Post-release dispersal of the introduced lygus bug parasitoid Peristenus relictus in California

Lygus spp. (Heteroptera: Miridae) are serious pests of numerous field and fruit crops in North America. In an effort to reduce crop damage, parasitoids known to attack these species in Europe were introduced into the USA beginning in the 1970s. Permanent populations of the nymphal endoparasitoid Peristenus relictus (Hymenoptera: Braconidae) were established at two locations in California during the 2000s. Both populations are associated with significant reductions in lygus bugs attacking alfalfa, commercially produced strawberries grown organically and in wild vegetation. Beginning in 2009, in an effort to determine the extent to which P. relictus has spread from the Sacramento Valley and Monterey Bay region, populations of lygus bug were sampled at increasing distances from their original release sites. P. relictus has dispersed at least 213 km in the central region of California and 150 km along the coastal mountains. These respective populations have averaged 16.6 km/year and 17.7 km/year since they were released. While not directly examined, the spread of P. relictus south into the central and major growing region of California, the San Joaquin Valley, where previous releases have failed, suggests this population may be evolving greater heat tolerance, relative to the populations originally introduced into California.