Developmental plasticity of melanisation in a beetle reveals sex-specific responses and performance costs

Dark pigmentation of adults resulting from cold exposure during development is ubiquitous among insects. Yet the costs associated with the developmental plasticity of melanisation have been explored only for a handful of species. We explored two hypotheses relating to colour plasticity of insects. First, if melanin pigments are costly to synthesise and require allocation of resources, increased melanisation should be negatively associated with performance traits (the cost hypothesis). Second, given the positive association between melanisation and immune responses or desiccation resistance in some species, melanisation and performance may co-vary positively (the positive covariance hypothesis). Furthermore, given the typical differences in reproductive effort and activity between females and males, we expect these performance-melanisation relationships to differ between sexes. We assessed whether temperature-induced melanisation of the globally invasive beetle Harmonia axyridis f. succinea (Pallas) (Coleoptera: Coccinellidae) was related to standard metabolic rate and flight performance while accounting for developmental temperature and family affiliation. Melanisation explained little variation in standard metabolic rate at most temperatures tested (15, 20, 25, 30 and 35 degrees C). For flight performance and after the cold-rearing regime, lighter coloured female beetles had increased flight speed compared with darker females, whereas the opposite was found in males. In addition, in warm-reared beetles, darker individuals had lower mean flight speeds. The results suggest that at least some flight performance costs are associated with developmentally induced melanisation, supporting the cost hypothesis. Disparities in these relationships between males and females suggest that the drivers underlying variation in melanisation differ between sexes. Importantly, variation in melanisation was more acute for females reared in the warm regime compared with equally treated males and to cold-reared individuals. We propose new directions to test hypotheses pertaining to the maintenance of colour variation and performance between sexes in nature. A cold exposure during development increased melanisation of adult beetles of Harmonia axyridis f. succinea compared with a warm exposure but the extent of melanisation varied more in females than males. In the warm treatment, darker beetles (more black spots and % melanic elytron area) had lower average flight speed than lighter beetles. In the cold treatment, the relationship between flight performance and melanisation was sex dependent. image