Order Thysanoptera - Thrips

• Pea Thrips

Minute, slender-bodied insects, often found in flowers. Antennae rather short, with 6-9 segments, and placed close together on the front of the head. Mouthparts for piercing and sucking. Usually with a pair of very narrow wings, fringed both front and back with long hairs, but many species are wingless, and wing development is often very variable, so that even within a species there may be wingless, short-winged and fully-winged individuals. The prothorax is distinct, but the other two thoracic segments are completely fused together. Legs with very short, one or two segmented tarsi, ending in claws and a protrusible vesicle. These vesicles act like small suction pads and enable the insect to walk on almost any kind of surface. Cerci absent. Metamorphosis is simple, with two nymphal stages of the usual type, followed by two or three non-feeding, but mobile, 'pupal' stages. Although thrips pass through these so called pupal stages during their development, their wings develop externally and they are clearly exopterygotes and not closely related to the other (endopterygote) Orders with pupal stages. Some 5,000 species of thrips are known worldwide, of which about 150 are native to the British Isles.

Adult thrips (left), showing the feather-like wings, and a wingless young stage or nymph (right).
Most thrips are only 1-2 mm long, but some tropical species reach about 14 mm long.
Illustration: adapted from Imms 1957

Some thrips suck the juices of other insects and some live on fungi and decaying material, but the majority feed on the sap of living plants and, although the individual insects are small, they often exist in such immense numbers that several species are agricultural pests. One such pest is the Pea Thrips (Kakothrips robustus), which is illustrated below. Other species damage cereal crops and ornamental plants. The nymphs and young adults of the Corn Thrips, Limothrips cerealium, feed in the ears of wheat and other cereals, causing the grain to shrivel. In addition to the direct damage done by thrips, some species are able to transmit plant diseases. However, the damage caused by some thrips is offset, at least to some extent, by the pollinating services of others as they crawl about among flowers.

In view of the delicate structure of their wings, the thrips are surprisingly good fliers and many of them take to the air on warm, still days. This is when they get into our eyes and hair, and may cause considerable irritation. These flying thrips are often called 'thunder flies' or 'thunder bugs' because of their association with sultry and thundery weather. They are particularly common in mid-summer when cereal thrips are leaving their host plants, as the cereal crops ripen.

Description. Up to 2 mm long. Body mainly dark brown. Forewings heavily tinged with brown, except at the base. Hindwings almost transparent. Tarsi yellow.

Biology. These thrips are responsible for the mottled silvery appearance of pea and bean pods. The insects pierce and scrape the outer cells to reach the sap and the damaged, collapsed cells give the surface its silvery appearance. When they attack the flowers and young pods there can be considerable crop losses. Adults emerge from pupae in the soil during late spring and early summer, and fly up on to the plants. Females lay eggs in flowers and the nymphs hatch after a week or so, feed for 2-3 weeks and then pupate in the soil to overwinter. Usually only one generation a year.

Distribution. Most of Europe, including the British Isles.

Photo: V.J. Stanek ©

Winged adult