds are perennial herbs that grow each year from a thick root or deep rhizome. The stems are simple or sparingly branched with opposite or whorled leaves [one of the Canadian species-Butterfly-weed (Asclepias tuberosa)- has alternate leaves (Gleason, 1968)]. The flowers are generally small and arranged in clusters at the top of the main stem and sometimes also at the ends of secondary branches. The flowers of all species have a characteristic shape-a ring of five drooping petals overtopped by five erect hood-shaped petals. All species except Butterfly-weed have milky juice-hence the common name "Milkweed".

TABLE 1: Summary of Milkweed Provincial Distribution and Status

Milkweed Species Distribution and Status

BC AB SK MB ON QC NB NS PE NF

Asclepias exaltata - - - - Unc. Rare - - - -

A. hirtella - - - - Rare - - - - -

A incarnata - - - Unc. Com. Com. Unc. Unc Unc -

A. lanuginosa - - - Rare - - - - - -

A. ovalifolia Rare Unc. Unc. Unc. Rare - - - - -

A. purpurascens - - - - Rare - - - - -

A. quadrifolia - - - - Rare - - - - -

A. speciosa Unc. Unc. Unc. Unc. - - - - - -

A. sullivantii - - - - Rare - - - - -

A. syriaca - - Rare Unc. Com. Unc. Unc. Unc. Unc. -

A. tuberosa - - - - Unc. Rare - - - -

A. variegata - - - - Ex. - - - - -

A. verticillata - - Rare Unc. Rare - - - - -

A. virdiflora Unc. Rare Unc. Unc. Rare - - - - -

Total Species 3 3 5 7 12 4 2 2 2 0

AB = Alberta, BC = British Columbia, MB = Manitoba, NB = New Brunswick, NF = Newfoundland, NS = Nova Scotia, ON = Ontario, PE = Prince Edward Island, QC = Quebec, SK = Saskatchewan. Com. = common, Ex = extirpated, Unc. = uncommon. If a cell is empty, it indicates that species does not occur in that province. The status assessments are based on the most current floras and rare plant publications.

Although most mature Milkweed plants have many flowers, only a few of the flowers on a plant will produce fruit (Gleason, 1968). The fruit consists of a long pod containing many flat round seeds attached to silky threads capable of transporting the seeds long distances on the gentlest of breezes. Some species of Milkweed can also reproduce vegetatively by means of shoots that can develop on their extensive roots. The flowers are insect-pollinated and have a very specialized floral structure (Gleason, 1968). The complex flower structures have evolved to facilitate cross-pollination by insects and most species are self-incompatible, however, a few species can produce some viable seed by self-pollination (Kephart, 1981).

1.3 Biological and Economic Significance of Milkweeds

Milkweeds have both positive and negative attributes. North American native people used the seed "floss" and stem fibers (Woodson, 1954). In colonial New England, settlers used the floss for stuffing pillows and cushions (Woodson, 1954). The stem fibers were considered as a substitute for flax and hemp (Woodson, 1954) but it was considered inferior to them. During and shortly after the Second World War, a number of investigations were carried out to determine the usefulness of the latex of various species of Milkweed as a possible source of rubber and the seed floss as a replacement for "kapok" in life preservers (Groh 1943; Groh & Dore, 1945; Moore 1947; Woodson, 1954). The investigations evidently ended sometime after the Second World War when it became apparent the Milkweed could not compete with existing crop plants (Woodson, 1954.)

Milkweeds are a nectar source for the Honeybee (Apis mellifera) (Robinson & Oertel, 1975) and for many other insects (Wilson et al., 1979) including the well-known Monarch Butterfly. Due to their complex flower structure that is regarded as second in complexity only to the Orchid family (Wyatt, 1976), Milkweeds have been the subject of many studies into pollination mechanisms and life history evolution.

On the negative side of the ledger, some Milkweeds - especially Common Milkweed (Asclepias syriaca) - can be aggressive and persistent weeds in agricultural land and the plants are toxic to livestock (Kingsbury, 1964). This situation has resulted in some Milkweeds being declared "noxious weeds" in several jurisdictions.

Status, Distribution, and Potential Impact from Noxious Weed Legislation

2.0 SPECIES ACCOUNTS
The following section presents information on the 14 species of Milkweeds (Asclepias spp.) known to occur in Canada. The species are arranged in alphabetical order and the format generally follows the style of COSEWIC Status Reports (Haber and Members of the Subcommittee for Vascular Plants, Mosses, and Lichens, 1994) except the following treatments are more brief than formal Status Reports. Plant nomenclature follows Kartesz (1994). The species descriptions are based on Gleason (1968) and herbarium specimens. Total range many are provided for all species. Provincial range maps are provided for rare species where recent maps available. To illustrate each species, there is a photocopy of a herbarium specimen from the Canadian Museum of Nature, Ottawa. The species habitats are based on relevant floras and data from herbarium specimens. The total range maps are based on Woodson (1954), and the most recent floras and rare plant publications. A literature search of relevant journal articles was performed using the library computer database AGRICOLA and BIOSIS.

Asclepias exaltata L. - Poke Milkweed

Description

This Milkweed grows to 1.5 m tall and has opposite leaves that are broadly elliptic and taper at both ends. The flower clusters are loosely few-flowered on spreading or drooping stalks. The flower color is white to pink or pale dull purple and the hoods of each flower support a crown of five incurved horns.

Illustration

An illustration can be found in Gleason (9168) vol 3, page 76.

Biological and Economic Significance

Poke Milkweed is an uncommon to rare plant of open woods whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its limited range, scarcity, and woodland habitat, it is probably not an important source of food.

Distribution

Asclepias exaltata has a wide range in the eastern United States from New England to Minnesota and south to Georgia. In Canada it occurs only in southern Ontario and southwestern Quebec.

Figure: North American distribution of Asclepias exaltata

General Habitat

Open woods and clearing in moist to dry upland woods and thickets.

Population Biology

In southwestern Ontario, Oldham et al. (1991) report Poke Milkweed to be very localized, often with only a few plants at each locality. Wilbur (1976) reports colonies of from 10 to several hundred plants in Michigan. Shannon and Wyatt (1986) reported on four colonies in Virginia with 16 to 40 plants each.

Poke Milkweed does not reproduce vegetatively (Shannon & Wyatt, 1986). This Milkweed suffers extensively from herbivore damage but it has a low mortality due to its large root storage capacity (Wilbur, 1976).

Evidence of Threats to Survival

No imminent threats are known. Due to its woodland habitat, it is not likely to be targeted by "weed control" programs even though it bears a superficial resemblance to Common Milkweed (Asclepias syriaca).

