In a comprehensive study recently published in Ecology and Evolution, CEID members Maria L Müller‐Theissen, Nicole L Gottdenker, and Sonia M Altizer explore how closely related milkweed butterflies manage infections by their protozoan parasites. The researchers conducted cross-infection experiments between two sympatric species—the migratory monarch (Danaus plexippus) and the more resident queen butterfly (Danaus gilippus)—by exposing larvae to parasites originating from both their own species and from the other. By manipulating the type of parasite exposure and the larval host plant (with differing levels of cardenolide toxins), the study aimed to disentangle the evolutionary trade‐offs between resistance (the ability to block or slow infection) and tolerance (the capacity to endure infection without severe fitness costs).
The results revealed stark differences between the two butterfly species. Monarchs were highly susceptible to both conspecific and heterospecific parasites, suffering higher pre‐adult mortality, heavier parasite loads, and increased incidences of wing deformities. In contrast, queen butterflies were largely resistant to parasites from monarchs and exhibited stronger innate immune responses, such as enhanced melanization activity and higher hemocyte concentrations. These differences suggest that queens can better mitigate the deleterious effects of infection, likely due to a more robust immune investment, while monarchs may face a tradeoff where the benefits of migratory success come at the cost of reduced immune defenses.
With the prevalence of the debilitating protozoan Ophryocystis elektroscirrha on the rise—especially in migratory monarch populations—this research underscores the potential fitness costs and population declines associated with parasite infections. The study not only sheds light on the dynamics of cross‐species parasite transmission but also highlights the complex evolutionary balancing act between resistance and tolerance in natural host populations. Ultimately, these insights could help guide future efforts to mitigate disease impacts and preserve these iconic butterfly species.
Please find the study here.