Habitat eradication and cropland intensification may reduce parasitoid diversity and natural pest control services in annual crop fields

Authors: LETOURNEAU, D. - University Of California; BOTHWELL, ALLEN, S. - University Of California; Kula, Robert; SHARKEY, M. - University Of Kentucky; STIREMAN, J. O - The Ohio State University

Submitted to: Elementa: Science of the Anthropocene
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2015
Publication Date: 10/7/2015
Citation: Letourneau, D.K., Bothwell, A.G., Kula, R.R., Sharkey, M.J., Stireman, J.I. 2015. Habitat eradication and cropland intensification may reduce parasitoid diversity and natural pest control services in annual crop fields. Elementa: Science of the Anthropocene. 3:1-13.

Interpretive Summary: Parasitic wasps and flies attack crop, forest, and horticultural pests that cause billions of dollars of damage to agricultural commodities and natural resources annually. The wasps and flies treated in this paper attack a broad spectrum of herbivorous insects, including vegetable crop pests. The relationship between vegetational surroundings and natural enemy species richness was tested for parasitic wasps and flies foraging in cole crops. As intensification of the landscape for crop production increased, overall richness and abundance of parasitic wasps and flies in the focal crop field decreased. Parasitic wasp and fly richness overall was positively associated with the amount forest and grassland surrounding crop fields. Principal components analysis revealed a positive association of landscape complexity with braconid and ichneumonid wasp richness. Parasitism of cabbage aphid pests was positively correlated with grassland cover. Cabbage looper parasitism was either positively or negatively correlated with landscape vegetational complexity depending on natural enemy species. This paper will be useful to scientists conducting research on vegetable crop pests, as well as vegetable crop producers and agricultural extension agencies.

Technical Abstract: California’s central coast differs from many agricultural areas in the U.S., which feature large tracts of monoculture production fields and relatively simple landscapes. Known as the nation’s salad bowl, and producing up to 90% of U.S. production of lettuces, broccoli and Brussels sprouts, this region is a mosaic of fresh vegetable fields, coastal meadow, chaparral shrubs, riparian and woodland habitat. We tested for relationships between the percent cover of crops, riparian and other natural landscape vegetation and the species richness of parasitic wasps and flies foraging in crops, such as broccoli, kale and cauliflower, and interpreted our results with respect to the decrease in natural habitat and increase in cropland cover prompted by a local microbial contamination event in 2006. Our key results are that: (1) as cropland cover in the landscape increased, fewer species of parasitoids were captured in the crop field, (2) parasitoid richness overall was positively associated with the amount of riparian and other natural vegetation in the surrounding 500m, (3) different groups of parasitoids were associated with unique types of natural vegetation, and (4) parasitism rates of sentinel cabbage aphid and cabbage looper pests were correlated with landscape vegetation features according to which parasitoids caused the mortality. Although individual species of parasitoids may thrive in landscapes that are predominantly short season crops, the robust associations found in this study across specialist and generalist parasitoids and different taxa (tachinid flies, ichneumon wasps, braconid wasps) shows that recent food safety practices targeting removal of natural vegetation around vegetable fields in an attempt to eliminate wildlife may harm natural enemy communities and reduce ecosystem services. We argue that enhancing biological diversity is a key goal for transforming agroecosystems for future productivity, sustainability and public health.