Adaptation and heritability of diapause timing in a range expanding biocontrol agent

Adaptation to local environments during range expansion is crucial for establishment of invasive pests and their biological control agents. The rate of adaptation depends on heritable variation, which depends on the environment. Thus, as populations spread to new environments, the heritability of important traits can change. Populations spreading across latitude encounter different photoperiod regimes, which are used by many insects to cue winter dormancy (diapause). Appropriate timing of diapause in the local photoperiod regime allows individuals to survive the winter. Understanding the role of adaptation and its limits during range expansion into novel photoperiod environments will improve predictions of the rate and extent of range expansions. Here, we use the southward range expansion of Diorhabda carinulata, the biocontrol agent of the invasive plant saltcedar, to study adaptation to photoperiod and limits to the rate of adaptation. We measured heritability of diapause timing in one population in local and novel environments. We also measured the responses of eight populations from across the range to two photoperiod environments. We found that diapause timing was highly heritable in the local environment, but not in the novel environment, and that populations across the range were locally adapted to photoperiod. These results suggest that D. carinulata has the genetic variation to adapt to local photoperiod regimes, but adaptation is only possible when the range expansion happens gradually. Our results can be used to predict the rate of spread of D. carinulata and to improve our understanding of the ecological factors important to range expansions.