Comparing the roles of climate, predation and phylogeography in driving wing colour variation in Ranchman's tiger moth (Arctia virginalis)

In Lepidoptera, as an explanation for darker phenotypes occurring in colder areas, wing melanism has been proposed to increase solar thermal gain. Alternatively, trade-offs with aposematic signalling and ultraviolet protection have been proposed as explanations for variation in melanism. To investigate the roles of temperature, humidity, solar radiation and predation in driving melanism in the Ranchman's tiger moth (Arctia virginalis), we characterized wing melanism in 23 populations across the range. We also conducted predation experiments using artificial moths and carried out genetic analyses to examine population structure and to test whether wing coloration was hereditary. We found that wing melanism was positively associated with mean temperature during the flight season, which was the best predictor of melanism rates. Wing melanism also exhibited a negative association with humidity and a weak positive association with insolation. We also found two loci weakly associated with wing melanism and showed that melanism is likely to be highly hereditary but not closely associated with population differentiation. Our results contrast with previous findings that melanism is associated with colder conditions and higher predation risk and suggest that humidity and protection against ultraviolet radiation are potential drivers of variation in wing melanism that have been overlooked.

Automated summary

In Lepidoptera, wing melanism is proposed to increase solar thermal gain and has been associated with colder areas. However, this study found that wing melanism in the Ranchman's tiger moth is positively associated with temperature during the flight season, negatively associated with humidity, and weakly positively associated with insolation. Genetic analyses showed that melanism is hereditary but not closely linked to population differentiation. These findings suggest that humidity and protection against ultraviolet radiation may be overlooked drivers of wing melanism variation.