Status Report on the Monarch Butterfly (Danaus plexippus) in Canada

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

Table of Contents

A. Abstract	G. Limiting Factors
B. Distribution	H. Special Significance of the Species
C. Protection	I. Recommendations and Management Options
D. Population Size and Trend	J. Evaluation
E. Habitat	K. References
F. General Biology	L. Acknowledgments

A. ABSTRACT
The Monarch, Danaus plexippus (Linnaeus), is a large showy butterfly that is famous for its spectacular migration between breeding areas in the United States and Canada and overwintering sites in Mexico and California. The small size and fragile nature of the overwintering sites has brought the species into public attention, and made the Monarch a symbol of natural history conservation. Recent disasters and human pressures at the overwintering sites have made the Monarch susceptible to a catastrophic decline in numbers that could potentially push the North American populations of the Monarch to the brink of extinction. Within the breeding areas in the United States and Canada, the species has greatly declined in its former range in the Great Plains, while expanding into the eastern United States and southeastern Canada with the clearing of the forest s and creation of open habitat suitable for Monarch breeding. Because of the vulnerability of the species, it has become increasingly necessary to preserve remaining Monarch breeding habitat, especially in the eastern United States and southeastern Canada. This is complicated by the fact that milkweed is considered a noxious weed in most areas. Recommendations for the recognition and preservation of remaining habitat, and the removal of milkweed from noxious weed acts are discussed.

B. DISTRIBUTION
North American Distribution
The Monarch is widely distributed across North America from Central America northwards to southern Canada, and from the Atlantic Coast westwards to the Pacific Coast. Within this broad area of occurrence, there are three geographically distinct populations of the Monarch (the eastern, western and Central American populations) which collectively make up the total North American range of the species. The appearance of the adults of these three populations is so similar that no-one has proposed treating them as distinct subspecies; however, these populations obviously differ genetically in that their migratory patterns are distinct and preclude little if any gene exchange through interbreeding.

The Central American population of the Monarch occurs in Guatemala, El Salvador, Honduras, British Honduras, Nicaragua, Costa Rica, Panama and southern Mexico. Unlike the eastern and western populations, the Central American Monarch population is relatively sedentary, although short distance (10 to 100 km) seasonal migrations in Costa Rica between highland and lowland regions, in an east to west direction, have recently been documented (Haber 1993). In contrast to the other North American populations, Central American Monarchs are reproductively active throughout the year, and the migration of the species in Costa Rica is dictated by dry and wet seasonal conditions and their favourability for breeding. This is the only North American population that has a relatively fixed year-round distribution, and it does not contribute to the numbers of Monarchs found in Canada and the United States each year.

The eastern population of the Monarch is the largest of the three North American populations, and includes all Monarchs that occur each year east of the Rocky Mountains in the United States and Canada. The present annual breeding range of the eastern population extends from the Gulf Coast States (Texas, Louisiana, Mississippi, Alabama, Georgia and Florida) northwards to southern Canada (Alberta to New Brunswick and Nova Scotia), and from the Great Plains States and Prairie Provinces eastwards to the Atlantic Coast and the Maritime Provinces. The entire eastern population annually migrates to a small number of overwintering sites in central Mexico where they congregate in vast numbers.

The western population includes all Monarchs found west of the Rocky Mountains in the United States and Canada. The present annual breeding range of western Monarchs extends from the southwestern United States (Arizona and New Mexico) northwards to southern Canada (British Columbia) and from the Rocky Mountains westwards to the Pacific Coast. The entire western North American population overwinters each year at numerous sites along the coast of California.

In the last 150 years there has been a major shift in the North American distribution of the eastern population of the Monarch (see Brower 1995). Until the 1880's, the prairie region of central North America appears to have been the main breeding area of the eastern Monarch population. The native prairie flora includes about 22 species of habitat-specific milkweeds (Asclepias), many of which can serve as larval hostplants, and an abundance of flowering plants that provide a diverse array of nectar resources for adult Monarchs.

In the latter half of the nineteenth century, plowing destroyed 433 million acres of the midwestern prairie, and by 1910 most of the native prairie had been converted to crop land (Brower 1995). Concurrent to this widespread destruction of the prairie flora, the deciduous forests of eastern North America were being cleared on a vast scale. Most of the deciduous forest in the eastern United States had been cleared by 1860, and from 1860 to 1890 an additional 50 million acres of forest in the Great Lakes region was cut (Brower 1995). One result of the opening of the eastern deciduous forest was a rapid and widespread proliferation of the weedy Common Milkweed (Asclepias syriaca) in cleared lands across the northeast. The destruction of the native prairie flora of the midwest (with its associated milkweeds), and the concurrent rapid spread and increase in abundance of Common Milkweed in the northeast, appears to have resulted in a major shift in the main breeding range of the eastern population of the Monarch, from the Great Plains to northeastern North America This expansion into the northeast was characterized by a shift from utilization of a variety of prairie milkweeds as larval hostplants, to the utilization of Common Milkweed as the primary larval host of the eastern population of the Monarch in North America.

The historically cleared eastern deciduous forest region corresponds to the principal area of concentrated breeding of the eastern population of the Monarch in the present day. This consists of a large area of the eastern United States and southeastern Canada, extending from 32 ^o north latitude northwards to 48^o north latitude, and from 95 ^o west longitude eastwards to the Atlantic Coast (Urquhart 1960).

Distribution in Canada
The distribution of the Monarch in Canada is shown in the distribution map below. The map incorporates both historic and current collections and observations of the Monarch, based on information in the database of the Canadian National Collection (CNC) of Insects at Agriculture Canada in Ottawa. Each dot represents a location in Canada where the Monarch has been recorded, and an individual dot may be based on one or many records of occurrences at a given location. The majority of records shown are adult butterflies, but, since the Monarch is easily recognized as both a caterpillar and a pupa, records of Monarchs encountered in all stages of its life cycle are indicated.

