Managing Lygus in seed alfalfa

C Shannon Mueller¹ and B Peter Goodell²

¹University of California Cooperative Extension, Fresno, CA, USA, Statewide IPM Program, Kearney Agricultural Center, Parlier, CA, USA

Correspondence: scmueller@ucdavis.edu

In the San Joaquin Valley of California, Lygus bugs (Lygus spp.) are a pest throughout the season, shifting between crops within the landscape as plants become unsuitable hosts due to maturity or harvest. Lygus are the key insect pest and by far the most difficult insect to manage in alfalfa seed fields, and when present in high numbers, may completely destroy the crop. Both adults and nymphs feed on the alfalfa plant, attacking reproductive parts, and causing premature drop of buds and flowers (stripping), seed deformation, and reduced seed viability. Control of Lygus is critical to the economic production of alfalfa seed. Action thresholds vary with the stage of crop development. The greatest period of Lygus bug activity is from June through August. Degree-days can be used to forecast seasonal Lygus development, especially hatch and migration dates. Pesticide applications should be timed to coincide with the hatching of Lygus broods. Treatment can be delayed until egg hatch is complete, but should take place before the nymphs reach the fourth and fifth instar since these older instars and adults are more difficult to control with insecticides than younger instars. Few insecticides are available for the control of Lygus in alfalfa seed fields. Because options are limited, and there is a high potential for the development of resistance, it is critical to maintain and preserve the efficacy of currently registered chemicals. Key components in the ability of Lygus to develop resistance to insecticides include short life cycle with many generations per year, a wide host range, and exposure to many insecticide applications each year, not only in seed alfalfa but in other susceptible crops as well. The best insurance against development of insecticide resistance is rotating chemical controls and maintaining the insect's natural enemies in the field. Research continues to develop alfalfa germplasm with resistance to Lygus.

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