Thrips

Thrips are minute (less than 0.20 in or 5 mm) slender-bodied insects of the order Thysanoptera, characterized by two pairs of veinless, bristle-fringed wings, which are narrow and held over the back when at rest. Although thrips have wings, they do not fly. There are 4,500 species of thrips worldwide. In North America, there are 694 species of thrips in a number of families in two suborders, the Terebrantia and the Tubulifia. Greenhouse thrips (Heliothrips haemorrhoidalis) in the suborder Terebrantia deposit their eggs into slits made on plant tissues. Black hunter thrips (Haplothrips mali) in the suborder Tubulifera lay their eggs on protected plant surfaces. Red-banded thrips (Heliothrips rubrocinctus) are widespread in North America, sucking plant juices and possibly spreading plant viruses.

Several species of predatory thrips feed on mites, aphids, and other thrips. The majority of thrips are fond of trees and shrubs, roses, onions, and many other herbaceous plants. Pale or white flowers are most attractive to thrips. Their rough mouthparts break open plant tissues, allowing them to suck up plant juices. The most obvious signs of thrip damage are tiny black specks of feces, silvering foliage, distorted flowers and leaves, or buds that fail to open. The insects themselves are rarely seen without a hand lens.

In warm weather, thrips eggs hatch in days, although cold weather may delay hatching for weeks to months. Thrips undergo incomplete metamorphosis, with eggs hatching into a series of nymphs (which resemble adults), which finally metamorphose into adult thrips. The wingless thrip nymphs eat their way through two larval stages, then find a protected crevice (either on the plant or in soil) to complete the transition to a winged adult. Female thrips can reproduce parthenogenetically—that is, without mating with males. This strategy allows rapid population explosions when conditions are optimal. The complete life cycle can occur in as little as two weeks. Adults are relatively long-lived.

Adult thrips are attracted to yellow and blue sticky traps hung a few feet above plants. Conventional pesticides (such as Malathion) used to control aphids and whiteflies also kill thrips. Less toxic control can be accomplished by maintaining plant health, for water-stressed plants and weedy areas are especially attractive to thrips. Treatment of infested plants involves removing infested plant tissues and mulching with plastic or newspaper to prevent successful development, followed by surrounding the plant stems with aluminum foil to prevent reinfestation. Predatory mites (Neoseiulus mackenziei, Amblyseius cucumeris) eat thrip eggs and larvae. Larval lacewings (Chrysoperta carnea) will also control greenhouse thrips as long as aphids, the preferred prey of lacewings, are not abundant. Predatory nematodes also consume larval thrips found in soils. Other control measures such as tobacco water spray, garlic/hot pepper spray, pyrethrin, and insecticidal soap sprays can be effective in controlling thrips.