Monarch butterflies vary their medication strategy based on the level of parasite risk
Parasites select for host defenses, and hosts may acquire resistance through environmental factors in addition to genetic factors. For instance, monarch butterflies gain resistance to a common protozoan parasite, Ophryocystis elektroscirrha, through their larval milkweed diet. Higher concentrations of toxic cardenolides in milkweeds correlate with greater levels of resistance. In eastern North America, adult female monarchs use this to their advantage by selectively ovipositing on medicinal (high cardenolide) plants when infected, thereby reducing parasite burdens and virulence in their offspring. Uninfected females do not demonstrate this preference, indicating that this trans-generational medication is a form of phenotypic plasticity. However, this phenotypically plastic strategy may not be universally adaptive. Monarchs in this eastern migratory population experience relatively low parasite prevalence. Theory predicts a shift from a plastic response to a fixed response when the threat of parasitism is high. To test this prediction, we compared medication behavior in adult females derived from the eastern North American population (low to intermediate parasite prevalence) and a population in south Florida where parasite prevalence is high.
When given a choice between a medicinal and non-medicinal host plant, female monarch oviposition strategies differed by host population. As previously demonstrated, eastern North American monarchs used a plastic medication strategy, with infected females preferring medicinal plants for oviposition, but uninfected females displaying no preference. In contrast, south Florida monarchs showed a fixed medication strategy, whereby all females, whether infected or not, had a significant preference for oviposition on medicinal plants. These differences in behavior are consistent with theory that predicts plasticity in populations with low parasite prevalence and fixed medication in populations with high prevalence.