In Canada, the Monarch has been recorded in all ten provinces and in the Northwest Territories. Its breeding range occupies the area lying mainly south of 50 ^o latitude, although in the Prairie Provinces the range extends north to about 54 ^o latitude. This northern range limit in Canada represents the northern limit of the Monarch's breeding range in North America, which corresponds with the northern range limit of milkweeds (Asclepias). The western population of the Monarch occurs in Canada only in southern British Columbia. Western Monarchs generally reach British Columbia only in summers with protracted periods of warm, sunny weather in the Pacific Northwest, when conditions are most favorable for northward bound migrants. When they do occur, because they are strong, high fliers, flights of Monarchs spreading out over the province will usually reach available patches of milkweed. Breeding occurs in scattered locations in the province, particularly in the Okanagan Valley and along the Fraser River.

The eastern population of the Monarch occurs in Canada from Alberta eastwards to Newfoundland, and accounts for over 90% of the Canadian distributions of the species. Two occurrences from northern British Columbia and one from Great Slave Lake in the Northwest Territories are records of adult Monarchs from the eastern population that wandered well north of the normal breeding range.

Eastern Monarchs reach Canada with regularity each year, although numbers can very dramatically from year to year depending on the success of overwintering in Mexico, and the size of spring generations produced annually in the Gulf Coast States during the spring migration. Distribution east of the Rocky Mountains is varied, but can be divided into three principal regions where occurrence is similar: the Prairie Provinces (Alberta, Saskatchewan and Manitoba), southern Ontario and southern Quebec, and the Maritime Provinces (New Brunswick, Nova Scotia, Prince Edward Island and Newfoundland).

In the Prairie Provinces extensive breeding occurs only in the southern portion, and the abundance of Monarchs decreases northwards and westward from Manitoba to Alberta. Southern Ontario and southern Quebec represents the most extensive area of breeding in Canada. Abandoned farmland, the prime habitat for Common Milkweed, is widespread in southern Ontario and Quebec, and this region supports the main breeding population of Monarchs in Canada each year. Monarchs reach the Maritime Provinces with less regularity, and breeding occurs only at scattered locations due to the limited distribution o f milkweeds. In New Brunswick, breeding occurs mainly along the banks of the Saint John River, and in Nova Scotia breeding is confined mainly to the Annapolis Valley. The Monarch reaches Newfoundland as a migrant, sometimes in considerable numbers, but does not breed there since milkweeds do not occur in the wild and do not fare well when planted. In Prince Edward Island, Swamp Milkweed occurs in the wild, and Common Milkweed has been introduced, but both have a very limited distribution in the province, and no records of Monarch breeding could be found.

At present there is abundant habitat for Monarchs in southern Ontario and Quebec, consisting mainly of abandoned farms and roadside verges. This habitat can easily be lost in one of three ways:

1. Abandoned farms being put into active production.
2. Succession of abandoned farmlands into wooded and brushy habitats.
3. Active programs to eliminate milkweed.

The continued availability of milkweeds growing on abandoned farmlands can be ensured by encouraging farmers to cut abandoned fields in the late fall (mid-September to early October) when Monarch breeding has ended. This would restrict the growth of woody plants (trees and shrubs) - which normally crowd out milkweeds in the succession of old fields to brushy and wooded habitats - while allowing milkweeds and nectar-producing wildflowers to continue to thrive each year. Weedy and brushy fields in parks (including provincial and national parks in southern Ontario which contain areas of abandoned farmland) could be maintained in the same way through selective cutting or controlled burning.

Rather than maintaining grasses along the median strips and verges of highways, wildflower mixes including milkweeds could be planted instead, as has been done in some areas of the United States. While the maintenance of grassy borders along roads and highways requires the application of herbicides and repeated mowings, strips of wildflowers would require mowing only once a year, in the late fall, to restrict the growth of woody plants. In addition to creating increased breeding habitat for Monarchs, maintaining wildflowers along highways would be more economical, and would be beneficial to many other species of butterflies and flower-visiting insects. Maintaining powerline corridors through selective cutting rather than herbicide spraying would similarly increase the availability of hostplant and nectar resources for Monarchs.

The creation of butterfly gardens and milkweed areas in communities would also increase the availability of Monarch habitat, and would partially compensate for the loss of habitat resulting from urban sprawl. The creation of butterfly gardens could be undertaken as part of public education and awareness programs in schools and communities, highlighting the uniqueness of the Monarch and the role that community and backyard gardens can play in ensuring its continued survival in North America. There has been a great increase in public awareness of the Monarch and its annual migration in the last few years (see section H) and many communities would probably respond positively if the connection between milkweed in gardens, and the spectacular migration and overwintering in Mexico, were drawn. The increasing popularity of butterfly gardening in North America, and the growing number of seed houses now dealing in wildflowers, suggests that community programs to develop Monarch habitat would be well received.

Milkweed are presently considered noxious weeds in most areas of Canada. The decision to control milkweeds in left to the provinces, and ultimately to regional municipalities. In most areas there appears to be no active program to eliminate milkweeds, except where there are specific complaints. However, lepidopterists contacted in both Manitoba and Nova Scotia reported that milkweeds were currently being sprayed and eliminated in their areas (see section D). The removal of milkweeds from provincial noxious weed acts is a necessary first step to ensuring the continued existence of breeding habitat for the Monarch in Canada. An obvious conflict is created by encouraging communities, farmers and various agencies to maintain milkweed areas and plant butterfly gardens, while officially milkweeds are still recognized as noxious weeds. Milkweeds are included in noxious weed acts because they are considered to be poisonous to livestock. However, the level of toxicity varies among milkweed species, and Common Milkweed, which is one of the least toxic species, was apparently listed by the Ontario Weed Control Act based only on circumstantial evidence (Alex 1992). In many habitats where they occur milkweeds pose no direct threat to livestock, and they could be safely removed from noxious weed acts, while recognizing that in some situations where there is a real concern it may still be necessary to eradicate them.

