Colonization and establishment of Peristenus relictus (Hymenoptera: Braconidae) for control of Lygus spp. (Hemiptera: Miridae) in strawberries on the California Central Coast

Two braconid parasitoids (Hymenoptera) of European origin, Peristenus relictus and Peristenus digoneutis, were released into non-crop vegetation at four locations in the Monterey Bay region of coastal Central California for their permanent establishment and control of Lygus spp. (Hemiptera: Miridae) infesting strawberries. At two sites, parasitoids were released into wild vegetation known to support Lygus spp. near conventionally-managed strawberry fields. Parasitoids were also released into Lygus spp. specific alfalfa, Medicago sativa, trap crops intercropped in two different organic strawberry fields. P. relictus has persisted for over 4 years since last released into the original release site of wild vegetation and for two years at the first organic strawberry release site. Populations of P. relictus were significantly correlated with Lygus spp. collected from alfalfa trap crops from 2005 to 2007 (r 2 = 0.60; p < 0.005). At this organic strawberry farm, mean densities of Lygus spp. in strawberries have fallen significantly (p < 0.05) from a pre-release seasonal high of 2.7 nymphs per 50 suctions (bug-vac machine) in 2003 to 0.8 nymphs in 2007. Bordering wild vegetation composed of winter/spring annuals at three different sites supported both native Lygus spp. and Closterotomus (=Calocoris) norvegicus, an exotic mirid in California that is attacked by P. relictus in Europe. C. norvegicus dominated sampled vegetation in spring to early summer, when Lygus spp. were nearly absent, allowing for the persistence and early build-up of P. relictus at the edge of the strawberry agroecosystem, with no detectable damage to strawberries. An overwintering population of P. digoneutis has not been found in the four release sites.