We are finding large numbers of alfalfa plant bugs showing up in alfalfa fields throughout South Dakota. By directly feeding on alfalfa plants, alfalfa plant bugs can become an issue for seed production as well as forage quality. While these insects are currently below threshold levels, it is important to scout in case they reach damaging populations.
Profile & Behavior
Alfalfa plant bugs are an introduced pest in the United States. The overwintering eggs hatch in late May to early June and the nymphs begin feeding on alfalfa. Alfalfa plant bugs are active for the duration of the growing season and can have up to 2 generations each year. Adults are approximately 3/8 of an inch long and pale green to brown in color with black speckles on the legs (Figure 1). Alfalfa plant bugs may be confused with tarnished plant bugs, but the alfalfa plant bugs are about twice as long. Nymphs vary in size and appear bright green with black speckles on the legs (Figure 2). Both nymphs and adults feed on the newest growth of alfalfa plants, including the leaves, buds, and flowers. This feeding injury can lead to stunting and reduced forage quality of the affected crop.
Fig. 1. Adult alfalfa plant bug.
Credit: P. Wagner
Fig. 2. Alfalfa plant bug nymph. Notice the distinct spots on the legs. Courtesy: S. Bauer, Bugwood.org
Scouting & Management
Use a sweep net to scout for alfalfa plant bugs. For each sample, take 10 pendulum sweeps and count the number of alfalfa plant bugs that are captured. Management is recommended when populations reach an estimated 20-30 alfalfa plant bugs per 10 sweeps (this includes both nymphs and adults). Several samples should be taken throughout the field to ensure accuracy.
Management options include early harvest and insecticide applications. Early harvests can cause adults to leave the field and increase mortality of developing nymphs. For a list of labeled insecticides, refer to the South Dakota Pest Management Guide: Alfalfa and check under the products approved for managing lygus bugs. Note that insecticides should be applied prior to bloom to minimize non-target impacts on pollinators.
Reference: Blodgett, S. 2006. Lygus and Alfalfa Plant Bugs. High Plains IPM Guide.