In addition to ensuring that breeding habitat for Monarchs in Canada remains widely available, consideration should also be given to the importance of certain areas of Canada to the success of the Monarch's annual migration. The main population of Monarchs breeding in eastern Canada funnels through southern and eastern Ontario each year, and the availability of roosting sites and nectar sources is crucial to the success of their fall migration. Staging areas along the north shores of Lake Ontario and Lake Erie where Monarchs gather into migratory swarms and form large aggregations at overnight roosting spots should be recognized and protected. While it is probable that specific trees are not crucial as roosting sites, the presence of both suitable roosting trees and adjacent open areas where the butterflies can forage for nectar is essential. Critical stands of trees where overnight roosts form each year should be identified and protected from disturbance such as the development of campgrounds, picnic grounds, and roadways. Weedy fields adjacent to these areas, that support large numbers of wildflowers suitable for nectaring, should be maintained through selective cutting. Most staging areas in Canada lie within protected areas (e.g. Presquile Provincial Park, Point Pelee National Park, etc.) but this does not ensure the maintenance of suitable habitat for the monarch. For example, most areas of former abandoned fields in parks are rapidly changing to brushy and wooded habitats that are not suitable for nectaring.

D. POPULATION SIZE AND TREND
Based on estimates of the number of butterflies overwintering in Mexico and California, the eastern population of the Monarch presently numbers in the tens of millions, while the smaller western population numbers in the millions. Historical data indicates that the size of both populations fluctuates regularly, and often dramatically, as a result of winter storm mortality, poor breeding conditions, predation, parasitism, disease and other pressures in combination with each other. Numbers can vary by a factor of 10 over time, and in the past both the eastern and western populations of the Monarch have suffered losses approaching 90% and still recovered when the 10% surviving experienced optimal conditions in the breeding range. However, while a fluctuating population size appears to be the norm for Monarchs, human degradation of the overwintering sites in Mexico is resulting in consistently higher levels of mortality, which may be pushing the population size below a threshold level from which it can recover itself. Widespread and increasing use of herbicides throughout North America may also result in dwindling fall migrations to the overwintering sites in the next few years (Brower 1995), due to the eradication of hostplants and nectar sources in the breeding range. Without real protection of the Mexican overwintering sites, as well as protection of breeding habitats and nectar resources along migration routes in Canada and the United States, the eastern population of the Monarch may become extirpated in North America early in the next century (Brower 1995).

The number of Monarchs reaching Canada each year is entirely dependent on the success of overwintering in Mexico and California, and on weather conditions in the United States during the northward remigration in the spring and early summer. Severe winter storms with freezing temperatures at the Mexican overwintering sites resulted in extremely high mortality in the eastern population of the species in the winter of 1991-92, with 80% mortality being recorded at one site (Brower 1995), and high mortality has also been reported in the recent winter of 1995-96. At the overwintering sites in California, possibly as a result of disease, numbers of western Monarchs crashed in 1992, dropped to the lowest level on record by the winter of 1994-95, then rebounded to near-normal levels in the winter of 1995-96.

Few Monarchs reached Canada in 1992 due to the high mortality at the overwintering sites in Mexico, and numbers remained low over much of the country in 1992 and 1993 due to unusually cool, wet summers. Numbers in Canada increased in 1994 and 1995 to about average. Monarch occurrence in each region of Canada in the last five years is described in more detail below.

British Columbia
Monarchs occur erratically in British Columbia at the best of times and in recent years there have been few reports. In the last five years, breeding has been observed near Lillooet (adults and larvae in numbers) and in the Okanagan Valley (larvae on milkweed).

Prairie Provinces
In Alberta, Monarchs occur very rarely, and breeding usually occurs only in the southern region of the province, and rarely as far north as Edmonton. In 1994 about a dozen Monarchs were found breeding near Taber in the Old Man River area.

In Saskatchewan, where Monarchs are also scarce and irregular, none was recorded in 1993, but in 1994 a small migration occurred with a total of 20 Monarchs being reported in the province (Minno & Minno 1995).

In southern Manitoba, Monarchs occur regularly and in greater numbers than elsewhere in the prairies, although they are less common in the southeastern part of the province, which is more extensively wooded and supports less Monarch habitat. Monarchs were common to abundant in 1991 in southern Manitoba. In 1992 they were uncommon, and weak flights with very few Monarchs being reported occurred in 1993 and 1994. In 1995 Monarchs were again fairly common, but still below the levels of 1991. In southern Manitoba, milkweeds are being sprayed with herbicides in roadside ditches in response to complaints from farmers who were concerned that the plants would spread from roadsides into their fields.

Ontario and Quebec
In southern Ontario and southern Quebec, Monarchs occur annually, and frequently in abundance. In 1991 numbers in Ontario and Quebec were at normal levels with Monarchs being common in most milkweed patches. In 1992, after the disastrous winter kill in Mexico, Monarch numbers were very low over much of Ontario, making it one of the worst years for Monarchs ever recorded (McKown 1993). Monarchs increased to some degree in 1993, and a large migratory flight was noted at Point Pelee (see Species Movement in section F). The Monarch was more common in 1994 and 1995, almost back to normal levels, reflecting the gradual recovery of the species from the 1992 disaster. In spite of generally low numbers in much of Canada over the last few years, Point Pelee in extreme southern Ontario (the primary concentration point for migrating Monarchs in Canada) continued to report large numbers of Monarchs.

Maritime Provinces
In New Brunswick, Monarchs occur irregularly and breeding is largely restricted to a limited area of the province where Common Milkweed grows along the banks of the Saint John River. Dr. Anthony Thomas of Forestry Canada (personal communication) has seen about 100 Monarchs in the province in 20 years of field work; 1991 was a good year for Monarchs in New Brunswick, but only one or two per year were recorded form 1992 to 1995.

