Density-Dependent Melanism in Winter Moth Larvae (Lepidoptera: Geometridae): A Countermeasure Against Parasitoids?

Abstract

Density-dependent melanism, i.e., the phenomenon that individuals at high population densities develop into a dark-colored phenotype, is often explained as a defense against density-dependent responses of natural enemies, in particular, disease organisms. In the work reported here, we wanted to test whether density-dependent melanism in insects may yield protection against parasitoids, which are important regulators of many outbreak populations. This was carried out by collecting comprehensive field data on parasitoid attack rates and overall mortality of both larvae and pupae of the outbreak species Operophtera brumata L. (winter moth) in relation to degree of cuticular melanism. As expected, the results showed that parasitoids were the dominating mortality factor, but unexpectedly, parasitoid attack was positively associated with the degree of melanism. Also, mortality caused by unknown factors seemed to be highest in melanic larvae. These results indicate that density-dependent melanism, which is known to protect several species of insects against disease agents, does not confer protection against parasitoids in this outbreak species, which is largely regulated by parasitoids in nature.