Alfalfa and pastures: sources of pests or generalist natural enemies?

Authors: Sisterson, Mark; Uchima, Sean; Dwyer, Donal

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2018
Publication Date: 2/27/2018
Citation: Sisterson, M.S., Uchima, S.Y., Dwyer, D.P. 2018. Alfalfa and pastures: sources of pests or generalist natural enemies? Environmental Entomology. 47:271-281.

Interpretive Summary: Pierce’s disease and almond leaf scorch disease are both caused by the bacterial pathogen Xylella fastidiosa. The Central Valley of California is the only region in California that commercially produces almonds and grapevines. In the Central Valley of California, the most common vector of X. fastidiosa is the green sharpshooter. Green sharpshooters are known to develop large populations in pastures and weedy alfalfa fields. As a result, it is recommended that almond and grape growers distance orchards and vineyards from pastures and alfalfa fields to reduce risk of Pierce’s disease and almond leaf scorch disease. In this study, the potential risk of pastures and alfalfa fields serving as sources of green sharpshooters was compared to the potential benefit of pastures and alfalfa fields serving as sources of generalist natural enemies. Green sharpshooter abundance was low in well maintained alfalfa fields, whereas generalist natural enemy abundance was high. Accordingly, well maintained alfalfa fields were more likely to serve as a source of generalist natural enemies than green sharpshooters. In contrast, green sharpshooter abundance was high in pastures, whereas generalist natural enemy numbers were low. Thus, pastures were more likely to serve as a source of green sharpshooters than generalist natural enemies. Results provide growers with information needed to evaluate potential risks and benefits of cultivating almonds or grapevine in proximity to alfalfa fields and pastures.

Technical Abstract: Pierce’s disease of grapevine and almond leaf scorch disease are both caused by the bacterial pathogen Xyllela fastidiosa. In the Central Valley of California, the green sharpshooter is the most common vector of X. fastidiosa. As alfalfa fields and pastures are considered source habitats for green sharpshooters, it is recommended that almond orchards and vineyards should be distanced from alfalfa fields and pastures. Here, the potential for alfalfa fields and pastures to serve as sources of green sharpshooters was compared to the potential benefit of alfalfa fields and pastures serving as sources of two generalist natural enemies; lacewings and ladybugs. Green sharpshooters were abundant in pastures, and trap catches of green sharpshooters in almond orchards and vineyards coincided with peaks in trap catches in pastures. Lacewings were less abundant in pastures than in vineyards or almond orchards, and ladybugs were only moderately more abundant in pastures than in vineyards or almond orchards. Accordingly, the potential for pastures to serve as a source of green sharpshooters was not offset by any potential benefit of pastures serving as a source of lacewings or ladybugs. Green sharpshooters were not abundant in well maintained alfalfa fields. In contrast, lacewings and ladybugs were abundant in alfalfa fields. Thus, well-maintained stands of alfalfa are unlikely to serve as a significant source of green sharpshooters, but may serve as a source of lacewings and ladybugs. During sampling, threecornered alfalfa hoppers were observed in vineyards and were abundant in alfalfa fields and pastures. The threecornered alfalfa hopper was recently identified as a vector of the virus causing grapevine red blotch disease. Thus, a full evaluation of the risk of planting vineyards near alfalfa may require considering risk associated with movement of threecornered alfalfa hoppers.