In Nova Scotia, Monarchs are never very common; none was encountered in 1994, and only a few were found in 1995. Common Milkweed is rare and localized in Nova Scotia, and officers spray for it when they find it. As a result, Monarch breeding in Nova Scotia does not produce large numbers of adults.

In both Nova Scotia and New Brunswick, limited breeding occurs on localized patches of Swamp Milkweed.

E. HABITAT
In Canada, suitable breeding habitat for Monarchs exists wherever milkweeds (Asclepias) occur. In eastern Canada, the primary larval hostplant of the Monarch is Common Milkweed (Asclepias syriaca), which accounts for 95% of the milkweed found in the east. Common Milkweed grows widely on abandoned farmland, along roadsides, and in other open areas where weedy species proliferate. Swamp Milkweed (A. incarnata) and Butterfly Weed (A. tuberosa) are also utilized by Monarch in eastern Canada, though to a much lesser extent. Swamp Milkweed occurs in small stands in open wetlands and wet ditches, while Butterfly Weed is largely confined to very dry sandy areas in southern Ontario.

In western Canada, Showy Milkweed (A. speciosa) is the primary larval hostplant. Low Milkweed (A. ovalifolia) and Green Milkweed (A. viridiflora) are also sometimes used by Monarchs in the west (Bird et al. 1995, Klassen et al. 1989). In the Prairie Provinces milkweed is widespread in areas of native short grass and long grass prairie, and also occurs in agricultural areas, particularly along roadsides, river banks and irrigation ditches. In British Columbia milkweed grows at scattered locations in arid valleys and on south-facing hillsides.

Wildflowers, which are particularly prevalent in abandoned farmlands and roadsides, are used as nectar sources by the adult butterflies and are also an important component of Monarch habitats. They are especially important during the fall migration, when sugars obtained from nectar are converted to the fat that is essential for the butterflies to complete their migration and overwinter successfully. Goldenrods (Solidago) and asters (Aster), as well as Purple Loosestrife (Lythrum salicaria) and milkweeds, are the nectar sources used most frequently by Monarchs.

Trends
There have been several habitat trends that have affected Monarch breeding range and productivity. There has been a great increase in Monarch habitat due to the clearing of the deciduous forest region of the eastern United States and southeastern Canada. This, in combination with the lost of extensive areas of habitat in the eastern great plains as a result of agriculture, has shifted the overall range of Monarchs into eastern North America (see North American Distribution in section B). During the middle and latter part of the twentieth century it has become increasingly uneconomical to maintain small farms, and as a result there has been a great increase in abandoned farmland in the east, creating suitable areas for Monarch breeding and nectaring. In rural areas adjacent to large cities there also has been a trend for estate housing to occupy large acreages of unused farmland. As a result there is probably more Monarch habitat now than there has ever been. Over the next several decades the trend will be for a lot of this habitat to be lost, as this farmland gives way to the growth of trees and shrubs.

In southern Canada, eastern Monarchs produce two to three generations (or broods) each year from June to September. Development from egg to adult butterfly takes about 30 days, although the length of time required for development can be reduced or prolonged by favorable or adverse conditions. In response to factors such as day length, temperature, and the availability and quality of the foodplant, the rate of development may range from about 20 to 45 days.

The success of Monarch breeding from season to season varies greatly from year to year due to climatic conditions, resulting in tremendous swings in the number of Monarchs overwintering in Mexico. This variable reproductive success, in combination with predation and potential weather disasters in Mexico, make the species highly vulnerable.

Species Movement
Monarchs of the eastern and western populations annually migrate south in the fall (to Mexico and California respectively), in order to escape the lethal low temperatures and predictable disappearance of the hostplants in the breeding range during the winter months. The main southward migration begins in Canada in early August and continues through to mid-October, although an extreme southern Ontario single migrants have been recorded as early as July, and ill-fated stragglers have remained as late as December 17. Migrating Monarchs enter a state of reproductive inactivity and avidly seek nectar from flowers to fuel their migration, and to build up a critical fat reserve necessary to sustain the butterflies through the winter (see Gibo and McCurdy 1993). This stored fat is also essential during the northward migration to the Gulf Coast States in the spring, when nectar sources are not available. In order to minimize energy expenditure during migration, Monarchs employ a soaring flight, riding columns of rising warm air in the same way as migrating hawks do, which greatly reduces the need for sustained flapping. They frequently reach high altitudes where strong prevailing winds speed their flight, and glider pilots have observed migrating Monarchs flying over a kilometer above the ground (Gibo 1986).

During the fall migration in southern Ontario, Monarchs cluster together on trees to form overnight roosts in a manner similar to the dense aggregations formed at the overwintering sites. Overnight roosts may contain a few hundred to several thousand individuals, and Monarchs usually form clusters in the same areas year after year. The location of overnight roosts is largely determined by topography, and proximity to abandoned farmlands with abundant nectar resources such as fall Composites (asters and goldenrods). Monarchs migrating south in the fall through southern Ontario build up in numbers along the north shores of Lake Ontario and Lake Erie. Their apparent reluctance to fly over large bodies of water, coupled with the desire to continue moving southwards, probably causes the butterflies to fly southwestward following the shoreline. This inevitably results in large concentrations of Monarchs accumulating on peninsulas jutting out into the lakes, where they have little choice but to eventually proceed southward over open water. Large aggregations and overnight roosts occur at many locations on peninsulas and at other locations along the lakeshores, including Presquile Provincial Park, Long Point, Rondeau Provincial Park and Point Pelee National Park. On September 6, 1993, over 96,000 Monarchs were recorded at Point Pelee, including two overnight roosts of 7,500 and 3,000 individuals (Wormington 1994).

