Milkweed Patch Size Effects on Monarch Butterfly Oviposition within Iowa Prairies and Roadside

Monarch butterfly, Danaus plexippus, overwintering populations in Mexico have declined over the past 20 years. One reason attributed to this decline is the loss of milkweed (Asclepias spp.) in corn and soybean fields due to growers' widespread use of herbicide-tolerant crops. Reestablishing milkweed and nectar resources is critical to combat the population decline. Roadsides and prairies are important habitat resources and could help compensate for this loss if conservation efforts in these habitats are successful in providing milkweed and forbs for ovipositing adults and hungry caterpillars. There are many factors to consider when establishing habitat for the monarch, including milkweed patch size. If monarchs have a patch size preference for oviposition, then optimizing patch size could help maximize egg and caterpillar production. We hypothesized that monarch oviposition is not random and is influenced by milkweed patch size.;To test this hypothesis, eight gravel roadsides in Story and Boone Counties, Iowa were monitored for milkweed with GPS units. Over 14,000 milkweed stems were mapped and examined for eggs and larvae. These data were spatially analyzed in ArcMap(TM) 10.4, GeoDa, and R to measure the correlation between stem density and egg and larval density. A similar study was conducted in several prairies where common (A. syriaca) and swamp (A. incarnata) milkweed were transplanted into a replicated block design with three patch densities. Plants were examined weekly for eggs and larvae over three summers. Results from both studies are used to determine if there is an "optimal" patch size for female monarch oviposition.