Crowding can result in greater disease transmission, yet crowded hosts may also remove infectious propagules from the environment, lowering the encounter rate and infectious dose of conspecifics. We combined experimental and modeling work to examine the impact of crowding of butterfly larvae on per-capita infection risk 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 mimicked 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 infection rates on plants with ten caterpillars were reduced by at least 50% compared to single caterpillars. High density reduced individual infection risk and parasite load and extended the lifespan of all hosts, as crowded hosts removed parasites from the environment. Modeling suggested that the lower consumption rate due to crowding can lower infection prevalence by as much as 20%; however, the total number of infected individuals increases. Our results highlight that the expected positive relationship between host density and infection prevalence breaks down when crowding removes infectious particles from the environment.

Funding provided by: National Science FoundationROR ID: https://ror.org/021nxhr62Award Number: 1754392 Funding provided by: National Science FoundationROR ID: https://ror.org/021nxhr62Award Number: 1911925 Funding provided by: National Institute of General Medical SciencesROR ID: https://ror.org/04q48ey07Award Number: 5K12GM000680-19