The entire eastern population of the Monarch overwinters annually at about 12 overwintering sites in the Transverse Neovolcanic Belt, a belt of mountain ranges extending across central Mexico. The overwintering sites are all located within a small area of roughly 800 square kilometers, and occur only in Oyamel Fir (Abies religiosa) forest. Oyamel Fir forests are specialized high altitude ecosystems that occur only between elevations of 2,400 to 3,600 meters, and are currently restricted to 13 "islands" of habitat on the highest peaks in Mexico, making up less than one half of 1% of the country's total land area (Brower 1995). It is likely that in the past Monarch overwintering was more widespread in the Oyamel Fir forests; a survey conducted in 1984 found over 60 potential overwintering sites in areas where commercial logging had destroyed much of the original habitat (Brower 1995). The survival of the eastern population of the Monarch is now completely dependent on the preservation and enhancement of the few remaining overwintering areas.

Migrating eastern Monarchs reach the overwintering sites in Mexico in November and late December, where they form dense aggregations made up of tens of millions of individuals, and remain relatively inactive through the winter months. Individual colonies at the overwintering sites occupy less than 0.5 hectares to nearly 3.5 hectares of forest, with a density of approximately 10 million Monarchs per hectare (Calvert & Brower 1986). The overwintering colonies break up in March and early April and the butterflies migrate north to the Gulf Coast of the southeastern United States where the females lay their eggs on milkweeds. One to two generations of Monarchs are produced in the Gulf Coast States in the spring, and it is these offspring of the overwintering generation that continue the northward migration to recolonize the northern breeding range. The continued migration northwards from the southeastern United States in late spring is necessary since southern milkweeds die out in June. Continuing north allows Monarchs to exploit the milkweed resources of central and northeastern North America, enabling the species to produce up to three additional summer generations. These generations are critical to the build up of sufficiently large numbers for survival of the population after losses at the overwintering sites in Mexico. The first returning migrants reach southern Canada near the end of May and in the first week of June. In extreme southern Ontario, at point Pelee, single Monarchs have occasionally been recorded as early as late April, and may represent a minority of individuals that are able to recolonize the breeding range in a single flight from Mexico (see Brower 1995).

Monarchs of the western population undertake a similar migration and overwinter at numerous sites along nearly 1000 kilometers of the California coast, from north of San Francisco to the Mexican border. Over 200 overwintering sites have been recorded in California, and individual colonies may support dozens to tens of thousands of individuals (Lane 1984, 1993). Although colonies in California are much smaller than those in Mexico, even the smallest permanent overwintering sites are believed to be essential to the continued survival of western Monarchs (Malcolm 1993). The vast majority of the overwintering sites are associated with stands of non-native Australian Eucalyptus trees, which were introduced to California in the 1850s (Lane 1993). Eucalyptus were widely planted for landscaping, as windbreaks, and for use as fuel, resulting in a dramatic increase in the amount of suitable Monarch overwintering habitat in California. Monarch colonies in California have dispersed to take advantage of the widespread availability of overwintering habitat following the introduction of Eucalyptus . This dispersal also coincided with the cutting of dense coastal stands of native tree species, such as Monterey Pine (Pinus radiata) and Monterey Cypress (Cupressus macrocarpa), used by overwintering Monarchs prior to the introduction of Eucalyptus, and to a lesser degree in the present day (Lane 1993).

Most overwintering sites in California are not protected, although several are located within state, county or town parks. However, this does not guarantee protection or even recognition of the overwintering sites by those managing the parks. There has been a city ordinance against molestation of overwintering Monarchs in the town of Pacific Grove since 1938. Nevertheless, it affords little protection since it rules only against disturbing the butterflies themselves and does not protect the overwintering habitats, some of which have since been destroyed (Lane 1984). A number of coastal counties have passed ordinances protecting overwintering trees (Malcolm 1993), and in 1988 the California state government allocated two million dollars for acquisition of Monarch overwintering habitat (Snow & Allen 1993).

In Mexico, federal officials have recognized the importance of the overwintering sites, and five forested mountaintops where Monarchs overwinter have been defined as protected natural areas. However, almost all of the land in these protected areas belongs, through land grants that cannot be bought, sold or transferred, to the local residents who live off the land (Snook 1993). Federal agencies in Mexico have to date had little control at a municipal level in protecting the overwintering areas from logging and other forms of disturbance by residents living in these areas. As a result, although legislative protection exists in Mexico, it currently affords little or no protection to the overwintering eastern population. A forest conservation plan, integrating the needs of both the Monarchs and the local populace, has yet to be implemented.

Behaviour/Adaptability
The Monarch is a highly adaptive species, which has been able to withstand dramatic ecological changes in North America resulting from human disturbance in the last 150 years. The clearing of forested lands for agriculture and the creation of roadways, rail lines and other right-of-ways have greatly increased the abundance of Common Milkweed in eastern Canada, and the Monarch has steadily expanded its breeding range to make use of the increasing availability of milkweeds in areas where they were not formerly present. In 1940, Monarch breeding in Ontario was largely confined to the lower Great Lakes region of southern Ontario, south of 44 ^o latitude. By 1975, following the expansion of Common Milkweed into the forested areas of northern Ontario along roads, highways and powerline corridors, large concentrations of Monarchs were reported along the north shore of Lake Superior from as far north as Thunder Bay, Sault Ste. Marie, and Sudbury (Urquhart & Urquhart 1979). Monarchs are wide-ranging, powerful fliers and are able to seek out and find even the smallest patches of milkweed, including plants growing in city gardens, and along busy roads and highways. They are not limited by human disruption and, as long as the plants are healthy, they will breed readily on milkweeds in area where there is a high level of human activity. Thus, if milkweeds are planted specifically to encourage breeding in urban areas, parks, gardens, and along roadways, the butterflies will undoubtedly find and make use of them.

