Whitney Cranshaw, Colorado State University, Bugwood.org
If not adequately controlled, serious economic damage from immature and adult stages often results. Estimates of crop damage and losses to the cabbage industry have averaged 3 to 5 percent during the past decade.
Onion thrips are small, less than 2 mm in length, and because of their minute size usually go unnoticed by growers. Adults are pale yellowish to brownish, with four long, narrow wings fringed with long hairs. Immature thrips are similar in shape to the adults, but smaller in size, wingless, and lighter in color.
Onion thrips overwinter as adults in alfalfa, clover, and wheat fields. In late spring, as the daily mean temperature rises above 60o, thrips migrate to one of their many hosts. They lay eggs singly and indiscriminately in incisions made in plant tissue by a saw-like tube. More eggs are laid during the hot summer months than in cooler days. Eggs hatch and young larvae develop into adults in about two weeks. Adults live for three weeks, during which females may lay over 100 eggs. A warm, sunny, dry summer encourages reproduction and survival.
Thrips build up on alfalfa, small grains, and weeds. As alfalfa is cut for hay, and small grains are maturing and cut for threshing, movements of thrips to and from large acreages are intensified. When cabbage is nearby, thrips can infest it.
Cabbage are blistered, scarred, and bronzed by thrips feeding on leaves; with thrips present, heads are unmarketable. Thrips feed on leaves by puncturing and rasping the outer leaf tissue and sucking the sap as it exudes from the cells. Where many thrips have fed, the discolored areas coalesce to form large brownish, blister-like areas.
Nonpersistent systemic insecticides are effective against thrips. Nonpersistent contact insecticides are usually unsatisfactory for controlling thrips unless used on a regular schedule during head formation. Applications of insecticidal sprays during head formation is important for control.