Description of the Pest
Thrips are small insects, about 0.04 inch (1 mm) long. Adult thrips have two pairs of narrow wings fringed with hairs. Younger thrips (nymphs) are wingless, whitish to yellowish, and are most commonly found in buds, flowers, or on the underside of leaves. Nymphs molt to adults continuously throughout the warm months. Adults and nymphs are found on beans throughout the growing season. Thrips lay eggs in plant tissue. Nymphs hatch after about 5 days during the summer months and complete development into adults in about 5 to 7 days.
Thrips are most noticeable and of greatest concern on young seedling plants. The plants look increasingly ragged as the thrips feed on the young leaves and buds. A common sign of a heavy thrips infestation is distorted leaves that turn brownish around the edges and curl upward. Plants typically recover from this damage as they age.
Biological control and weather conditions that promote rapid plant growth generally reduce numbers of thrips before an insecticide application is necessary. Additionally, plants usually recover from thrips injury with little impact on dry bean yield or quality.
Natural enemies play a major role in controlling thrips, especially minute pirate bugs (Orius tristicolor). Avoid the use of broad-spectrum pesticides known to harm natural enemies.
Weedy areas can support high numbers of thrips. Cultivate nearby weedy areas before beans emerge to reduce the potential of a thrips infestation. Avoid cultivating weedy areas after bean emergence or letting weeds dry out, since this will cause the thrips to migrate from the weeds to the beans.