Hygiene conditions explain larval density-dependent performance in Plutella xylostella with sufficient food

Larval density is an important factor influencing insect growth, development and population dynamics. Under conditions of limited food resources, higher larval density leads to reduced food availability per individual, causing adverse outcomes. When food was abundant, how larval density influences insect performance and its underlying mechanism have been understudied. We compared fitness traits of Plutella xylostella (Lepidoptera: Plutellidae), over a wide range of rearing densities (1, 4, 7 and 10 larvae per Petri dish) provided with sufficient common cabbage leaves. Plutella xylostella reared at 10 larvae per container had significant reductions in the pupal weight, realized fecundity, and adult life span when compared with those reared at lower densities. To find out the underlying mechanism, we further tested the hypothesis that the hygiene condition in part mediates larval density-dependent performance of larvae with sufficient food. We investigated the effect of 3 hygiene conditions (clean, control, mess: indicated by feces accumulation rates) on fitness of P. xylostella at the same rearing density (10 larvae per Petri dish) provided with sufficient common cabbage leaves. Results confirmed the important role of hygiene conditions in mediating the responses of larva fitness to larval density that the mess condition significantly reduced the pupal weight, total survival rate, realized fecundity, adult life span, and the development rates of completing a life cycle in comparison to cleaner treatments. These findings highlight that, even with sufficient food, rearing insects in different densities and hygiene conditions can largely affect the fitness outcomes. This provides critical insights for understanding the ecology of insects feeding in high densities.