Interest in the population dynamics and geographic distribution of common milkweed (Asclepias syriaca) has recently increased due to the importance of common milkweed in the life cycle of the monarch butterfly {Danaus plexippus (Lepidoptera:Danaidae)}. Distribution and patch characteristics of common milkweed in Iowa were determined by a plant census conducted during June and July of 1999. A random, representative sample of the state was generated by selecting forty 100 km2 cells using a coordinate reference system. Within each 100 km2 cell ten arbitrarily selected 0.5 ha (50 by 100m) areas were surveyed for common milkweed. Common milkweed was found in 71% of the roadsides and approximately 50% of the corn and soybean fields surveyed. Pastures had the lowest infestation rate (28%) of habitats surveyed. Roadsides had 48 common milkweed patches ha-1, whereas corn and soybean fields averaged 7 patches ha-1. Land maintained in the Conservation Reserve Program (CRP) had a relatively low number of patches (9 ha-1). Patches in CRP were larger than those in other settings, resulting in the highest percentage (2%) of area infested with common milkweed. Land maintained in corn, soybean or pasture had the lowest average area infested (less than 0.3%); however, this land use encompasses 78% of the Iowa land mass and therefore may be an important resource for monarch butterflies. This data, combined with information concerning the behavior of monarchs and toxicity of pollen from Bt corn, can be used to access the potential impact of genetically modified crops on monarch populations.

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