Crowding-induced plasticity in Epirrita autumnata (Lepidoptera: Geometridae): Weak evidence of specific modifications in reaction norms

Abstract

We sought to distinguish between two potential effects of larval crowding on developmental schedules in Epirrita autumnata, an outbreaking folivorous moth. We asked if consequences of crowding can be entirely attributed to deterioration of environment, caused by mutual disturbance of larvae, or if there are specific modifications of reaction norms involved. In a laboratory rearing experiment, we found that most of the consequences of larval crowding could be explained as unspecific environmental effects. However, there was some evidence of crowding-related modifications of reaction norms. In particular, crowding during early development more strongly affected further growth of larvae than expected from the direct influence of inferior environmental quality: Larvae crowded early in development had shorter growth periods in the final instar and attained lower final weights. It is not clear if these modifications of reaction norms are adaptive. Nevertheless, these observations document a type of plasticity which is different from unidimensional responses to variation in environmental quality, and may thus contribute to understanding evolution of body size in insects. However, recorded modifications of reaction norms are apparently weak enough to have only a minor role in population dynamics of E. autumnata.