Conserving natural enemies to control lygus in Washington State alfalfa fields

Ann Elizabeth Jorgensen

Lygus bugs (Lygus spp.) are the most important insect pests in alfalfa grown for seed. Lygus are particularly damaging to seed production because they feed on the reproductive parts of the plant. Typically, Lygus bug outbreaks have been treated, in seed fields, with broad-spectrum insecticides. However, alternatives to broad-spectrum insecticides are being sought for many reasons, but most importantly because of new legislation that will limit the use of broad-spectrum insecticides. Conservation biological control is a promising alternative to insecticide use in alfalfa seed. Thus, a survey of alfalfa (Medicago sativa) seed and hay fields was conducted to identify insect predators that will be beneficial for controlling Lygus bugs. Samples were taken from hay fields to determine insect densities in alfalfa with low insecticide input. The survey of alfalfa hay and seed fields was conducted during 2003 and 2004. Samples were taken from fields in Touchet, WA and Warden, WA in May, June and July of each year. Insect samples were collected using an insect suction sampler (D-vac) and pitfall traps. Lygus populations were most abundant in July, when blooms are forming on alfalfa plants. Another important pest of alfalfa is the pea aphid (Acyrthosiphon pisum), which also had high densities in July. The most common insect predators in July were Hippodamia convergence, Coccinella septempunctata, Calosoma spp., Staphylinid beetles, Nabis spp., Thomisid spiders and Linyphiid spiders. Predator populations tended to be higher in hay fields than in seed fields. In a Petri dish assay, the six most common insect predators during July were tested for Lygus and pea aphid predation. Damsel bugs and crab spiders ate the most Lygus bugs. A microcosm experiment was performed with these two predators to determine Lygus consumption in the presence and absence of pea aphids. In the presence of alternative prey, Lygus predation was reduced suggesting a positive prey-prey interaction. In conclusion, Lygus control is most important during July when populations peak. Predator populations in hay fields suggest that there are a sufficient number of predators for biological control of Lygus, in the absence of broad-spectrum insecticide use.

Conserving natural enemies to control lygus in Washington State alfalfa fields

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