Present Legal or Other Formal Status

Poke Milkweed has no legal protection, however, it is considered rare in Quebec in Bouchard et al., (1983). Its status in the United States is unknown.

Assessment of Status

Asclepias exaltata is rare in Quebec and uncommon in southern Ontario.

Asclepias hirtella (Pennell) - Prairie Milkweed

Description

This Milkweed grows to one metre tall and has opposite or sub-opposite leaves that are linear or lance-shaped and taper at both ends. The flower clusters are densely-flowered and occur in the upper leaf axils. The hoods of each flower lack horns but the former are separated from the drooping petals by a distinct column.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 80.

Biological and Economic Significance

This Milkweed is a very rare plant whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its very limited range and extremely rarity, it is probably not a significant food source.

Distribution

Asclepias hirtella has a moderate range in east-central United States from Michigan and Minnesota south to Texas. In Canada it occurs only in extreme southwestern Ontario.

Figure: North American distribution of Asclepias hirtella

Figure: Ontario distribution of Asclepias hirtella

General Habitat

Dry, sandy soil.

Population Biology

In southwestern Ontario, Oldham et al., (1991) report it to be very localized, often with only few plants at each locality.

Evidence of Threats to Survival

No imminent threats are known. Due to its woodland habitat, it is not likely to be targeted by "weed control" programs.

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Ontario in Argus & Keddy (1984). It is considered to be rare in Canada and is given the highest priority ranking of "1" in Argus & Pryer (1990) because of its absence or rarity in bordering states and because it is considered to be "N1" nationally and "S1" in Ontario where "1" indicates "critically imperiled because of extreme rarity". Also regard as "S1" in Ontario (extremely rare" in Oldham (1994).

In the United States, it is regarded as a "G5" species (Argus & Pryer, 1990) which means that it is "abundant and demonstrably secure with many occurrences" - Mississippi; and status uncertain or unknown - Georgia and Tennessee.

Assessment of Status

Asclepias hirtella is very rare in southwestern Ontario - its only Canadian occurrence.

Asclepias incarnataL. - Swamp Milkweed

Description

This Milkweed grows to 1.5 m tall and has opposite leaves that are lance-shaped and taper at both ends. The flower clusters are on upright or spreading stalks. The flower colour is pink to red and the hoods of each flower support a crown of five incurved horns.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 75.

Biological and Economic Significance

Swamp Milkweed is a common to uncommon plant of wet woods, ditches, and wetland edges whose natural significance is largely unknown. The species is used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its scattered occurrence and wetland habitat, it is probably not an important food source.

Distribution

Asclepias incarnata has a wide range in the United States from Maine to North Dakota and south to Florida and Texas. In Canada, it occurs in southern Nova Scotia, Prince Edward Island, southwest New Brunswick, southern Quebec, southern Ontario, and southeastern Manitoba.

Figure: North American distribution of Asclepias incarnata

General Habitat

Swamp Milkweed prefers the moist soil of wet woods, ditches, riverbanks, and wetland edges.

Population Biology

In eastern Ontario, it occurs singly or with only a few plants at each locality (DJW, pers obs.). Wilbur (1976) reports it to be generally of scattered occurrence in a study area in Michigan, but reports one linear colony along a wetland edge consisting of from 400 to 500 plants. Vegetative reproduction occurs infrequently, if at all (Kephart, 1981). Individual plants tend to be short-lived--five to ten years at most--because its habitat of floodplains and wetland edges is very dynamic and prone to erosion, slumping, etc., (Wilbur, 1976). Swamp Milkweed tends to suffer little damage from herbivores and may bloom in its first year after germination (Wilbur, 1976).

Evidence of Threats to Survival

No imminent threats are known. Due to its wetland habitat, it is not likely to be targeted by "weed control" programs.

Present Legal or Other Formal Status

Swamp Milkweed has no legal protection but it is considered to be rare in Nova Scotia (Maher et al., 1978). It is considered common or uncommon in the rest of its Canadian range. Its status in the United States is unknown.

Assessment of Status

Asclepias incarnata is rare in Nova Scotia, but is relatively common from New Brunswick to Manitoba.

Asclepias lanuginosa Nutt. - Hairy Milkweed
(Asclepias otariodes auct. non Fourn.:Acerates lanuginosa (Nutt.) Dcne.)

Description

This generally hairy Milkweed grows to 30 centimetres tall and has alternate or sub-opposite leaves that are oblong or lance-shaped. The single flower cluster is terminal and the hoods of each greenish flower are without horns.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 80.

Biological and Economic Significance

This Milkweed is a very rare plant whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to its very limited range and extreme rarity, it is probably not a significant food source.

Distribution

Asclepias lanuginosa has a limited range in the central United States from North Dakota to Illinois and south to Kansas. In Canada it occurs only in a small portion of southwestern Manitoba. According to Morton & Venn (1990), reports of this species from Ontario--such as in Scoggan (1978-1979)--are based on misidentifications of Butterfly-weed (Asclepias tuberosa).

Figure: North American distribution of Asclepias lanuginosa

Figure: Manitoba distribution of Asclepias lanuginosa

General Habitat

Sandhills.

Population Biology

The average population size for this Milkweed is unknown.

Evidence of Threats to Survival

No imminent threats are known. Due to its rarity and sandhills habitat, it is not likely to be targeted by "weed control" programs.

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Manitoba in White & Johnson (1980). It is considered rare in Canada and is given a priority ranking of "4" in Argus & Pryer (1990) despite being considered to be "N1" nationally and "S1" in Manitoba where "1" indicates "critically imperiled because of extreme rarity."

In the United States, it is regarded as a "G?" species (Argus & Pryer, 1990) which means that its status is "unknown".

The status in individual states from Argus & Pryer (1990) is as follows: "S1" (Critically imperiled) - Iowa and Illinois; "S2" (Imperiled because of rarity) - Wisconsin.

Assessment of Status

Asclepias lanuginosa is very rare in southwestern Manitoba--its only Canadian occurrence.

Asclepias ovalifolia Dcne. - Oval-leaved Milkweed

Description

This slender Milkweed grows to 50 centimetres tall and has opposite leaves that are oblong or elliptic shaped. The single flower cluster is terminal although occasionally there may be a few additional flower clusters in the upper leaf axils. The hoods of each greenish flower are without horns.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 77.

Biological and Economic Significance

Distribution

Asclepias ovalifolia has a limited range in the central United States from Illinois and Wisconsin to South Dakota. In Canada it occurs in southeastern British Columbia, southern and central Alberta, southern Saskatchewan, southern Manitoba, and extreme northwestern Ontario.

