Thrips
Flower thrips, Frankliniella tritici (Fitch); Soybean thrips, Sericothrips variabilis (Beach); Tobacco thrips, Frankliniella fusca (Hinds), Thripidae, THYSANOPTERA


DESCRIPTION

Adult - Thrips are 1.25 mm or less in length and have two pairs of long, narrow wings fringed with long hairs. The flower thrips is yellowish-brown to amber with an orange thorax. The soybean thrips has a yellow body with a dark blotch on the thorax and two distinct crossbands on the forewings. The tobacco thrips is dark brown or black.

Egg - About 0.2 mm long, thrips eggs are cylindrical and slightly kidney shaped with a smooth white or yellow surface.

Larva - The two larval instars range from 0.6 to 1.0 mm long. Larvae are white when newly hatched, then gradually turn yellow with age. The soybean thrips' larva eventually turn orange with some red pigmentation, though the body sometimes has a greenish tint due to ingested chlorophyll.

Prepupa and Pupa - Pupal stages resemble larvae in shape and color but have short (prepupae) to long (pupae) wing pads. The prepupa is about 1.2 mm long and shrinks to about 1.0 mm as it becomes a pupa. The pupa usually remains motionless unless disturbed.


BIOLOGY

Distribution - All three thrips species are common throughout the eastern U.S. Soybean thrips also occur in California, Arizona, Utah, and Texas. Flower thrips migrate in frontal wind systems, especially in June.

Host Plants - Flower and tobacco thrips have been collected from many plant orders, including a wide range of flowers, trees, grasses, field crops, vegetables, vines, and weeds. Flower thrips seem to prefer grasses and yellow or other lightly colored blossoms. Soybean thrips feed primarily on soybean, bean, and other legumes, but also infest cotton, cucumber, smartweed, and a number of grasses.

Damage - Among damaging thrips species, soybean thrips have been ranked ninth in economic importance, yet they are relatively minor pests. These thrips feed on the underside of bean and soybean leaves throughout the growing season reaching maximum densities about a month after planting. Six to 10 thrips per leaf may cause some yellowing but relatively little economic damage. Early season drought may cause an ordinarily harmless thrips population to become a problem. Thrips damage occurs primarily during periods of vegetative growth and is difficult to distinguish from that of a wide range of pests and diseases which cause yellowing and browning of leaves in late summer. On soybeans, populations of 30 to 60 thrips per leaf have been reported to cause substantial injury.

Life History - In North Carolina, thrips overwinter as hibernating adults in sheltered areas, as larvae on plants or as pupae in the soil. They resume development in spring. Adults emerge and fly in search of suitable host plants. Though weak fliers, adult thrips are capable of flying from plant to plant and may be carried long distances by wind.

The number of eggs laid by each female soybean thrips has not been determined, but most species of thrips insert 10 to 100 eggs, singly, into plant tissue. About 4 days later, eggs hatch. At temperatures of 22 degrees C (72 F), larvae feed for about 7 days on the underside of leaves, often hiding near large leaf veins. Mature second instar larvae drop to the soil and make chambers in the center of small dirt clods. Here they turn into quiescent nonfeeding prepupae. The next day prepupae transform into pupae. About 3 days later a new generation of adults emerges. Most thrips species complete five or more generations in North Carolina.

Though soybean thrips develop most rapidly at temperatures of 32 degrees C (90 F), 26 degrees C (78 F) is considered optimum for their development because at this temperature, thrips mortality is lowest. In addition to temperature, several predators have an effect on the size of thrips populations. These include a cocoon-spinning thrips, Aeolothrips fasciatus (Linnaeus) (about 1.5 mm long, dark as an adult, yellow as a larva), the insidious plant bug, Orius insidiosus (Say) (a tiny chinch-bug-like insect), and several phytoseiid mites.


CONTROL

For up-to-date chemical recommendations, consult the current North Carolina Agricultural Chemicals Manual.

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