East meets west: adaptive evolution of an insect introduced for biological control.
A possible explanation for low success rates when introducing natural enemies to new regions for biological control of insect pests is that they fail to adapt to their new conditions. Therefore it has been widely recommended that biological control practitioners increase the probability of local adaptation by maximizing the genetic variation released. An alternative recommendation is to use climate matching to identify native populations that may already possess traits suited to the new region. However, support for these recommendations is weak through lack of empirical evidence that local adaptation is important to biological control. This study examined how genetic drift and selection influenced the population frequencies of two asexually reproducing, genetically differentiated parasitoid biotypes that were introduced to New Zealand from South America for biological control. Other than by mutation, the biotypes were genetically fixed due to the absence of recombination both within and between biotypes. This meant that adaptive evolution could occur only if selection acted on any traits that varied between the biotypes introduced from South America. The two parasitoid biotypes were released simultaneously at 14 sites and their frequencies were monitored for up to 10 years. Changes in biotype frequency were consistent with strong directional selection favouring one of the South American biotypes, thus generating established parasitoid populations that were better adapted to New Zealand conditions than those that had originally been released. This local adaptation of the control agent contributed to greater mortality of the pest. Synthesis and applications. This study provides the first clear demonstration of the importance of releasing natural enemy genetic variation in new regions to foster adaptive evolution and improve success rates in classical biological control. However, the benefit to biological control of maximizing the genetic variation released needs to be balanced against possible risks to non-target species. The results do not support the concept of choosing sampling sites for putative biological control agents based solely on climatic similarities between the source location and the intended region of introduction.