Figure: North American distribution of Asclepias ovalifolia

Figure: Ontario distribution of Asclepias ovalifolia

Figure: British Columbia distribution of Asclepias ovalifolia

General Habitat

Silty riverbanks, open woods, thickets, roadside ditches, sand dunes, and slopes of valleys.

Population Biology

The average population size for this Milkweed is unknown.

Evidence of Threats to Survival

No imminent threats are known. Due to its riverbank and thicket habitat, it is not likely to be targeted by "weed control" programs even though it bears a superficial resemblance to Common Milkweed (Asclepias syriaca).

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in British Columbia in Straley et al., (1985) and rare in Ontario in Argus & Keddy (1984). Since it is not rare in all of its Canadian range, it has no priority ranking in Argus & Pryer (1990). It is regarded as "SH" in Ontario (known from old records only) in Oldham (1994). Its status in the United States in unknown.

Assessment of Status

Asclepias ovalifolia is very rare in northwestern Ontario and in southeastern British Columbia. It is relatively common in Manitoba and in southern Alberta.

Asclepias purpurascensL. - Purple Milkweed

Description

This Milkweed grows to one metre tall and has opposite or sub-opposite leaves that are broad in the middle and taper at both ends. The flower clusters are many-flowered and are terminal or sometimes also in the uppermost leaf axils. The hoods of each flower hide the much-shorter, incurved horns. The flowers are a distinctive, deep magenta-red. The seed pods are downy.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 77.

Biological and Economic Significance

Distribution

Asclepias purpurasens has a moderate range in east-central United States from Minnesota to New England and south to Arkansas and Kansas. In Canada it occurs only in extreme southwestern Ontario.

Figure: North American distribution of Asclepias purpurascens

Figure: Ontario distribution of Asclepias purpurascens

General Habitat

Dry to moist thickets, open woods, and prairies.

Population Biology

In Michigan, Wilbur (1976) reports a population of 32 plants--the only one in his study area. It rarity in the Michigan study area may be due to its low seed production and high level of damage by herbivores (Wilbur, 1976).

Evidence of Threats to Survival

No imminent threats are known. Due to its rarity and thicket habitat, it is no likely to be targeted by "weed control" programs even though it bears a superficial resemblance to Common Milkweed (Asclepias syriaca).

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Ontario in Argus & Keddy (1984). It is considered to be rare in Canada and is given the priority ranking of "3" in Argus & Pryer (1990) because of its absence or rarity in bordering states and because it is considered to be "N2" nationally and "S2" in Ontario where "2" indicates "imperiled because of rarity". Also regarded as "S2" in Ontario (very rare) in Oldham (1994).

In the United States, it is regarded as a "G4G5" species (Argus & Pryer, 1990) which means that it is "apparently secure to abundant and demonstrably secure with many occurrences".

The status in individual states from Argus & Pryer (1990) is as follows: "SH" (Historical-not-recorded since 1900) - Vermont; "S1" (Critically imperiled) - ?Georgia, New Hampshire, New York, and Wisconsin; "S3" (Rare or uncommon) - Maryland; "S4?" (Apparently secure with many occurrences) - Mississippi; and status uncertain or unknown - Minnesota and Tennessee.

Assessment of Status

Asclepias purpurascens is very rare in extreme southwestern Ontario--its only Candian occurrence.

Asclepias quadrifolia Jacq. - Four-leaved Milkweed

Description

Four-leaved Milkweed grow to 50 centimetres tall and has narrow to oval-shaped leaves that are broadest in the middle and taper at both ends. The leaf arrangement is quite distinctive because the leaves normally occur at three nodes--the upper and lower node each support a pair of small leaves while the middle node has a whorl of four much larger leaves. The flower clusters are many-flowered and are terminal or sometimes one or more clusters occur also in the uppermost leaf axils. The hoods of each flower hide the much-shorter, incurved horns. The flowers are white, pink, or lavender and the seed pods are very slender on erect stalks.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 78.

Biological and Economic Significance

Distribution

Asclepias quadrifolia has a moderate range in east-central United States from New England to Iowa and south to Oklahoma and Georgia. In Canada it occurs only in extreme south Ontario.

Figure: North American distribution of Asclepias quadrifolia

Figure: Ontario distribution of Asclepias quadrifolia

General Habitat

Open, dry woods.

Population Biology

The average population size for Four-leaved Milkweed is unknown. This species cannot reproduce vegetatively (Cabin et al., 1991).

Evidence of Threats to Survival

No imminent threats are known. Due to its rarity and woodland habitat, it is not likely to be targeted by "weed control" programs.

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Ontario in Argus & Keddy (1984). It is considered to be rare in Canada and is given the priority ranking of "3" in Argus & Pryer (1990) because of its absence or rarity in bordering states and because it is considered to be "N1" nationally and "S1" in Ontario where "1" indicates "critically imperiled because of extreme rarity". It is regarded as "SH" in Ontario (known from old records only) in Olham (1994).

In the United States, it is regarded as a "G5" species (Argus & Pryer, 1990) which means that it is "abundant and demonstrably secure with many occurrences".

The status in individual states from Argus & Pryer (1990) is as follows: "SH" (Historical-not-recorded since 1900) - Delaware; "S1" (Critically imperiled) - Kansas, New Hampshire, and Rhode Island; "S3" (Rare or uncommon) - Vermont and Iowa.

Assessment of Status

Asclepias quadrifolia is very rare in extreme southern Ontario - its only Candian occurrence.

Status, Distribution, and Potential Impact from Noxious Weed Legislation

Asclepias speciosa Torr. - Showy Milkweed

Description

This robust Milkweed grows to 1.5 meters tall and has opposite leaves that are oblong and blunt at each end. The flower clusters are few but large and terminal or in the uppermost leaf axils. The hoods of each large, greenish-purple flower hide the short, incurved horns. The pods are densely hairy.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 78.

Biological and Economic Significance

This Milkweed is an uncommon plant whose natural significance is largely unknown. The species is considered the primary food source of the larvae of Monarch Butterfly (Danaus plexippus) in western Canada (Crolla & Lafontaine, 1996).

Distribution

Asclepias speciosa has a moderate range in west-central United States from Washington to Minnesota and south to Oklahoma and California. In Canada it occurs in southern British Columbia, southern Alberta, southern Saskatchewan, and southern Manitoba.

Figure: North American distribution of Asclepias speciosa

General Habitat

Thickets, roadsides, and moist grassland.

Population Biology

The average population size for this Milkweed is unknown.

