Thrips

Thrips are tiny, slender insects less than 0.06 inch (1.55 mm) long. Western flower thrips are by far the most common thrips on cotton and may be found all season. They are light colored; adults have clear, slender wings. The bean thrips occasionally appear on cotton in summer, usually at field edges. Immature bean thrips have orange or red spots at either side on the end of their abdomen; adults are usually dark with white bands on their wings.

Although western flower thrips feed on leaves and buds and may sometimes cause seedling leaves to become distorted, their benefits as predators of mites generally outweigh any damage they may cause. While infested seedlings may appear severely deformed, they will grow out of the damage rapidly with the onset of hot weather. In unusual seasons when cool spring weather persists into June, damage to terminals and squares may be severe, particularly in the northern cotton growing areas of the San Joaquin Valley (Merced County). In the low desert growing areas where Bt cottons are used, Delta Bt cotton appears to be more susceptible to damage by western flower thrips.

Bean thrips feed on the cotton plant and may cause mature leaves to turn coppery brown or red and lower leaves to drop. Bean thrips injury may resemble spider mite injury, but affected leaves are covered with tiny black specks, which are the feces of the thrips. Usually injury is confined to field borders.

No treatment is generally recommended for western flower thrips. Young plants will rapidly recover from injury. Insecticides applied to control flower thrips are usually counterproductive, as they tend to promote outbreaks of mites. Only in situations where a prolonged thrips infestation is destroying seedling terminals is treatment justified.

Bean thrips outbreaks most commonly occur where there are abandoned fields or pasture areas with an abundant growth of prickly lettuce and morningglory, or field bindweed. Control of these weeds will reduce the probability of an outbreak. Spot or strip treatments may occasionally be needed; the bean thrips are controlled by most insecticides used against lygus bug.

Some Californian upland cotton varieties appear to be more susceptible to thrips than Acala varieties. These susceptible varieties should be monitored more closely for thrips populations and damage.

The critical time for monitoring thrips is from crop emergence through seedling stages. To improve the efficiency of your monitoring program, combine sampling of thrips with other pests. From crop emergence to seedling growth sample aphids, mites, and thrips together as described in MONITORING SPIDER MITES, APHIDS, AND THRIPS.