Density-dependent phase polyphenism in the fall armyworm, Spodoptera frugiperta

Phase polyphenism is a form of phenotypic plasticity in which the expression of a variety of traits can be affected by local population density. We tested for the expression of density-dependent phase polyphenism in the fall armyworm, Spodoptera frugiperta. Larval coloration, development time, pupal weight and pathogen resistance were compared between individuals reared under isolated versus crowded conditions. When reared at high densities, the larvae exhibited a smaller body size and darker cuticular melanization than larvae reared singly. The phenotypic switch to a rapidly developing dark phase at high densities could be interpreted as an adaptive responses to reach pupation before food supplies are depleted, thereby minimizing the effects of intraspecific competition when larval density is high. However, larvae reared singly appeared to be more cannibalistic than crowd-reared larvae. This is inconsistent with the prediction that cannibalism increases at high densities in response to resource limitation. The ecological consequences of cannibalism, along with density-dependent effects on pathogen resistance and adult flight performance remained to be elucidated.