Evidence of Threats to Survival

No imminent threats are known. Due to its grassland and roadside habitat, it could be targeted by "weed control" programs especially since it bears a superficial resemblance to Common Milkweed (Asclepias syriaca). It is listed as a noxious weed in Manitoba and is sometimes targeted for control.

Present Legal or Other Formal Status

This Milkweed has no legal protection, nor is it considered rare in any Canadian province. Its status in the United States is unknown.

Assessment of Status

Asclepias speciosa is uncommon from British Columbia to southern Manitoba.

Asclepias sullivantii Engelm. ex Gray - Sullivant's Milkweed

Description

This Milkweed grows to 1.5 metres tall and has oval-shaped leaves that are broadest in the middle and taper abruptly at both ends. This plant is quite similar to Common Milkweed (Asclepias syriaca) except that this species is entirely smooth and the leaf ends are very blunt. The flower clusters are compact and many-flowered and are terminal or sometimes one or more clusters occur also in the uppermost leaf axils. The hoods of each flower hide the slightly incurved horns. The flowers are white, pink, or lavender.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 79.

Biological and Economic Significance

Distribution

Asclepias sullivantii has a moderate range in east-central United States from Ohio to Minnesota and south to Oklahoma. In Canada it occurs only in extreme southern Ontario.

Figure: North American distribution of Asclepias sullivantii

Figure: Ontario distribution of Asclepias sullivantii

General Habitat

Wet meadows, moist prairies, railway embankments, and roadsides.

Population Biology

Fox (1944) reported that several colonies investigated on Walpole Island in extreme southwestern Ontario were "not large" in size. Notes on the DAQ herbarium specimens collected by M. J. Oldham report colonies of 10 plants and 20 plants.

Evidence of Threats to Survival

No imminent threats are known. Since it has been recorded in Ontario along roadsides and railway embankments, it could be targeted by "weed control" programs especially since its bears a superficial resemblance to Common Milkweed (Asclepias syriaca).

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Ontario in Argus & Keddy (1984). It is considered rare in Canada and is given the priority ranking of "3" in Argus & Pryer (1990) because of its absence or rarity in bordering states and because it is considered to be "N2" nationally and "S2" in Ontario where "2" indicates "imperiled because of rarity". It is regarded as "S2" in Ontario (very rare) in Olham (1994).

In the United States, it is regarded as a "G5" species (Argus & Pryer, 1990) which means that it is "abundant and demonstrably secure with many occurrences".

The status in individual states from Argus & Pryer (1990) is as follows: "SH" (Historical-not-recorded since 1900) - North Dakota; "S1" (Critically imperiled) - Michigan; "S1S2" (Critically imperiled or imperiled) - Wisconsin "S4" (Apparently secure) - Iowa; and status uncertain or unknown - Arkansas and South Dakota.

Assessment of Status

Asclepias sullivantii is very rare in extreme southern Ontario--its only Canadian occurrence. A 1991 collection made by M. J. Oldham (specimen at CAN) and 1987 and 1990 collections, also made by M. J. Oldham (specimens at DAO), are shown on the Ontario map. The new records occur within the known range of the species and do not change its rare status.

Asclepias syriacaL. - Common Milkweed

Description

This Milkweed grows to 1.5 metres tall and has oval-shaped leaves that are broadest in the middle and taper abruptly at both ends. This plant is more -or-less hairy throughout. The flower clusters are compact and many-flowered and are terminal and in the uppermost leaf axils. The hoods of each flower are quite spreading and don't hid the short incurved horns. The flowers are normally greenish-purple but white-flowered forms do occur (Dore, 1944). The pods are quite distinctive because of their densely hairy and "warty" appearance on stalks that are bent downward.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 77.

Biological and Economic Significance

This Milkweed is a very common plant in eastern North American whose natural significance is largely unknown. During the late stages of the Second World War, the leaves of Common Milkweed were gathered in Ontario for their possible use as a source for "rubber" and the mature pods were gathered for possible use as a replacement for kapok in life jackets (Groh & Dore, 1945). It is not known how useful these products were but there is no evidence that their collection and use developed beyond the "pilot project" stage. The species is the most abundant and widespread of the Canadian Milkweeds and is the main food source for the larvae of Monarch Butterfly (Danaus plexippus) in Canada (Malcolm et al., 1989; Urquhart & Urquhart, 1979).

Distribution

Asclepias syriaca has a moderate range in east-central United States from Maine to North Dakota and south to Kansas and Georgia. In Canada it occurs in Nova Scotia to southeastern Saskatchewan. Malcolm et al., (1989) report that the present range of this species is essentially the same as that shown in Woodson (1954), however, it is now more common in many areas where it was of sporadic occurrence. As in 1945, its main centre of abundance in Canada is in southern Ontario in the Sarnia, Hamilton, Toronto, Kingston, and Belleville areas (Dore & Groh, 1945; Urquhart & Urquhart, 1979). Since the 1945 surveys, the species has also become very common in portions of central and northern Ontario, especially in the vicinities of Sudbury, Sault Ste. Marie, and Thunder Bay (Urquhart & Urquhart, 1979).

Figure: North American distribution of Asclepias syriaca

Figure: Saskatchewan distribution of Asclepias syriaca

General Habitat

Fields, meadows, agricultural land, prairies, railway embankments, and roadsides.

Population Biology

Surveys were conducted for Common Milkweed in Ontario and Quebec in the mid 1940's (Groh & Dore, 1945). In these surveys, stands varied from a few isolated plants to patches of tens of thousands of stalks. In Michigan, Wilbur (1976) reports colonies of up to several thousand stems. Malcolm et al., (1989) studies Common Milkweed across its North American range and reported patch sizes from four to 25,000 stems with an average of 345 stems. This species spreads vegetatively by means of underground rhizomes that may form large clones of up to several thousand stems (Gleason, 1968). Normally, each patch represents a single clone (Wilbur, 1976). Even though other species of Milkweed are able to reproduce vegetatively, this species is the most prolific (Wilbur, 1976). Although the most important means of reproduction in this species is by vegetative means (Wilbur, 1976), Common Milkweed is able to produce large numbers of seeds compared to other Milkweed species. These air-borne propagules enable long-distance spread.

Evidence of Threats to Survival

It is considered to be a noxious weed in Nova Scotia, Manitoba, Quebec, and Saskachewan. All Milkweeds are considered noxious weeds in Ontario, however, this is the primary species of concern. In provinces--such as Nova Scotia--where there is a limited population, the species could be eliminated by an aggressive control program. No imminent threats are known in Ontario and Quebec because the species is so abundant.

