Anthropogenic changes of the natural environment can lead to changes in disease risk for wildlife. Planting a non-native variety of milkweed for monarchs, namely tropical milkweed (Asclepias curassavica) is associated with extremely high infection prevalence a debilitating specialist protozoan parasite, Ophryocystis elektroscirrha (OE). While this phenomena appears to restricted to southern coastal US, mechanisms responsible for high infection risks remain unclear. With combination of field observations and modeling, we examine the causes of high OE prevalence in monarchs utilizing gardens with A. curassavica for reproduction. We parameterize and compare the results of stage structured compartment models based on field observations of monarchs in (1) gardens of southern coastal US with A. curassavica and (2) monarchs in natural milkweed patches of Midwestern US.
Results/Conclusions
Field observations indicated that monarch lay eggs on the tropical milkweed plants more frequently than on native milkweeds found in natural sites across Midwestern US, resulting in crowding of caterpillars and a steady increase in OE parasite prevalence. We also documented a steady accumulation of spore parasites on the tropical milkweeds, which poses a risk for monarchs as all adults emerge infected by the Fall. Finally, we found that healthy individuals acquire spores by mating with infected individuals, indicating this is a significant transmission route for OE. Modeling work suggests that this sexual transmission route in combination with previously identified routes (namely vertical and environmental transmissions) can increase prevalence. We further examine how and when multiple transmission routes contribute to extremely high infection prevalence, and show that three transmission routes are necessary to reach high prevalence observed in the field.