Thrips

Adult bean thrips are minute slender-bodied insects that have a uniformly dark, grayish black body. Their forewings have two dark and two pale bands and visible under magnification, the legs and antennae are also banded light and dark. These should not be confused with the predaceous sixspotted thrips, which are pale-yellow and have six dark spots on the wing covers and primarily feed on mites.

Note: Western flower thrips, Frankliniella occidentalis, which vary from yellow to dark brown are very common in alfalfa, but have never been shown to cause economic damage in California. In fact, they often serve as alternate prey for a number of natural enemy species commonly found in alfalfa and they can be effective predators of spider mites.

Thrips feed by rasping and lacerating the plant tissue and then sucking-up the resulting juices. The rasping causes deformed and crinkled leaves from the uneven growth around the damaged parts of the plant. Feeding, particularly near the leaf mid-rib, causes curling and distortion of the leaves, which often have a cuplike or puckered appearance. Feeding damage also causes light tan spots on the leaf tissue that are accompanied by black fecal spots.

Bean thrips are not economically important pests of established stands, but they can on rare occasions be injurious to seedling stands in October in the low desert alfalfa growing areas. Puckering and stippling of leaves is not a problem. However, if temperatures are high (90ºF+) and plants are water stressed, high numbers of bean thrips along with heavy feeding damage may cause leaf drop. This problem may be worsened by an increased sensitivity to herbicides due to highly stressed plants. Bean thrips may also cause contamination problems (body parts and feces) in adjacent vegetable crops in low desert growing areas when they migrate after the alfalfa hay is cut.

Bean thrips rarely require pesticide applications for control, except perhaps for seedling stands when extensive leaf drop occurs.