Present Legal or Other Formal Status

This Milkweed has no legal protection and it is considered common or uncommon across most of its Canadian range, except is Saskatchewan where it is considered to be rare (Maher et al., 1979). Its status in the United States is unknown.

Assessment of Status

Asclepias syriaca is very common in southern Ontario--although less common elsewhere, it also occurs from Saskatchewan--where it is rare--to Nova Scotia.

Asclepias tuberosaL. - Butterfly-weed

Description

This distinctive Milkweed grows to 70 centimetres tall and has alternate to opposite, thick, fleshy leaves that are linear to lance-shaped. The entire plant is hairy and is often much-branched. It is the only Milkweed that does not have milky juice. The flower clusters are several-flowered and are terminal and also in many of the uppermost leaf axils. The hoods of each flower barely conceal the straight horns. The flowers are a distinctive yellow-orange colour and the seed pods are slender.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 75.

Biological and Economic Significance

This Milkweed is an uncommon plant whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of the Monarch Butterfly (Danaus plexippus), however, due to its limited range, it is probably not a significant food source.

Distribution

Asclepias tuberosa has a wide range in the United States from New England to North Dakota and south to Florida and Arizona. In Canada it occurs only in extreme southwestern Quebec and in Southern Ontario.

Figure: North American distribution of Asclepias tuberosa

General Habitat

Dry sandy soil of meadows, open woods, and prairies.

Population Biology

In Michigan, Wilbur (1976) reports that it occurs in colonies that range from isolated individuals to patches containing several hundred plants. This species is slow to reach flowering size (Woodson, 1954) but individual plants can be quite long-lived-one individual reported in Wilbur (1976) is known to be over 25 years old. Butterfly-weed is rarely affected by herbivores but only about 1% of flowers produce mature pods (Wilber, 1976).

Evidence of Threats to Survival

No imminent threats are known. Due to its lack of abundance and its "unmilkweed-like" appearance, it is not likely to be targeted by "weed control" programs.

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Quebec in Bouchard et.al., (1983). The Quebec record may not be native (Bouchard et.al., 1983). Its status in the United States is unknown.

Assessment of Status

Asclepias tuberosa is very rare in extreme southwestern Quebec and uncommon in southern Ontario-its only Canadian occurrences.

Asclepias variegataL. - Variegated Milkweed

Description

This Milkweed grows to one metre tall and has oval-shaped, opposite leaves that are broadest in the middle and generally taper abruptly at both ends. The flower clusters are compact and many-flowered and are terminal or sometimes one or more clusters occur also in the uppermost leaf axils. The hoods of each flower hide the short, flat horns. The flowers are white with a purplish centre.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 78.

Biological and Economic Significance

Distribution

Asclepias variegata has a moderate range in the southeastern United States from New England to Ohio and south to Florida and Texas. In Canada it is known only from an 1859 record in southern Ontario. Since this is a showy species that hasn't been recorded in the country in over 130 years, the record may be based on a cultivated plant or a short-lived escape from cultivation (Argus & Keddy, 1984; Morton & Venn, 1990). Cultivated or not, this Milkweed appears to be extirpated in Canada.

Figure: North American distribution of Asclepias variegata

Figure: Ontario distribution of Asclepias variegata

General Habitat

Upland woods and thickets.

Population Biology

The average population size for Variegated Milkweed is impossible to judge because the species' occurrence in Canada is based on a single, very old record.

Evidence of Threats to Survival

This Milkweed appears to be extirpated in Canada.

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Ontario in Argus & Keddy (1984). It is considered rare in Canada and is given the priority ranking of "3" in Argus & Pryer (1990) because of its absence or rarity in bordering states and because it is considered to be "NIX" nationally and "SX" in Ontario where "X" indicates "apparently extinct or extirpated without exception that it will be rediscovered". It is regarded as "SE?" in Ontario (possibly not native) in Oldham (1994).

In the United States, it is regarded as a "G5" species (Argus & Pryer, 1990) which means that it is "abundant and demonstrably secure with many occurrences".

The status in individual states from Argus & Pryer (1990) is as follows. "S1" (Critically imperiled)-Connecticut, New York, and Ohio, "S2" (Imperiled)-Indiana and New Jersey.

Assessment of Status

This Milkweed appears to be extirpated in Canada. It is not clear whether the only known record (made in 1859) is from garden material, from a garden escape, or from a naturally occurring stand that no longer exists.

Asclepias verticillataL. - Whorled Milkweed

Description

This slender, distinctive Milkweed grows to 50 centimetres tall and has numerous linear leaves that are arranged on the stem in whorls of three to six. The several flower clusters are terminal and in the upper leaf axils. The hoods of each greenish-white flower are surpassed by the curved horns.

Illustration

An illustration can be found Gleason (1968) vol 3, page 75.

Biological and Economic Significance

This Milkweed is a relatively rare plant whose natural significance is largely unknown. The species could be used occasionally as a food source by the larvae of Monarch Butterfly (Danaus plexippus), however, due to somewhat limited range and rarity, it is probably no a significant food source.

Distribution

Asclepias verticillata has a moderate range in the east-central United States from New England and Florida across to North Dakota and Texas. In Canada it occurs in southeastern Saskatchewan, southern Manitoba, and southwestern Ontario.

Figure: North American distribution ofAsclepias verticillata

Figure: Ontario distribution of Asclepias verticllata

Figure: Saskatchewan distribution of Asclepias verticllata

General Habitat

Dry fields, roadsides, and open sandy woods.

Population Biology

In southwestern Ontario, Oldham et al. (199l) report it to be very rare and localized, but with weedy tenancies. Wilbur (1976) described colonies in Michigan as dense but widely scattered, however, he did not report actual plant numbers.

The species can reproduce vegatatively under certain circumstances (Kephart, 1981) resulting in colonies several metres across that consist of just a few genetic individuals (Wilson et al., 1979). Whorled Milkweed is largely self-incompatible but some seeds may be produced by self-pollination (Wilson et al., 1979). Plants tend to be long-lived and are not affected much by herbivores (Wilbur, 1976).

Evidence of Threats to Survival

No imminent threats are known. Due to its roadside and field habitat, it could be targeted by "weed control" programs.

Present Legal or Other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Saskatchewan in Maher et al., (1979), rare in Ontario in Argus & Keddy (1984), and uncommon in Manitoba. Since it is not rare in all of its Canadian range, it has no priority ranking in Argus & Pryer (1990). Its status in the United States is unknown.

