THRIPS: Thysanoptera

PHYSICAL CHARACTERISTICS

The common name "thrips" refers to a single insect or many individuals. It comes from Latin and Greek words that mean "woodworm," a reference to the fact that many species live on dead branches. Thrips are long, slender, flat insects measuring
0.02 to 0.6 inches (0.5 to 15 millimeters) in length. Depending on the species, males are either larger or smaller than females. They are usually black in color, but many species range from whitish to yellow. Other species are black, red, and white. Both adult and larval thrips are unique among insects in that they have only the left jaw in their head. The body absorbs the right jaw while the thrips is still developing in the egg. The remaining left and right mouthparts form a sucking tube with a single channel inside. Both food and saliva flow through this channel. The compound eyes are well developed, but may have as few as ten lenses in some wingless species. Winged species have three simple eyes located between the compound eyes, but they are absent in thrips that are wingless as adults. The antennae (an-TEH-nee), or sense organs, are short and have four to nine segments.

Winged adults have four slender wings that lie side by side flat over the back when at rest. The wings have few, if any, veins and are fringed with long, hairlike structures. Their legs are all similar to one another in appearance. The feet have one or two segments and lack claws. Instead each foot has a sticky, inflatable, pouchlike sac. In some species, the ten-segmented abdomen is tipped with an ovipositor, or egg-laying tube.

GEOGRAPHIC RANGE

Thrips are found on every continent except Antarctica. There are about 5,500 species known worldwide, with approximately seven hundred species in the United States and Canada.

HABITAT

Nearly half of all thrips species live on dead branches or in leaf litter. The remaining species are evenly divided between living on green leaves or on flowers. Leaf feeders often produce galls (gawls), abnormal swellings on plants. These unusual plant growths provide thrips with both food and shelter. Many species live inside leaves curled or deformed by their feeding activities, or inside bundles of leaves they attach together by glue or silk. Many flower-feeding species prefer grasses. A few species are found on mosses. Pest species eat a wide range of plants in a variety of habitats, but most thrips prefer just a few plants in specific habitats. Most species are found in tropical forests, but the largest populations are found in open habitats, such as mountain meadows.

DIET

The feeding habits of most thrips are unknown. Most species eat funguses. Others feed on leaves or flowers and pollen. Some species prey on small insects and spiders.

BEES DO IT AND THRIPS DO TOO!

The importance of thrips as pollinators is often overlooked by botanists, scientists that study plants. They are often found in large numbers on many different kinds of flowers. They continually fly from flower to flower, their bodies carrying ten to fifty grains of pollen. Thrips pollinate a wide range of flowers, including heather plants in North America and Eurasia, as well as numerous rainforest trees in Malaysia and eastern Australia.

BEHAVIOR AND REPRODUCTION

Thrips are thigmotactic (THIG-mo-TAK-tik), animals that always seek the security of confined and narrow spaces. Both larvae (LAR-vee) and adults live together. The larvae are young of the animal that must change form before becoming adults. Males of fungus-feeding species will defend a single female or groups of females they have mated with as well as the egg masses they produce. These males will compete with one another for females by flicking their abdomens at one another or by attempting to stab each other to death with spines on their legs. In other species, males form groups, or leks (lehks), that attract females that are ready to mate.

Eggs usually hatch in just a few days. In species that reproduce by mating, males develop from unfertilized eggs. Some species reproduce by parthenogenesis (PAR-thuh-no-JEH-nuh-sihs), a process where the larvae also develop from unfertilized eggs. However, these eggs always develop into females.

The development of thrips is unique. Each larval stage ends with the molt, or shedding of the exoskeleton, or hard outer covering. The first two larval stages are active and feed for 2 to 5 days. In winged species, neither of these larvae shows any sign of wing development on the outside of the body. The larval stages are followed by two or three pupal (PYU-pul) stages. The pupa (PYU-pah) is the stage of insect development when the transition between larva and adult becomes obvious. The pupa cannot walk and does not feed. In thrips, the antennae are very small, and the developing wings can clearly be seen on the outside of the pupa. Depending on species, the pupae are sometimes found on leaves, but most species pupate on bark or in leaf litter.

Thrips found on flowers usually produce only one generation per year, but most species reproduce whenever conditions are good. Pest species usually reproduce all the time, with a new generation developing every few weeks.

THRIPS AND PEOPLE

Thrips are seldom noticed except when they become pests on garden and crop plants. Their feeding activities damage leaves, flowers, and fruit. However, they cause the most damage by spreading plant diseases. Although generally considered pests, less than ten percent of all known species are known to damage or infect crops.

When their populations reach high numbers, some thrips may bite humans by probing the skin with their mouthparts. Their thigmotactic behavior has resulted in thrips setting off smoke detectors as they enter these devices to seek shelter during mass flights in late summer.

CONSERVATION STATUS

No thrips are endangered or threatened. Their survival depends on the conservation of their food plants and habitats. Developed and agricultural habitats have few or no native species of thrips.

WESTERN FLOWER THRIPS (Frankliniella occidentalis): SPECIES ACCOUNT

Physical characteristics: Western flower thrips are yellow to dark brown and measure about 0.04 inches (1 millimeter) in length.

Geographic range: Originally from the western United States, they now live throughout the temperate parts of North and South America, Europe, Africa, Asia, Australia, and New Zealand.

Habitat: They live on the flowers and leaves of many different kinds of plants.

Diet: They eat pollen as well as new flower and leaf tissues. They will occasionally prey on mites.

Behavior and reproduction: Western flower thrips fly up to one hundred yards at a time when their food plants are disturbed. Their distribution across oceans, however, is a result of hitchhiking on plants sold around the world. Males will compete with other males for territory on a leaf, but only when the population density of thrips is low.

Thrips reproduce by mating, with males developing from unfertilized eggs.

Western flower thrips and people: The western flower thrip is one of the most important pests in the world. Their feeding activities cause serious damage to flower crops, tomatoes, capsicums and cucumbers, as well as stone fruits and table grapes. They also infect many plants with disease as they feed.

Conservation status: This species is considered a pest. It is not listed as endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Lewis, T., ed. Thrips as Crop Pests. Wallingford, U.K.: CAB International, 1997.

Tavolacci, J., ed. Insects and Spiders of the World. Volume 9: Stonefly-Velvet worm. New York: Marshall Cavendish, 2003.

Web sites:

Thrips. Thysanoptera. http://www.biokids.umich.edu/critters/information/Thysanoptera.html (accessed on October 11, 2004).

"Thysanoptera. Thrips." Ecowatch. http://www.ento.csiro.au/Ecowatch/Thysanoptera.htm (accessed on October 11, 2004).

CD-ROM:

Moritz, G., D. C. Morris, and L. A. Mound. Thrips ID: Pest Thrips of the World. Melbourne: CSIRO Publishing, 2001.