For some species of migratory animals, migration acts as a mechanism for moderating parasitic infection by both removing infected individuals from the population and by allowing migrants to seasonally escape contaminated habitats. However, recent studies shown that supplementation of resources and/or climate change may result in reductions in migratory movements. Investigating how host-pathogen dynamics are subsequently altered by these changes is needed to determine future population-level impacts. Monarch butterflies are an excellent study system for investigating the interaction between migratory behavior and disease dynamics. Although the monarch butterfly (Danaus plexippus) is famous for its annual long distance migration from southeastern Canada to central Mexico, observations indicate that a subset of the population is ending migration in order to breed along the Gulf Coast. Previous studies have also found an increased prevalence of the protozoan parasite Ophryocystis elektroscirrha (OE), in winter-breeding populations.
To determine how seasonal and urban spatial variability may influence infection dynamics of OE in winter-breeding monarch populations, a preliminary monitoring project was established in October of 2017 within the city of New Orleans, a documented site of winter-breeding. Temporal and spatial data on parasite and host populations, as well as plot-level environmental variables were collected bi-weekly. All analyses were completed in R using package "nlme".
Results/Conclusions
Preliminary results indicated that there was a markedly higher abundance of severely infected individuals sampled during winter breeding (October-December 2017) than during early spring, when migrants returned to the region (March-May 2018). Our highest observed incidence of infection occurred in December 2017 with 86% of sampled individuals severely infected. The reduction in spring infection levels was likely caused by two primary factors. First, the eradication of the winter-breeding population during the freezes of January and February of 2018 may have led to a reduction in the potential for transmission from winter-breeding individuals and habitats to migratory individuals as they returned in spring. Second, migratory individuals have been observed to have low prevalence of OE infection, likely due to the harmful symptoms of OE infection resulting in reduced migratory success. Thus, individuals sampled in spring were likely not winter-breeding individuals. Over the next year, the preliminary monitoring of winter-breeding will be expanded through the recruitment of citizen scientists, with the goal of gathering more information on the potential disease risk winter-breeding activity poses to spring migrants.