Two-way Monarch Migration Map

With the assistance of noted graphic artist Paul Mirocha of Tucson, Arizona we have created a two-way monarch migration map:

Our goal was to produce a comprehensive map representing both the fall and spring migrations both east and west of the Rocky Mountains. To the extent possible, the map is based on data. Nevertheless, our knowledge of the distribution and abundance of monarchs in some seasons is lacking for certain areas of the country. In these cases, our interpretations are based on what monarchs are doing in surrounding areas.

There are two question marks on the map; one represents the possibility that some monarchs move north through the Sierra Madre Occidental into California and Arizona and the other the uncertainty concerning the spring movement of monarchs out of Florida. Although it is clear that monarchs build up in Florida each spring, it is not clear whether the offspring of the spring generation moves northward along the east coast. Tagging has failed to resolve this issue. In fact, the origin of the spring monarchs in Florida is not absolutely clear. Do they originate from populations in extreme southern Florida that have matured on naturalized* and cultivated Asclepias curassavica, from monarchs returning from overwintering sites in the mountains of Cuba, or butterflies that have returned from overwintering in Mexico, or some combination of these? The Mexico connection is particularly puzzling since it is unclear why spring migrants, that are generally on a N/NE track, would take an easterly or even slightly southeasterly track to reach central Florida. My hope is that these question marks will inspire a resolution of these issues.

Similarly, you will notice that the corn belt is roughly outlined on the map. Historically the corn belt has been an area of high monarch reproduction, a point emphasized by the isotope study conducted in 1996 (1) that showed roughly 50% of the monarchs that reached the Mexican overwintering sites originated in the corn belt and the subsequent finding published in 2000 (2) showing that corn and soybean fields were the most productive breeding habitat for monarchs in the Midwest. Subsequent to these studies (but beginning in 1997) Roundup Ready soybean varieties, typically used in rotation with corn, were introduced to the American farmer as a cost and energy effective means of weed control. Unfortunately, the widespread planting of these genetically modified seeds has eliminated milkweed from at least 100 million acres of row crops. Clearly, there is less milkweed/monarch habitat available in the corn belt than there was in the past but has this reduced monarch reproduction in these areas or have monarchs simply shifted their reproduction to common milkweed at other disturbed sites? Habitats for monarchs are becoming increasingly fragmented and there are more and more areas where monarchs are unable to reproduce due to the lack of milkweed and nectar sources. We need to know how monarchs respond to these changes to mitigate habitat losses.

Lastly, the map is not perfect in that it doesn’t represent all that we know about monarchs. For example, there is an area (roughly 36 to 40N) of overlap of reproduction by butterflies returning from Mexico in April with a region of continuous summer reproduction. We tried to represent this overlap on the map but failed. The map became cluttered and confusing. Similarly, there are three migratory generations (did you know that?): the fall migration; the first generation offspring of the returning fall migrants that move north from late April to mid June; and an as yet poorly defined migration southward in late July and early August, about a month before the fall migration, that recolonizes the southern Midwest and south. This third migration, which I’ve given the unfortunate name of the “pre-migration migration” (and it truly is a migration), is not represented on the map nor is the late season reproduction in the south that sometimes results from this flight southward.

Please note that this map is copyrighted and may only be used with permission from Monarch Watch. We plan to create a poster based on this map for display in classrooms, nature centers, at events, etc. – this will be available via the Monarch Watch Shop.

References

1. Wassenaar, L.I. and Hobson, K.A. 1998. Natal origins of migratory monarch butterflies at wintering colonies in Mexico: new isotopic evidence. Proc. Nat’l Acad. Sci. U.S.A., 95:15436-15439.

2. Oberhauser, K.S., Prysby, M.D., Mattila, H. R., Stanley_Horn, D. E., Sears, M.K., Dively, G., Olson, E., Pleasants, J.M., Lam, W. F., & Hellmich, R. Temporal and spatial overlap between monarch larvae and corn pollen. Proc. Natl. Acad. Sci. USA 2001 98: 11913-11918.

For more on Roundup Ready Crops and Milkweeds see:

Roundup-Ready Crops and Resistant Weeds (Monarch Watch article)

Effects of Transgenic Crops on Milkweeds (Monarch Watch article)

Farmers Cope With Roundup-Resistant Weeds (New York Times article)

*Asclepias curassavica has been recorded in 17 counties in Florida.

Addendum

The above map resembles, but differs in many details from, maps published by Lincoln Brower in 1995. The Brower maps were not consulted in the production of this map.

The two-way map is based on my 18 years of experience following the monarch migration in great detail through the monarch tagging program, reports to Dplex-L and Journey North and from hundreds of emails and phone calls. The inclusions, omissions, and errors are mine.

The Brower maps appear in two publications:

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. 1996. Monarch butterfly orientation: missing pieces of a magnificent puzzle. Journal of experimental biology 199:93-103.