Weather conditions are the principal factor determining the size and breeding success of the Monarch population (Brower 1995). Cold, wet, overcast conditions in the spring and summer breeding range can prevent adult butterflies from dispersing, mating, laying eggs and nectaring, since they require warm and clear conditions to be active. Adults remain inactive in periods of such weather, and many die without having mated or laid eggs, resulting in fewer and smaller generations of Monarchs. Harsh winters at the overwintering sites in Mexico, with severe winter storms and freezing temperatures, can cause extremely high mortality in the Monarch population. Following winters in which high mortality occurs in Mexico, if the decreased number of Monarchs leaving the overwintering sites in the spring are met with cold, wet weather along the Gulf Coast of the southeastern United States, the number of Monarchs produced in the spring generation and continuing northwards to Canada will be greatly reduced. If the same weather conditions are encountered in the summer breeding range in Canada and the northern United States, the size of the Monarch population will decrease dramatically, producing a greatly diminished fall migration. Dry, hot summers with periods of drought are detrimental to the growth of herbaceous plants, and these conditions will also limit the success of Monarch breeding by reducing the quality and availability of milkweeds for egg laying and larval food, and of nectar sources for adults. Mild winters in Mexico, followed by warm, sunny conditions with moderate amounts of rainfall in the eastern United States and southern Canada provide the best conditions for Monarch breeding success, and produce large fall migrations (Brower 1995).

G. LIMITING FACTORS
Present decline in the eastern population of the Monarch can be attributed to three main factors:

1. Logging and human disturbance at the overwintering sites in Mexico.

The Oyamel Fir forest of Mexico are under rapidly increasing pressure from human disturbance. Legal and illegal commercial logging, harvesting of wood by local inhabitants, livestock grazing, fire, a developing cottage charcoal industry and a number of other human and biological factors are severely threatening the continued existence and integrity of the overwintering sites (see Snook 1993 for detailed discussion). Trees are presently being cut down in the so-called buffer zones around almost all of the core overwintering areas (Brower 1995). This is altering the unique ability of the Oyamel Fir Forest to shelter Monarchs from snow, hail and rainfall, and to insulate them from freezing temperatures (see Calvert & Brower 1981). Current logging practices are creating openings and thinned areas in the forest, which are exposing overwintering Monarchs to winter storms resulting in massive mortalities of millions of butterflies. These openings are also making overwintering Monarchs more vulnerable to predation from birds (Snook 1993). In addition to causing extraordinary mortality in overwintering Monarchs, human disturbance may also be disorienting the butterflies, resulting in unusual clusterings and migratory movements; the butterflies appear to have left the overwintering sites unusually early in the springs of 1994 and 1995 (Brower 1995).

2. Widespread and increasing use of herbicides and insecticides across North America.

Changing agricultural practices and land use patterns in central and eastern North America are resulting in widespread use of herbicides to sustain crops at the expense of all other competing plants, as a method of weed control along roadways, and to specifically eradicate milkweeds. This is contributing to a general decline of breeding habitat and nectar resources essential to the continued survival of the migratory North American Monarch populations. The use of insecticides and biological control agents (e.g.Bacillus thuringiensis or "Bt") to combat lepidopterous pests such as the Gypsy Moth (Lymantria dispar) has also had a negative effect on Monarchs and other non-target species.

3. Predation at the overwintering sites.

Two species of birds, the Black-headed Grosbeak (Pheucticus melanocephalus) and the Black-backed Oriole (Icterus galbula abeillei), feed extensively on Monarchs roosting at the Mexican overwintering sites. Both birds are able to circumvent the toxic properties of the Monarch (see section H), and predation is estimated to occur at a rate of up to 34,000 butterflies killed per day (Snook 1993). At one overwintering colony comprising approximately 20 million Monarchs, nearly 10% (2 million), were killed by bird predation over the winter (Arellano G. et al. 1993). In smaller colonies, mortality due to bird predation may reach as high as 44%, since the birds feed on the perimeter of a colony, and smaller colonies have a proportionally greater surface area, exposing more butterflies to attack (Calvert et al. 1979). The Black-eared Mouse (Peronmyscus melanotis) also feeds extensively on both live and dead Monarchs (Glendinning 1993).

Heavy predation at the overwintering sites may be a fairly recent phenomenon (Fink & Brower 1981); a large proportion of the Monarchs in the eastern population have low levels of toxicity which enable the birds and mice to feed on them (Brower 1995). This reduced toxicity is also a characteristic of Common Milkweed (Brower 1995), which has increased in abundance in the last 150 years due to changing agricultural and land use patterns, and has become the primary hostplant of the eastern population in that time. Heavy predation becomes of increasing concern as Monarch numbers decline due to other factors.

Three factors threaten the continued survival of the western population of the Monarch:

1. Real Estate of development along the California coast.

Almost all of the overwintering sites in California, except for a small number that lie within state, county, and town parks, are threatened by real estate development (Brower 1995). Twenty-one overwintering sites in California have been destroyed, and an additional 7 have been badly damaged, as a result of the building or expansion of motels, hotels, resort complexes, new houses, roads, and even to clear picturesque ocean views desired by wealthy landowners (Malcolm 1993).

2. Active programs to eliminate introduced Eucalyptus trees.

State and federal officials responsible for public lands management favor the eradication of exotic vegetation in California (which would include introduced Australian Eucalyptus trees presently harboring the majority of overwintering colonies of western Monarchs), in order to return the landscape to its original condition and native vegetation (Lane 1993). While only a minority of coastal overwintering sites are on public lands, the possibility of the eradication of Eucalyptus places these sites in jeopardy. In some state parks, Eucalyptus are currently being eradicated, despite the fact that the possibility of eliminating these trees in California is remote (Malcolm 1993).

3. Disease

The impact and significance of a recently discovered protozoan disease has yet to be determined. This protozoan may be a significant factor in the extreme fluctuations in numbers of populations at the California overwintering sites, but such fluctuations have long been recorded and the disease may not be a new threat to Monarch survival (John Lane personal communication).

