Spore persistence in the environment drives infection dynamics of a butterfly pathogen

Mary-Kate Williams, from the University of Arkansas at Little Rock, examined parasites of Monarch butterflies with Dr. Sonia Altizer, Dr. Richard Hall and graduate student Dara Satterfield.

Mary-Kate Williams1, Sonia Altizer2, Richard Hall2, Dara Satterfield2

1University of Arkansas at Little Rock, 2Odum School of Ecology, University of Georgia

Environmentally transmitted parasites commonly infect humans and wildlife. Environmental transmission is particularly important for insect pathogens, yet the factors affecting the persistence of infectious stages in the environment are poorly understood. Monarch butterflies are commonly infected by Ophryocystis elektroschirrha (OE); recent years have seen an increase in pathogen prevalence at the same time monarch populations in eastern North America have declined. OE is transmitted both vertically (from infected females to their progeny) and environmentally (when infected adults scatter spores onto milkweed leaves that are consumed by unrelated larvae). By using a combination of a mathematical modeling and an experimental study, we examined (1) how environmental conditions affect persistence of a free-living stage pathogen and (2) how pathogen shedding rate and environmental persistence time affect pathogen prevalence and host population size during the summer breeding season. We found that increased time spent fully exposed to environmental conditions (sun, rain, heat) reduced average infection severity induced by parasites, but did not reduce the fraction of monarchs infected by spores; therefore, parasites were able to remain viable after 15 days outdoors. Consistent with the experimental results, modeling findings showed that, parasite spores must persist for at least 20 days, in combination with a high shedding rate onto host plant leaves, for predicted prevalence to match the minimum prevalence observed in prior field studies.

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