Landscape diversity and crop vigor outweigh influence of local diversification on biological control of a vineyard pest

Abstract

The influence of local and landscape habitat diversification on biological control of the Western grape leafhopper (Erythroneura elegantula Osborn) by its key parasitoids Anagrus erythroneurae S. Trjapitzin & Chiappini and Anagrus daanei Triapitsyn was studied in wine grape vineyards. At the landscape scale, Anagrus rely on alternative host species in non‐crop habitats outside of the vineyard to successfully overwinter, while at the local scale vineyard diversification can provide resources, such as shelter and floral nectar, which improve parasitoid performance. In a two‐year experiment, plots with and without flowering cover crops were compared in vineyards representing a gradient of landscape diversity. While the cover crops did attract natural enemies, their populations were unchanged in the crop canopy and there was no difference in parasitism rate, leafhopper density, crop quality, or yield. Vineyards in diverse landscapes had higher early‐season abundance of Anagrus spp., which was linked to increased parasitism and decreased late‐season populations of E. elegantula. Leafhopper densities were also positively associated with crop vigor, regardless of landscape or cover crops. Flowering cover crops did increase abundance of some natural enemy species as well as parasitism rate in vineyard landscapes with intermediate levels of diversity, indicating a local × landscape interaction, although this did not lead to reductions in E. elegantula densities. These findings indicate that, in this agroecosystem, landscape diversity mediates and in many ways outweighs the influence of local diversification and that E. elegantula densities were regulated by a combination of biological control and crop vigor.

Native grass ground covers provide multiple ecosystem services in Californian vineyards

“…Our results are in accordance with Costello and Daane () and Wilson et al. (, ), who reported similar associations of lowered leafhopper densities on vines with lowered leaf nitrogen.…”

“…Numbers of spiders also increased slightly—but significantly—on vines in the non‐irrigated native grass treatment. Studies using exotic plant species as ground covers also reported increases in generalist predator numbers (Nicholls et al., ; Wilson et al., ). Spiders are sensitive to habitat structure and the grasses tested may have provided additional structure that benefited spiders and translated into higher numbers on vines.…”

“…(Settle & Wilson, ). Studies in coastal California vineyards, where E. elegantula was the dominant leafhopper species, found that a summer cover crop consisting of purple tansy ( Phacelia tanacetifolia ), Bishops flower ( Ammi majus ) and wild carrot ( Daucus carota ) had no influence on leafhopper densities or egg parasitism rates (Wilson et al., ). Similarly, Nicholls et al.…”

“…Leafhopper densities were previously shown to be negatively correlated with reduced vine vigour associated with lower applied water amounts (Daane & Williams, ). Also, ground covers that remain green throughout the growing season may provide superior habitat and alternate food to natural enemies (English‐Loeb, Rhainds, Martinson, & Ugine, ; Wilson et al., ).…”

“…Many vineyard growers use ground covers, either in the form of sown cover crops or resident floor vegetation, but effects on pest densities have not been consistent and/or economically significant (Daane, Vincent, Isaacs, & Ioriatti, 2018). Increases in natural enemy densities on grape vines have been recorded in the presence of cover crops, with either positive (Nicholls, Parrella, & Altieri, ) or neutral (Wilson, Miles, Daane, & Altieri, ) effects on suppression of leafhopper pests. On the other hand, Costello and Daane () reported ground vegetation reduced leafhopper numbers but had no effect on spider numbers, and suggested that competition from weedy grasses influenced leafhoppers in a bottom‐up manner by reducing vine vigour.…”