Robust associations between wild perennial vegetation in coastal landscapes and on-farm parasitic wasp diversity

Background/Question/Methods

Growers producing short-cycle, organically-managed crops in California depend on natural biological control for curbing pest outbreaks. However, these agroecosystems are challenging for effective conservation biological control because frequent disturbance precludes the establishment of resident communities of natural enemies of pests. We tested for a relationship between perennial, non-crop vegetation in agricultural landscapes and parasitoid diversity within Brassicacrop fields, for different functional taxa of parasitic wasps in the Braconidae and Ichneumonidae. Malaise trap samples were taken in three seasons in 35 organic farm fields situated within a mosaic of land uses in coastal California. Using a GIS, we characterized vegetative cover within 500m and 1500m, including annual croplands, grasslands, chaparral, oak woodlands, and coniferous forests.

Results/Conclusions

The mean species richness of ichneumonid and braconid parasitoids captured in field crops was, like our previous finding for tachinid flies, positively associated with the cover of wild perennial vegetation. This pattern held for some functional groups, such as braconid parasitoids of lepidopterans, coleopterans, and dipterans whereas mean richness of braconid species that parasitize aphids had no relationship with non-crop landscape features. Many studies now show a positive effect of wild, perennial habitat fragments as refugia to support parasitoids that provide ecosystem services in annual and short-cycle crop fields, but to stem the tide of agricultural intensification and expansion, actual valuations of these conservation measures and their economic advantages may be needed for growers, land managers and policy-makers.