Alfalfa and Pastures: Sources of Pests or Generalist Natural Enemies?

Pierce's disease of grapevine and almond leaf scorch disease are both caused by the bacterial pathogen Xylella fastidiosa Wells et al. In the Central Valley of California, Draeculacephala minerva Ball (Hemiptera: Cicadellidae) is the most common vector of X. fastidiosa. As alfalfa fields and pastures are considered source habitats for D. minerva, it is recommended that almond orchards and vineyards should be distanced from alfalfa and pastures. Here, risk of alfalfa and pastures serving as sources of D. minerva was compared to the potential benefit of alfalfa and pastures serving as sources of generalist natural enemies belonging to the families Chrysopidae and Coccinellidae. Populations of D. minerva were greatest in pastures, whereas chrysopids were least abundant in pastures, and coccinellids were only moderately more abundant in pastures than in vineyards or almond orchards. Accordingly, risk of pastures serving as a source of D. minerva was not offset by any potential benefit of pastures serving as a source of chrysopids or coccinellids. Abundance of D. minerva in alfalfa was low, whereas abundance of chrysopids and coccinellids in alfalfa was high. Thus, well-maintained alfalfa fields were a minor source of D. minerva that may contribute chrysopids and coccinellids to surrounding habitats. Spissistilus festinus (Say) (Hemiptera: Membracidae), a recently identified vector of grapevine red blotch virus, was abundant in alfalfa fields and was observed in vineyards. Thus, a full evaluation of the risk of planting vineyards near alfalfa may require considering risk associated with movement of S. festinus.