Temperature, density, and phenotypic plasticity of melanism in Callosobruchus maculatus (Coleoptera: Chrysomelidae)

Variation in dark coloration due to melanin, i.e., melanism, in insects is often phenotypically plastic and adaptive. However, the pigment melanin and/or components of the melanin synthesis pathway, have many functions in insects beyond just coloration, such as roles in immune defense and cuticular hardening. For these reasons, melanism responses may sometimes be complicated, particularly if melanin-based coloration trades off with other melanin-based functions; such trade-offs may be related to resource availability. We manipulated the temperature and density (number of beetles per bean) of cowpea weevils, Callosobruchus maculatus (Fabricus) (Coleoptera: Chrysomelidae) during the larval and pupal period and found both variables affected the coloration and size (measured as area of elytra) of the emerging adults. Lower temperatures produced darker and larger beetles in both sexes, with females also being darker than males. Increased numbers of beetles per bean reduced both the size and darkness of the beetles. The effects of temperature on melanism are consistent with the adaptive thermal melanism hypothesis while the effects of per-bean density on melanism may reflect resource-based trade-offs. However, both effects were modest and left considerable variation in color unexplained.