The mysterious migration of the Monarch in North America, and the concentration of millions of butterflies into a small region of Mexico each year, appears to have captured the public imagination. Many people with little knowledge of butterflies, or nature in general, recognize the Monarch and know something about its annual migration. The large size, striking appearance and frequent occurrence of the Monarch in cities and gardens have also contributed to making it one of the most familiar North American insects. The initial discovery of the Monarch's overwintering in Mexico was reported in National Geographic Magazine in 1976 and, since that time, the Monarch has received increasing attention from the media. The discovery of the overwintering sites was itself the result of thousands of volunteers tagging Monarchs and reporting Monarch observations to Canadian scientist Fred Urquhart from across Canada and the United States. Recent articles in prominent newspapers such as The Globe and Mail and The New York Times on the plight of the Monarch clearly demonstrate the continued high level of public interest in the fate of this butterfly. A traveling exhibit called "Monarca" was developed by the Canadian Museum of Nature, in cooperation with scientists in Mexico, the United States and Canada, in 1992. This exhibit is currently touring the United States and will eventually reside permanently in Mexico City.

The annual migration of the Monarch in North America has been recognized as an endangered biological phenomenon by various authors (e.g. Malcolm 1993, Brower & Malcolm 1989), and in 1983 the spectacular winter roosts in both Mexico and California were designated as threatened phenomena by the International Union for the Conservation of Nature and Natural Resources (IUCN) in the IUCN Invertebrate Red Data Book (Wells et al. 1983). This was the first such designation in the history of international conservation, and this new status category was created to recognize the fact that the millions of Monarch's migrating and overwintering in North America each year are imperiled, while acknowledging that the species as a whole is not in danger of extinction. The Monarch is indigenous to the New World, and occurs throughout North and South America. In the last 150 years it has also established breeding populations in Australia, a number of Atlantic and Pacific Islands including Hawaii and Bermuda, and recently on the east coast of Spain.

The Monarch is one of the classic examples of butterfly mimicry. Monarch caterpillars ingest toxins (cardenolides) from milkweeds that make them, and the butterflies they develop into, unpalatable to birds and other vertebrates. In contrast to the cryptic coloration of most caterpillars, monarch caterpillars are boldly patterned with striking yellow, black and white stripes which advertise their presence and unpalatability to predators. The bold orange and black pattern of Monarch butterflies similarly acts as a warning to predators and, in laboratory experiments, Blue Jays refuse to eat Monarchs. The Viceroy (Limenitis archippus), whose relatives are purplish-black butterflies with prominent white bands, gains protection from predators by mimicing the Monarch, and has a bold orange and black pattern that closely resembles a Monarch.

I. RECOMMENDATIONS/ MANAGEMENT OPTIONS
1. Consideration should be given to removing milkweeds from provincial noxious weed acts, and de-legislating municipal by-laws for their eradication, while recognizing that in some situations it may still be necessary to actively eliminate milkweeds.

2. The continued availability of healthy populations of milkweeds as larval hostplants for Monarchs would be improved by:

• maintaining wildflower mixes along highway verges and median strips instead of grasses (which require the use of herbicides and repeated mowing).
• encouraging communities to create butterfly gardens and milkweed areas.
• encouraging farmers to cut abandoned fields late in the fall (mid-September to early October) to limit the growth of woody plants.
• encouraging parks (national, provincial and local) that have weedy fields to maintain them through selective cutting and controlled burning.

3. It is important that staging areas where migrating monarchs mass are protected from cutting of critical trees, and from various kinds of human disturbance. Parks with staging areas within their boundaries should be made aware of this fact and encouraged to protect them. Abandoned farmlands adjacent to overnight roosts that sustain an abundance of nectar sources should be included in protected areas.

4. Public awareness and education campaigns highlighting the uniqueness of the Monarch and Canada's role in protecting its annual migration should be encouraged.

5. Canada, as co-signatory to the North American Free Trade Agreement (NAFTA) with Mexico, could work more closely with federal and state governments in Mexico to enhance protection and management of the overwintering sites.

J. EVALUATION
The main reasons for the decline in Monarch populations have been periodic disasters at the Mexican overwintering sites. Some sites may sustain losses of anywhere from 30% to 90% during winter storms (Lincoln Brower personal communication). Alteration of the habitat by man, particularly the opening of forests by logging, has greatly increased the negative effect of winter storms on overwintering Monarchs, and made them more susceptible to predation from birds and mammals. In the past, these disasters have been counterbalanced by the increase in Monarch breeding habitat in eastern North America in abandoned farmland and roadsides. However, the more widespread use of herbicides and insecticides, and the loss of milkweed habitats in this region, threatens to eliminate the ability of the Monarchs to produce the numbers necessary to ensure the survival of the population through the winter. Ways to reverse this trend are discussed in section C and summarized in section I.

K. REFERENCES
Literature Cited

Alex, J. F. 1992. Ontario weeds. Ontario Ministry of Agriculture and Food, Toronto. Publication 505. 304 pp.

Arellano G., A., J. I. Glendinning, J. B. Anderson and L. P. Brower. 1993. Interspecific comparisons of the foraging dynamics of black-backed orioles and black-headed grosbeaks on overwintering Monarch butterflies in Mexico. pp. 315-322. In Malcom, S. B. and M. P. Zalucki (eds.), Biology and conservation of the Monarch butterfly. Publications of the Natural History Museum of Los Angeles County, Los Angeles, California. Science series no. 38.

Bird, C. D., G. J. Hilchie, N. G. Kondla, E. M. Pike and F. A. H. Sperling. 1995. Alberta butterflies. Provincial Museum of Alberta, Edmonton. 347 pp.

Brower, L. P. 1995. Understanding and misunderstanding the migration of the Monarch butterfly (Nymphalidae) in North America: 1857-1995. Journal of the Lepidopterists Society 49: 304-385.

