Crowding reduces per-capita parasite infection risk in a butterfly host

Crowding can result in greater disease transmission, yet crowded hosts may also remove infectious propagules from the environment, thereby lowering the encounter rate and infectious dose received by conspecifics. We combined experimental and modelling work to examine the impact of crowding of butterfly larvae on the per-capita risk of infection by a protozoan that is transmitted via the larval food plant, and the resulting infection load in adult butterflies. We reared larvae at different densities and exposed them to low and high doses of parasites. We modified an existing model to include effects of conspecific density on food (and thus parasite) consumption rate and infected adult mortality rate. Experimental work indicated that the proportion of infected hosts on plants with ten caterpillars were reduced by at least 50% compared with single caterpillars. High density reduced per-capita infection risk and parasite load and extended lifespan of all hosts, as crowded hosts removed parasites from the environment. Modelling suggested that the lower consumption rate due to crowding can lower infection prevalence by as much as 20%, although the number of new cases increases with larger population size. Our results highlight that the expected positive relationship between host density and infection prevalence breaks down when crowding results in removal of infectious propagules from the environment.