Do host plant toxic compounds predict parasite infection in migratory monarch butterflies?
Many herbivorous insects derive chemical defenses from their food plants. Secondary chemical compounds vary widely among plant species and can have significant fitness effects on herbivores. Thus, for herbivorous insects, plant species matters. Larval monarch butterflies sequester toxic cardiac glycosides (cardenolides), which vary among milkweed species, to deter predators and provide resistance against a protozoan parasite. More toxic milkweed species confer greater parasite resistance. In recent years, the relative abundance of milkweed species has shifted in the U.S. due to the planting of the exotic milkweed Asclepias curassavica (a high-cardenolide species) and the widespread loss of A. syriaca (a low-cardenolide species) in response to intensive agriculture. This shift in host plant availability could have consequences for monarch chemical defenses. To investigate whether host plant cardenolides affect protozoan parasite infections in wild monarchs, we collected 342 migratory monarchs at overwintering grounds in Mexico in 2013. Deriving cardenolide profiles from monarch tissues and quantifying parasite infection allowed us to ask (a) if a greater proportion of monarchs originated from A. curassavicathan historically (i.e., 1993), (b) if parasite load or infection status varied for monarchs originating from different host plants species, and (c) if parasite load was correlated with cardenolide characteristics.
Cardenolide profiles indicated that about 9% of the 342 monarchs sampled used A. curassavica as a natal host plant. These data suggest that monarchs are using A. curassavica more frequently than in the past, and that at least some of these individuals are migrating to Mexico. Monarchs originating from A. curassavica were just as likely to be infected with the protozoan parasite as monarchs from other native host plant species. Among infected monarchs, parasite load was weakly negatively associated with cardenolide diversity and non-polarity.