Assessment of Status

Asclepias verticillata is rare in southwestern Ontario and in southeastern Saskatchewan. It is uncommon in Manitoba. A 1992 collection made by M.J. Oldham (specimen at CAN) is shown on the Ontario map. The new record occurs within the known range of the species and does not change its rare status in Ontario.

Asclepias viridifloraRaf. - Green Milkweed
(Acerates viridiflora(Raf.) Pursh ex Eat.)

Description

This Milkweed grows to 80 centimetres tall and has broad, opposite leaves that are blunt at both ends. The lateral flower clusters are densely-flowered in the upper leaf axils. The hoods of the greenish-white flowers lack horns.

Illustration

An illustration can be found in Gleason (1968) vol 3, page 80.

Biological and Economic Significance

Distribution

Asclepias viridiflora has an extensive range in the east-central United States from southern New England and Georgia across to Wyoming and New Mexico. In Canada it occurs in southern British Columbia, southeastern Alberta, southern Saskatchewan, southern Manitoba, and southern Ontario.

Figure: North American distribution of Asclepias viridiflora

Figure: Ontario distribution of Asclepias viridiflora

General Habitat

Sand dunes, dry prairie hillsides, and open sandy woods.

Population Biology

The average population size for this Milkweed is unknown. Green Milkweed suffers from herbivore damage but it has a low mortality due to its large root storage capacity (Wilbur, 1976).

Evidence of Threats to Survival

No imminent threats are known. Due to its habitat in woods and dunes, it is not likely to be targeted by "weed control" programs.

Present Legal or other Formal Status

This Milkweed has no legal protection, however, it is considered rare in Alberta in Argus &White (1978), rare in Ontario in Argus & Keddy (1984), and uncommon in Manitoba. Since it is not rare in all of its Canadian range, it has no priority ranking in Argus & Pryer (1990). Its status in the United States is unknown.

Assessment of Status

Asclepias viridiflora is rare in southern Ontario and in southeastern Alberta. It is uncommon in Manitoba.

Status, Distribution, and Potential Impact from Noxious Weed Legislation

3.0 NOXIOUS WEED LEGISLATION
Because some species of Milkweed act in an aggressive or invasive manner in agricultural land and along roadsides, some provinces have included one or more species of Milkweed on their list of "noxious" plants that should or must be controlled or eradicated whenever they are found. There is no federal legislation that is concerned specifically with noxious weeds, however, the federal Department of Agriculture administers the Seeds Act that is indirectly concerned with weeds because the seed grade of crops is partly determined by the content of weed seeds (White et al., 1993).

Provincial Weed Acts address both weeds and weed seeds (White et al., 1993) and they will be summarized below in terms of their possible application to species of Milkweed. Provincial Weed Acts are normally laid out in two parts. The first part addresses the implementation of the Act in terms of obligations, procedures, penalties for non-compliance, and exceptions to the Act. The second part generally lists the species to which the Act applies and may indicate methods for destroying particular species (White et al., 1993). Newfoundland does not have weed control legislation nor are there any Milkweeds known from that province.

3.1 Weed Legislation Summary

Federal Seeds Act

The following information on federal weed legislation was provided by Tom Hodgson of Agriculture Canada, Ottawa. The Weed Seeds Order is part of the federal Seeds Act that identifies a number of plants as noxious weeds. There are no Milkweeds on the Weed Seeds Order list and Milkweed seeds are not seen as a problem in crop seeds.

Alberta

The following information on Alberta weed legislation was provided by Walter Yarish of Alberta Agriculture, Edmonton. There are no Milkweeds listed on the Alberta Weed Act but individuals may try to eliminate Milkweeds where there is a local problem. In Alberta, Milkweeds are not considered to be a significant problem and there is presently no program to try to eliminate any species of Milkweed in the province.

British Colombia

The following information on British Columbia weed legislation was provided by Dave Ralph of the Ministry of Agriculture, Kamloops. There are no species of Milkweed on the British Columbia Weed Act and no programs in place targeting Milkweeds.

Manitoba

The following information on Manitoba weed legislation was provided by Jo Anne Buth of Manitoba Agriculture, Carman. The Manitoba Weed Act has been recently revised and now lists only Common Milkweed (Asclepias syriaca) and Showy Milkweed (Asclepias speciosa). Showy Milkweed is only on the list because of some minor, local problems in a few municipalities, generally it is not considered to be a significant problem. Common Milkweed is not seen as a major problem, however, some municipalities are attempting to control the species and have active eradication programs on a "complaint" basis. Its abundance in the province varies from year to year and from area to area but it is generally considered to be static or increasing somewhat.

New Brunswick

The following information on New Brunswick weed legislation was provided by Kevin McCully of the New Brunswick Department of Agriculture. Although the New Brunswick Weed Act was written in 1969, it has never been proclaimed into law and there are no plans to do so in the near future.

Newfoundland

There is no weed control legislation in Newfoundland.

Nova Scotia

The following information on Nova Scotia weed legislation was provided by Doug Doohan of the Nova Scotia Department of Agriculture, Truro. Common Milkweed (Asclepias syriaca) is the only Milkweed listed in the Weed Act of Nova Scotia. There is an active program to eradicate the species from the province because although it is presently known from only a limited number of infestations-mostly in the Annapolis Valley-and thus vulnerable to concerted action, it is likely to increase in the future if there is no control program because of a recent shift in agricultural practices to low-tillage management. Such a shift by Ontario farmers has contributed to the recent increase of the species in agricultural land in that province.

Ontario

The following information on Ontario weed legislation was provided by Howard Lang, Ontario Ministry of Agriculture, Food, and Rural Affairs, Guelph. All species of Milkweed are listed as noxious weeds in the province but it is Common Milkweed that is the main concern and the only species against which measures are normally taken. There is no active program to target Milkweeds, rather, the main thrust is to respond on a "complaint" basis to control a particular problem-there is no attempt at eradication. For a complaint to be acted on, a species must be causing a problem for a horticultural or agricultural operation-no action is taken under the weed act against Milkweeds growing along roadsides or a vacant land unless there is a clear threat to adjacent property involved with agriculture or horticulture. Municipalities can act against noxious weeds but they are also primarily concerned with problems affecting agricultural or horticultural operations. Common Milkweed has been around for a long time and it does not seem to be an increasing agricultural problem.

Prince Edward Island

The following information on Prince Edward Island weed legislation was provided by Sherry Ross of the Prince Edward Island Department of Agriculture, Fisheries, and Forestry, Charlottetown. There are no species of Milkweed on the Prince Edward Island Weed Act.

