Population density and diet type interactively affect individual growth of an omnivorous soil-dwelling insect (Anomala cuprea, Coleoptera: Scarabaeidae)

Density effects are a fundamental ecological question, but their impacts on the individual growth of insects are highly variable. Scarab larvae in soils often occur at high density, but density effects of their population are rarely reported. We examined how the density of the first instar larvae of the soil-dwelling omnivore Anomala cuprea Hope (Coleoptera: Scarabaeidae) affected their growth when fed two diet types in which carbon to nitrogen ratio (C:N ratio) differs. The C:N ratio was used as a parameter of diet quality in the context of ecological stoichiometry. The larvae were grown for 34 days at three population densities (one, three, or five larvae per cup) and fed two diet types (humus as a low C:N ratio diet or wood flakes as a high C:N ratio diet). An increase in population density reduced larval growth under the low C:N ratio diet, but it enhanced larval growth under the high C:N ratio diet. Larval mortality was always low, but it was observed only at a population density of three or five larvae. Compensatory growth, gut symbionts, and hormesis are discussed as possible mechanisms of these results. In nature, larvae occur at high density and the C:N ratio of their diets is low. Therefore, our results suggest that high population density will have positive effects under natural conditions.