The monarch butterfly (Danaus plexippus) is an iconic species, known for its annual migration to Mexico from the upper Midwest of the United States and Canada and back. Over the last decade, monarch populations have drastically decreased due to overwintering habitat loss in Mexico, inclement weather, and importantly, the loss of milkweed (Asclepias spp.) in the summer breeding habitat. The upper Midwest, including Iowa, produces a large percentage of overwintering monarchs. For this reason, monarch habitat restoration in Iowa is of great concern. Agricultural and conservation organizations, state agencies, companies, and Iowa State University have joined together to create the Iowa Monarch Conservation Consortium (IMCC). The IMCC supports a research-based framework for monarch conservation that includes refining practices for establishing breeding habitat and understanding monarch oviposition and larval feeding preference of Asclepias spp. Additionally, the IMCC is focused on understanding adult and larval landscape utilization by employing tracking and genetic markers. A spatially explicit agent-based model for adult movement and oviposition is also being developed to help determine optimal arrangement of restored and created habitat.
Results/Conclusions
The effects of plant density on oviposition preference were examined by monitoring natural monarch egg presence on transplanted plots of common milkweed (Asclepias syriaca) containing 1, 5, or 10 plants in 2m2 areas. There were no significant differences observed for these patch sizes (F = 0.05, df 2, 2116.6; P > 0.95). Naturally occurring common milkweed patches were also monitored for egg abundance along roadsides. It was found that the number of eggs per plant significantly declined as patch size increased (P < 0.0005, n = 30), but the total number of eggs increased as patch size increased (R2 = 0.357; P < 0.0005, n = 30). Nine native milkweed species were tested to determine acceptable larval host plants. All 9 species support larval growth, however larvae reared on Cynanchum laeve were significantly smaller and developed slower in comparison to other species. Furthermore, through modeling based on milkweed density, adult monarch movement pathways and cumulative eggs laid were simulated. The model suggested that egg density is higher in smaller patches of habitat, such as along roadsides. From the research conducted by the IMCC, we are beginning to understand the behavior of the monarch butterfly in the Midwest breeding grounds. Findings from these and other studies conducted by the IMCC will aid in developing a statewide monarch conservation plan for Iowa.