Brower, L. P. and S. P. Malcolm. 1989. Endangered phenomena. Wings 14(2): 3-9.

Calvert, W. H. and L. P. Brower. 1981. The importance of forest cover for the survival of overwintering Monarch butterflies (Danaus plexippus, Danaidae). Journal of the Lepidopterists Society 35: 216-225.

Calvert, W. H. and L. P. Brower. 1986. The location of Monarch butterfly (Danaus plexippus L.) overwintering colonies in Mexico in relation to topography and climate. Journal of the Lepidopterists Society 40: 164-187.

Calvert, W. H., L. E. Hedrick and L. P. Brower. 1979. Mortality of the Monarch butterfly (Danaus plexippus L.): avian predation at five overwintering sites in Mexico. Science 204: 847-851.

Fink, L. S. and L. P. Brower. 1981. Birds can overcome the cardenolide defense of Monarch butterflies in Mexico. Nature 291: 67-70.

Gibo, D. L. 1986. Flight strategies of migrating Monarch butterflies (Danaus plexippus L.) in southern Ontario. pp. 172-184. In Insect flight: dispersal and migration. Danthanarayana, W. (ed.). Springer-Verlag, Berlin.

Gibo, D. L. and J. A. McCurdy. 1993. Lipid accumulation by migrating Monarch butterflies, Danaus plexippus(Nymphalidae). Canadian Journal of Zoology 71: 76-82.

Glendinning, J. I. 1993. Comparative feeding responses of the mice Peromyscus melanotis, P. aztecus, Reithrodontomys sumichrasti, and Microtus mexicanus to overwintering Monarch butterflies in Mexico. pp. 323-333. In Malcom, S. B. and M. P. Zalucki (eds.), Biology and conservation of the Monarch butterfly. Publications of the Natural History Museum of Los Angeles County, Los Angeles, California. Science series no. 38.

Haber, W. A. 1993. Seasonal migration of Monarchs and other butterflies in Costa Rica. pp. 201-207. In Malcom, S. B. and M. P. Zalucki (eds.), Biology and conservation of the Monarch butterfly. Publications of the Natural History Museum of Los Angeles County, Los Angeles, California. Science series no. 38.

Hess, Q. F. and A. J. Hanks (eds.). 1986. Butterflies of Ontario and summaries of Lepidoptera encountered in Ontario in 1985. Toronto Entomologists Association Occassional Publications 17-86.79 pp.

Klassen, P., A. R. Westwood, W. B. Preston and W. B. McKillop. 1989. The butterflies of Manitoba. Manitoba Museum of Man and Nature, Winnipeg. 290 pp.

Lane, J. 1984. The status of Monarch butterfly overwintering sites in Alta California. Atala 9: 17-20.

Lane, J. 1993. Overwintering Monarch butterflies in California: past and present. pp. 335-344. In Malcom, S. B. and M. P. Zalucki (eds.), Biology and conservation of the Monarch butterfly. Publications of the Natural History Museum of Los Angeles County, Los Angeles, California. Science series no. 38.

Malcolm, S. B. 1993. Conservation of Monarch butterlfy migration in North America: an endangered phenomenon. pp. 357-361. In Malcom, S. B. and M. .P. Zalucki (eds.), Biology and conservation of the Monarch butterfly. Publications of the Natural History Museum of Los Angeles County, Los Angeles, California. Science series no. 38.

McKown, S. 1993. Season Summary 1992. News of the Lepidopterists Society 35(2): 42.

Minno, M. C. and M. F. Minno (eds.). 1995. Season Summary. News of the Lepidopterists Society 37 (June 1995): 19.

Opler, P. A. and G. O. Krizek. 1984. Butterflies east of the great plains, an illustrated natural history. The Johns Hopkins University Press, Baltimore, Maryland. 294 pp.

Snook, L. C. 1993. Conservation of the Monarch butterfly reserves in Mexico: focus on the forest. pp. 363-375. In Malcom, S. B. and M. P. Zalucki (eds.),Biology and conservation of the Monarch butterfly. Publications of the Natural History Museum of Los Angeles County, Los Angeles, California. Science series no. 38.

Snow, K. B. and M. M. Allen. 1993. The Monarch project: a program of practical conservation in California. pp. 393-394. In Malcom, S. B. and M. P. Zalucki (eds.), Biology and conservation of the Monarch butterfly. Publications of the Natural History Museum of Los Angeles County, Los Angeles, California. Science series no. 38.

Urquhart, F. A. 1960. The Monarch butterfly. University of Toronto Press, Toronto. 361 pp.

Urquhart, F. A. and N. R. 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. Canadian Field-Naturalist. 93(1): 41-47.

Wells, S. M., R. M. Pyle and N. M. Collins. 1983. The IUCN invertebrate red data book. International Union for Conservation of Nature and Natural Resources, Gland, Switzerland. 632 pp.

Wormington, A. 1994. A mass migration of Monarchs at Point Pelee, Ontario. pp. 26-27. In Hanks, A. J. (ed.), Butterflies of Ontario and summaries of Lepidoptera encountered in Ontario in 1993. Toronto Entomologists Association Occasional Publications 27-95.

L. ACKNOWLEDGMENTS
We would like to thank the following individuals who provided information critical to the completion of this report.

Lincoln Brower provided information on the status of the overwintering sites in Mexico. John Lane provided information on the status of the overwintering sites in California.

The following individuals provided information and insight on the status of the Monarch in various regions of Canada: Jon Shepard and Jim Troubridge (British Columbia); Ted Pike and Paul Klassen (the prairies); Alan Wormington (Ontario and Quebec); Tony Thomas, Ken Neil and Bernard Jackson (the maritimes).

Phil Shappert provided information on a variety of topics relating to Monarch biology and distribution.

Funding for this report was provided by the Canadian Wildlife Service and the North American Wetlands Conservation Council.