Countergradient vs. cogradient variation in growth and diapause in a lichen-feeding moth, Eilema depressum (Lepidoptera: Arctiidae).

Separating genetic and environmental causes of the latitudinal differences among populations is crucial when evaluating the potential for microevolutionary responses to the changing environment. We studied among-population and environmental components of variation in several life-history traits of a lichen-feeding moth Eilema depressum when offspring of replicate Swiss and Finnish females were reared in a common-garden factorial experiment. A partial second generation was produced only among Swiss larvae, more likely so at higher temperature regime and higher host quality, and more frequently among the offspring of particular females. Growth rates of larvae that chose the diapause development were higher in northern individuals. Our results thus reveal adaptive differences between latitudinal populations in studied life-history traits, allowing to expect rapid adaptation of the species to further environmental changes. In contrast, invariable responses of the growth rates of the larvae to temperature and host quality support the idea that some basic parameters of insect growth show a high degree of evolutionary conservatism.