Quebec

The following information on Quebec weed legislation was provided by Pierre Lavigne, Ministere de l'Agriculture, des Pecheries et de l'Alimentation, Sainte-Foy. Common Milkweed (Asclepias syriaca) is the only Milkweed listed in the Weed Act of Quebec. This species is a persistent problem in corn and soya bean fields but the population seems to be stable. There is no active program to control or eliminate Common Milkweed in Quebec.

Saskatchewan

The following information on Saskatchewan weed legislation was provided by Doug Billet of the Saskatchewan Department of Agriculture. Common Milkweed (Asclepias syriaca) is the only Milkweed listed in the Weed Act of Saskatchewan. This species is very sporadic in southeast Saskatchewan and the population seems to be stable. There is no active program to control or eliminate Common Milkweed because it is not seen as a problem.

TABLE 2: Summary of Weed Legislation Affecting Milkweeds

Milkweed Species Jurisdiction

CAN BC AB SK MB ON QC NB NS PE

Asclepias exaltata . . . . . X . . . .

A. hirtella . . . . . X . . . .

A. incarnata . . . . . X . . . .

A. lanuginosa . . . . . . . . . .

A. ovalifolia . . . . . X . . . .

A. purpurascens . . . . . X . . . .

A. quadrifolia . . . . . X . . . .

A. speciosa . . . . X . . . . .

A. sullivantii . . . . . X . . . .

A. syriaca . . . X X X X . X .

A. tuberosa . . . . . X . . . .

A. variegata . . . . . X . . . .

A. verticillata . . . . . X . . . .

A. viridiflora . . . . . X . . . .

AB=Alberta, BC=British Columia, CAN=Canada, MB=Manitoaba, NB=New Brunswick, NS=Nova Scotia, ON=Ontario, PE=Prince Edward Island, QC=Quebec, SK=Saskatchewan. An "X" in a cell indicates that the species is considered to be a noxious weed in that jurisdiction. If a cell is empty, it indicates that the species is not included in that jurisdiction's noxious weed legislation. Newfoundland does not have a weed control act.

3.2 Potential Impact from Weed Acts

There are 14 species of Milkweed that occur in Canada. Of these, only two species-Common Milkweed (Asclepias syriaca) and Showy Milkweed (Asclepias speciosa)-are considered to be problem weeds. Showy Milkweed (Asclepias speciosa) is listed as a noxious weed in Manitoba, however, it is not considered to be much of a problem in that province and is targeted only on a "complaint" basis.

Common Milkweed (Asclepias syriaca) is also listed as a noxious weed in Manitoba and although it is more common than Showy Milkweed, it is not considered to be a significant problem and it too is targeted only on a "complaint" basis. In Saskatchewan and Quebec, Common Milkweed is listed as a noxious weed but there is no active program in either province to target the species. In Saskatchewan the species is too rare to be a concern and in Quebec it is only regarded as a problem in two particular crops. Nova Scotia has a very localized population of Common Milkweed that the province considers to be a noxious weed and a threat to spread. For that reason, there is an active program in Nova Scotia to attempt eradication while the species is still confined and hence vulnerable.

The province of Ontario classifies all Milkweeds as noxious weeds, however, the primary concern is for Common Milkweed. Although there have been attempts to control Common Milkweed in particular areas, there is no broad-scale program to eradicate the species. Considering the abundant and spreading population of Common Milkweed in the province, it is unlikely that there are sufficient resources available to significantly reduce the species in Ontario even if a broad program was undertaken.

The Monarch Butterfly (Danaus plexippus) occurs across much of Canada as adults, however, its breeding range is mainly restricted to south and central Ontario, and southern Quebec. Although the Monarch larva can feed on a range of Milkweed species, it is largely dependent on Common Milkweed (Asclepias syriaca). From the point of view of the Monarch Butterfly, it is primarily the Ontario populations of Common Milkweed that are most important to its breeding in Canada. The enforcing of noxious weed legislation in some provinces-like Nova Scotia-could impact peripheral colonies of Monarch Butterflies. In Ontario, however, where there is a very abundant and spreading population of Common Milkweed, it seems unlikely that enforcing the noxious weed act could put the Monarch Butterfly at significant risk.

Status, Distribution, and Potential Impact from Noxious Weed Legislation

4.0 INFORMATION SOURCES
4.1 References

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Argus, G.W. and D.J. White. 1978. The rare vascular plants of Alberta. Syllogeus 17.47 pp.

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Cabin, R.J.,J. Ramstetter, and R.E. Engel. 1991. Reproductive limitations of a locally rare Asclepias. Rhodora 93:1-0.

Crolla, J.P. and J.D. Lafontaine. 1996. Status report on the Monarch Butterfly (Danaus plexippus) in Canada. Canadian Wildlife Service. 24 pp. Unpublished report.

Dore, W.G. 1994. White-flowered form of Asclepias syriaca. Rhodora 46:387.

Doyon, D. 1958. Etude de la distribution géographique de l' asclépiade commune (Asclepias syricaca L.) en Amérique due Nord. 40e rapport de la Société de Québec pour la Protection des Plantes, pp. 91-113.

Erskine, J. S. 1960. Flora of Prince Edward Island. Canada Department of Agriculture Publication No. 1088.

Fox, W.S. 1944. Botanical field notes-summer of 1943. Canadian Field-Naturalist 58:10-11.

Gleason, H.A. 1968. The new Britton and Brown Illustrated Flora of the Northeastern United States and Canada. 3 vols. New York, Hafner Press.

Gleason H.A. and A. Cronquist. 1963. Manual of vascular plants of northeastern United States and adjacent Canada. Van Nostrand Reinhold Co., New York. 810 pp.

Groh, H. 1943. Notes on common milkweed. Scientific Agriculture 23:625-632.

Groh, H. and W.G. Dore. 1945. A milkweed survey in Ontario and adjacent Quebec. Scientific Agriculture 25:463-48l.

Haber, E. and Members of the Subcommittee for Vascular Plants, Mosses, and Lichens. 1995. Guidelines for the preparation of COSEWIC Status Reports on vascular plants, mosses, and lichens. Committee on the Status of Endangered Wildlife in Canada (COSEWIC), Ottawa. 16pp.

Hill, G.a. 1959. A ready reference to the description of the Land of Ontario and its productivity. Department of Lands and Forests.

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Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. Second edition. Volume 1-checklist. Biota of North America Program and the North Carolina Botanical Garden. Timer Press, Portland, Oregon. 622 pp.

Kephart, S.R. 1981. Breeding systems in Asclepias incarnata L., A. syricaca L.; and A. verticillata L. American Journal of Botany 68:226-232.

