archives@tulane.edu / The Eastern migratory monarch butterfly undertakes one of the longest annual migrations known among animal taxa, journeying from as far north as Canada down to central Mexico. A small subpopulation of monarchs has been found dropping out of migration in favor of breeding year-round along the U.S. Gulf Coast. The majority of these dropouts, known as winter-breeders, are feeding and breeding on a non-native milkweed species called tropical milkweed (Asclepias curassavica) that has greater concentrations of toxic cardenolides. The effects of tropical milkweed on monarchs are not yet fully understood, but it is correlated to breakage of reproductive diapause and migratory drop-out. The drop-out phenomenon is concerning due to the increased prevalence of infection by the monarch’s specialized protozoan parasite, Ophryocystis elektroscirrha (OE). OE is often physically damaging and can be lethal to monarchs. Here, we investigate the effects of a colder climate that winter-breeders experience on the Gulf Coast in the winter, exposure to OE, and a diet of exclusively tropical milkweed on larval development and adult wing morphology and pigmentation. Morphology and pigmentation are often functional traits that enhance fitness; thus, we use these measures as proxies for fitness components such as flight performance and immune function. In the first chapter, we found that monarchs reared on tropical milkweed developed faster and had larger wings and higher aspect ratios than monarchs reared on a low-cardenolide native milkweed species, swamp milkweed (Asclepias incarnata). In the second chapter, we found that monarchs reared in a suboptimal temperature developed slower and emerged with smaller, darker (redder and more melanized) wings than monarchs reared in a warmer or ‘normal’ temperature. Additionally, exposure to OE affected wing shape such that exposed monarchs had rounder wings with a slightly lower aspect ratio than unexposed monarchs. Lower temperature did not significantly affect melanism in monarch wings, but monarchs exposed to OE had less melanin deposited onto their wings than unexposed monarchs. Together, the findings from both chapters suggest that the conditions experienced by winter-breeders on the Gulf Coast are detrimental to monarch morphology. Smaller, rounder wings with lower aspect ratios are not conducive to migratory success, and paler color or less melanin are potential negative responses to exposure or infection by OE. While monarchs reared on tropical milkweed were larger and slightly redder which is a seemingly positive result, it may be limited to nonmigratory monarchs or winter breeders who may be adapted to tropical milkweed. Thus, it remains unclear how tropical milkweed is affecting the biology of migratory monarchs versus winter-breeders, but we suggest that the colder winter and increased risk of OE infection are negatively affecting winter-breeding populations and possibly migratory behavior. Migratory monarchs are already of conservation concern, and further monitoring and studying of the winter-breeding population and migrant populations are necessary to ensure the stability of monarch populations if and when they encounter tropical milkweed and whether the milkweed is driving population-level shifts in migratory behavior. / 1 / Caitlin Ducat