Influence of landscape heterogeneity on biological control of the western grape leafhopper (Erythroneura elegantula Osborn) in northern California vineyards

Expansion and intensification of agricultural production has led to the simplification of individual crop fields (e.g., monocultures) as well as the landscapes that surround them (e.g., elimination/fragmentation of natural habitats). Natural enemies of herbivorous pests are particularly vulnerable to the effects of landscape simplification, as these changes most acutely impact higher trophic levels. Losses in natural enemy diversity or abundance can result in higher herbivore pest population densities, high rates of crop loss, and/or lower plant productivity.

This project evaluated how changes in the area and composition of natural habitats surrounding wine grape vineyards influenced biological control of the western grape leafhopper (Erythroneura elegantula Osborn) by its key egg parasitoids Anagrus spp. (Hymenoptera: Mymaridae). The 2-year study focused on 20 vineyards spread along a continuum of landscape heterogeneity. All field sites were located in Napa and Sonoma County, California. At each site data was collected on natural enemy and pest populations and parasitism rates. Landscape heterogeneity was quantified at multiple distances (0.5 – 4 km) around each site to determine the spatial scale at which changes in habitat diversity most significantly influenced local insect populations and biological control. A natural enemy exclusion study was also conducted in a subset of vineyard sites to evaluate whether natural enemy impact varied between contrasting landscapes.

These results are part of a larger study to understand how pests and their natural enemies utilize non-crop habitats surrounding vineyards and whether this subsequently influences the effectiveness of on-farm floral resource provisioning to enhance biological control.