Kephart, S.R. and C.B. Heiser, Jr. 1980. Reproductive isolation in Asclepias: lock and key hypothesis reconsidered. Evolution 34:738-746.

Kingsbury, J.M. 1964. Poisonous plants of the United States and Canada. Prentice-Hall Inc., New Jersey. 626 pp.

Logie, A. 1861. List of plants found growing in the neighborhood of Hamilton, during the years 1859 and 1860. Annals of the Botanical Society of Canada 1:90-108.

Maher, R.V., D.J. White, G.W. Argus and P.A. Keddy. 1978. The rare vascular plants of Nova Scotia. Syllogeus 18.38 pp.

Maher, R.V., G.W. Argus, V.L. Harms, and J.H. Hudson. 1979. The rare vascular plants of Saskatchewan. Syllogeus 20.55 pp.+ maps.

Malcolm, S.B., B.J. Cockrell, and L.P. Brower. 1989. Cardenolide fingerprint of monarch butterflies reared on common milkweed,Asclepias syriaca L. Journal of Chemical Ecology 15:819-853.

Marie-Victorin, F. 1995. Flore laurentienne. Troisiéme édition. Mise à jour et annotée par Luc Brouilet et Isabelle Goulet. Les Presses de l'Université de Montréal, Montréal.

Moore, R.J. 1947. Investigations on rubber-bearing plants. V. Notes on the flower biology and pod yield of Asclepias syriaca L. The Canadian Field-Naturalist 61:40-46.

Morton, J.K. and J.M. Venn. 1990. A checklist of the flora of Ontario-vascular plants. University of Waterloo Biology Series #34. 218 pp.

Oldham, M.J. 1994. Natural heritage resources of Ontario: rare vascular plants. Natural Heritage Information Centre, Peterborough. 48 pp.

Oldham, M.J., D. McLeod, W.G. Stewart, and J.M. Bowles. 1991. Preliminary annotated checklist of the vascular plants of Elgin, Middlesex, and Oxford Counties, Ontario Ministry of Natural Resources, Aylmer. 174 pp.

Robinson, F.A. and E. Oertel. 1975. Sources of nectar and pollen, pp. 283-302. In Dadant and Sons (eds). The hive and the honey bee. Dadant and Sons, Hamilton, Illinois.

Roland, A.E. and E.C. Smith. 1969. The flora of Nova Scotia. Nova Scotia Museum, Halifax 743 pp.

Rowe, J.S. 1972. Forest regions of Canada. Canadian Forestry Service. Publication No. 1300. 172 pp.

Scoggan H.J. 1957. Flora of Manitoba. National Museums of Canada. Bulletin 140.

Scoggan, H.J. 1978-1979. The flora of Canada. 4 parts. National Museums of Canada, Ottawa. 1711 pp.

Shannon, T.r. and R. Wyatt. 1986. Reproductive biology of Asclepias exaltata. American Journal of Botany 73:11-20.

Straley, G.B., R.L. Taylor, and G.W. Douglas. 1985. The rare vascular plants of British Columbia. Syllogeus 59. 123 pp. + maps.

The Nature Conservancy. 1988. Element state tracking database, Central Scientific Databases. The Nature Conservancy, Arlington, Virgina, U.S.A. Unpublished database printout.

Urquhart, F.A. and N.F. Urquhart. 1979. Breeding areas and overnight roosting locations in the northern range of the Monarch Butterfly (Danaus plexippus plexippus) with a summary of associated migratory routes. The Canadian Field-Naturalist 93:41-47.

White, D.J. and K.L. Johnson. 1980. The rare vascular plants of Manitoba. Syllogeus 27.52 pp. + maps.

White, D.J., E. Haber, and C. Keddy. 1993. Invasive plants of natural habitats in Canada-An integrated review of wetland and upland species and legislation governing their control. Canadian Wildlife Service, Ottawa. 115 pp.

Wilbur, H.M. 1976. Life history evolution in seven milkweeds of the genus Asclepias. Journal of Ecology 64:223-240.

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Wyatt, R. 1976. Pollination and fruit-set in Asclepias: a reappraisal. American Journal of Botany 63:845-851.

4.2 Collections Consulted

The following herbaria were consulted for records of Milkweeds in Ontario in 1983. All Ontario specimens from these herbaria were examined by C. Keddy for the Ontario Rare Plant Atlas (Argus & Keddy, 1984). The herbaria at CAN and DAO were also checked in 1996 by the present author for records of rare Ontario Milkweeds received after 1983.

CAN - Canadian Museum of Nature, Ottawa QFA - Laval University, Montreal.

DAO - Department of Agriculture, Ottawa. OK - Queens's University, Montreal.

HAM - Royal Botanical Gardens, Hamilton RPP - Herbarium of Rondeua Provincial Park.

LKHD - Lakehead University, Thunder Bay. SFS - Sherbrooke University, Sherbrooke.

MT - University of Montreal, Montreal. TRT - University of Toronto, Erindale College, Toronto.

MTJB - Montreal Botanical Garden, Montreal. UWO - University of Western Ontario, London.

OAC - Ontario Agricultural College, Guelph. WLU - Wilfred Lauier University, Waterloo.

PFM - Herbarium of P.F. Maycock, Erindale College, Toronto.

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The Milkweeds of Canada (Asclepias spp.)

Asclepias exaltata L. - Poke Milkweed

Asclepias hirtella (Pennell) - Prairie Milkweed

Asclepias incarnataL. - Swamp Milkweed

Asclepias lanuginosa Nutt. - Hairy Milkweed (Asclepias otariodes auct. non Fourn.:Acerates lanuginosa (Nutt.) Dcne.)

Asclepias ovalifolia Dcne. - Oval-leaved Milkweed

Asclepias purpurascensL. - Purple Milkweed

Asclepias quadrifolia Jacq. - Four-leaved Milkweed

Asclepias speciosa Torr. - Showy Milkweed

Asclepias sullivantii Engelm. ex Gray - Sullivant's Milkweed

Asclepias syriacaL. - Common Milkweed

Asclepias tuberosaL. - Butterfly-weed

Asclepias variegataL. - Variegated Milkweed

Asclepias verticillataL. - Whorled Milkweed

Asclepias viridifloraRaf. - Green Milkweed (Acerates viridiflora(Raf.) Pursh ex Eat.)

Asclepias lanuginosa Nutt. - Hairy Milkweed
(Asclepias otariodes auct. non Fourn.:Acerates lanuginosa (Nutt.) Dcne.)

Asclepias viridifloraRaf. - Green Milkweed
(Acerates viridiflora(Raf.) Pursh ex Eat.)