Rapid seasonal adaptation of an alien bruchid after introduction: geographic variation in life cycle synchronization and critical photoperiod for diapause induction

Abstract

Whether alien insects that are introduced into temperate regions adapt to seasonally changing environmental conditions is an important question in evolutionary biology. If rapid evolution has occurred in a non-native environment, a latitudinal cline in critical photoperiod for diapause induction (i.e., the photoperiod at which half of the individuals enter diapause) and in life cycle synchronization with host plant phenology should be evident among locations. The alien bruchid Acanthoscelides pallidipennis (Motschulsky) (Coleoptera: Bruchidae) is native to North America and introduced into Japan with the host plant Amorpha fruticosa L. (Fabaceae) in the late 1940s. To examine whether seasonal adaptation has occurred in A. pallidipennis, we conducted a laboratory experiment and phenological observations using three latitudinally different populations. We bred F1 eggs at 22 °C and five photoperiodic regimens – L:D = 10:14, 13:11, 14:10, 15:9, or 16:8 hours – and examined whether diapause was induced. The estimated critical photoperiod for diapause induction was longest in the most northern population and shortest in the most southern population. Life cycle was found to be synchronized with host phenology in each location. Also voltinism varied geographically, from univoltine in the northern population to bivoltine in the southern populations. These results showed that A. pallidipennis rapidly adapted to seasonal environmental conditions in Japan after its introduction.