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

We sought to distinguish between two potential effects of lar val crowding on developmental schedules in Epirrita autumnata, an outbreaking folivorous moth. We asked if consequences of crowding can be entirely attributed to d eterioration of environment, caused by mutual disturbance of larvae, or if there are specific modifications of reaction norms involved. In a laborator y rearing experiment, we found that most of the consequences of larval crow ding could be explained as unspecific environmental effects. However, there was some evidence of crowding-related modifications of reaction norms. In particular, crowding during early develop ment more strongly affected furth er growth of larvae than expected from the direct influence of inferior env ironmental quality: larvae crowded early in development had shorter growth periods in the final instar and attained lower final weights. It is not cle ar if these modifications of reaction norms are adaptive. Nevertheless, the se observations document a type of plasticity which is different from unidi mensional responses to variation in environmental quality, and may thus con tribute to understanding evolution of body size in insects. However, record ed modifications of reaction norms are apparently weak enough to have only a minor role in population dynamics of E. autumnata.