Stage-structured cannibalism reduces disease prevelance in a lepidopteran system
While cannibalism alone is unlikely to support the spread of an epizootic, it is often thought of as phenomenon which increases disease transmission. Here we describe a widespread insect pathogen system in which species that are cannibalistic can actually reduce the prevalence of the pathogen. Many lepidopteran species, some of which are cannibalistic as caterpillars, are infected by baculovirus pathogens. We tested whether cannibalistic behavior in the fall armyworm (Spodoptera frugiperda) can decrease the prevalence of epizootics and the amount of environmental virus produced. Baculoviruses are consumed by caterpillar hosts, replicate in and replace its tissues, then kill the host and dissolve it into a liquid of viral particles which can then be consumed by other feeding caterpillars to spread the infection. Importantly, while a host is infected, it will stop progressing through larval stages. We tested whether cannibalistic behavior in fall armyworm larvae reduced epidemic spread through stage-dependent cannibalism of larger healthy individuals on smaller infected individuals. We used no choice cannibalism assays with healthy and infected individuals to parameterize stage-structured models predicting the effects of cannibalistic behavior on population dynamics and disease spread.
Stage-structured cannibalism in the fall armyworm decreased the prevalence of baculovirus infections during epizootics under many realistic initial population structures. Fall armyworms never consumed larvae larger than themselves, tended to readily consume smaller larvae, and sometimes consumed similar sized (same stage) larvae. The relative stages of the larvae and their healthy or infected status affected the chance of cannibalism occurring with larger stage differences and higher instar larvae more likely to cannibalize smaller conspecifics. Disease transmission occurred through cannibalism, but appeared to be dose dependent, with much larger individuals eating smaller individuals (the most common case) rarely leading to disease transmission. In this way, as well as through thinning the population of susceptibles through consumption, fall armyworms can reduce the prevalence of baculovirus during an epizootic and reduce production of new viral particles by over 50% under many scenarios of population structure. Our results show that these two effects, consumption of infected smaller instars and thinning, have qualitatively different effects on disease and population dynamics. Baculoviruses are ubiquitous among lepidopteran species, and cannibalism is common. Thus we demonstrate a potentially widespread system in nature where cannibalism of infected individuals could significantly decrease the impact of a lethal disease.