No abstract

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.

The threecornered alfalfa hopper (Spissistilus festinus) is a pest of grapevine, with damage caused by transmission of grapevine red blotch virus. Because grapevine is not a preferred host of the threecornered alfalfa hopper, abundance in vineyards depends on proximity to source habitats and presence of preferred hosts in vineyard understories. The potential for alfalfa fields and pastures in the Central Valley of California to serve as sources of threecornered alfalfa hopper was evaluated by quantifying parameters associated with threecornered alfalfa hopper reproductive and nutritional status. Laboratory studies determined that the threecornered alfalfa hopper is synovigenic, emerging as an adult prior to initiation of oogenesis and that females have multiple rounds of egg production. Alfalfa fields, irrigated pastures, and vineyards were sampled monthly. Adults were observed year-round in alfalfa fields and pastures, with populations peaking in fall. Gravid females were observed from February through November. While rare, adult threecornered alfalfa hoppers were collected from 2 of 4 sampled vineyards. In spring, adults were observed in samples collected from vineyard ground cover. In fall, adults were observed in samples collected from vineyard ground cover and foliage samples. Samples collected from pastures and vineyards were male biased, whereas equal numbers of males and females were observed in alfalfa fields. Adults collected from alfalfa fields were larger, heavier, and had greater estimated energetic reserves than adults collected from pastures. Adults collected from vineyards were of above average size and had relatively high estimated energetic reserves. Results suggest that alfalfa fields are more likely to serve as sources of threecornered alfalfa hoppers than irrigated pastures and that differences in male and female behavior may affect rates of pathogen transmission.