Thrips

Thrips are very small, slender insects that are best seen with a hand lens. Mature western flower thrips are 0.06 inch (1.5 mm) long, while onion thrips are slightly smaller at 0.05 inch (1.3 mm) long. The most distinctive characteristic of thrips is two pairs of wings that are fringed with long hairs. Adults are pale yellow to light brown in color. Immature stages have the same body shape as adults but are lighter in color and are wingless. Western flower thrips adults have red-colored pigment in their simple eyes (ocelli) while onion thrips' simple eyes are gray.

Thrips have a very extensive host range, including cereals, onions, garlic, and broadleaved crops, but it is only the species of plants that are infected by tomato spotted wilt virus and on which the thrips can complete their entire life cycle that play an important role in the disease cycle. In California, the key crop hosts include tomato, pepper, lettuce, radicchio and fava bean. Important weed hosts include little mallow (Malva parviflora), sowthistle (Sonchus oleraceus), and prickly lettuce (Lactuca serriola).

The adults are the only life stage that can fly, but they are not strong fliers. Adult thrips can be carried on wind currents, on clothing, and in association with plants. The length of the thrips life cycle (from egg to adult) varies depending on environmental conditions, but is generally 30-45 days, though it can be as little as 14 days.

The primary damage caused by thrips to peppers is the vectoring of Tomato spotted wilt virus. The virus can only be acquired by the immature stage of thrips, whereas plant-to-plant transmission primarily occurs by adults. The adult thrips can transmit the virus for the remainder of their lives, which can last 30 to 45 days. However, the adults do not pass the virus to their progeny through the egg.

High populations of thrips can cause damage with their feeding, which distorts plant growth, deforms flowers, and causes white-to-silvery patches on emerging leaves that often have tiny black fecal specks in them.

If possible, avoid planting peppers next to onions, garlic, or cereals, because thrips often build up to large numbers on these crops. Also, avoid fields near greenhouses where ornamentals (cut flowers) are grown, as these plants serve as hosts for thrips and the virus they transmit.

Use reflective mulches early in the season to repel thrips. Put up yellow sticky traps, which are also useful for monitoring aphids, whiteflies, and tomato psyllid, and monitor the movement of thrips.

Insecticide treatments for thrips are usually not warranted in the Imperial Valley but may be needed for suppression of Tomato spotted wilt virus in the San Joaquin Valley and coastal growing areas. If thrips are present, treat seedlings and transplants with imidacloprid (treat transplants before placement in the field and at planting). Treating plants with foliar insecticide sprays throughout the season on a need basis may limit in-field spread of Tomato spotted wilt virus to some extent. Rotate classes of insecticides to minimize insecticide resistance in thrips.