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Thysanoptera

(Thrips)

Class Insecta

Order Thysanoptera

Number of families 9

Photo: Giant thrips of the Thysanoptera order, in

Java, Indonesia. (Photo by ©Simon D. Pollard/Photo

Researchers, Inc. Reproduced by permission.)

Evolution and systematics

Although recorded as fossils from the lower Permian, most fossil thrips prior to those in Cretaceous Lebanese amber are equivocal. Currently the group is considered part of the Paraneoptera, together with the Psocodea and Hemiptera. Two suborders are recognized: the Tubulifera with one family (Phlaeothripidae) and the Terebrantia with eight families (Merothripidae, Melanthripidae, Aeolothripidae, Adiheterothripidae, Fauriellidae, Heterothripidae, Thripidae, and Uzelothripidae). Species in the Merothripidae and Melanthripidae retain more ancestral features than other thrips, and the Uzelothripidae is represented by a single highly aberrant species. Currently, about 5,500 species are recognized, but there are many undescribed species in tropical areas.

Physical characteristics

Adult and larval thrips are unique among insects in retaining in the head only the left mandible, the right one being resorbed by the embryo. The maxillary stylets form a suctorial feeding tube, but this has only one channel for both food and saliva, unlike hemipterans. The wings are slender and fringed with long cilia, hence the ordinal name meaning “fringed-wings,” but similar narrow wings with long, fringing cilia occur in unrelated small insects, and many adult thrips are wingless. The German name for the group, Blassenfusse, refers to the adhesive tarsal pads found in adults, and the common name, thrips, is Greek for “woodworm,” refer-

ring to the fact that many species live on dead branches. Adults are usually flattened dorsoventrally, ranging in length from 0.02–0.6 in (0.5–15 mm), and although commonly black, many species are yellow to white, and others exhibit typical aposematic colors black, red, and white.

Distribution

Thrips are essentially tropical. For example, the combined area of Costa Rica and Panama is roughly equal in size to Britain, but despite limited study, more than 300 species are described from these two tropical countries, whereas 100 years of active study in Britain found less than 150 native thrips species. About 700 species are described from North America, 1,500 from South America, and 500 from Australia, but these figures probably represent scarcely 50% of the real fauna. The thrips fauna of tropical Africa is virtually unknown.

Habitat

About 40% of thrips species live on dead branches or in leaf litter, whereas about 30% live on green leaves. Most of the remaining 30% live in flowers, many being specific to grass florets. A few species live in mosses. Larvae and adults occupy the same habitat, but larvae commonly fall to the ground to pupate. Diversity is greatest in tropical forests, but populations are largest in open habitats, and vast numbers of individuals occur sometimes in alpine meadows.

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A group of black thrips (Echinothrips americanus) nymphs, some with secretion droplets. (Photo by ©Holt Studios Int./Photo Researchers, Inc. Reproduced by permission.)

Behavior

Fighting between males is probably widespread amongst thrips species, particularly in Phlaeothripidae, and the body size of males often varies greatly within species. In some fungus feeding species a male may fight to defend his mate, or alternatively to defend a clutch of eggs to which several females contribute after mating with him. Male lekking has been observed in two unrelated species, of which one, the Australian pest species Kelly’s citrus thrips (Thripidae), has females visiting a male aggregation briefly to copulate. Thigmotaxis, the habit of crawling into confined spaces, is widespread amongst thrips species.

Feeding ecology and diet

At least 2,000 described species feed on fungi (most on hyphae), but with about 700 taking whole fungal spores into their gut. Such species live on freshly dead leaves and branches, as well as in leaf litter. In the tropics, many species feed on the leaves of trees, some inducing galls, but in temperate regions thrips are usually associated with flowers, feeding on pollen and other floral tissues. Some species are predatory on other small arthropods, and a few pest species are so adaptable that by their feeding they may control pop-

ulations of pest mites on crop plants as well as damage such crops directly.

Reproductive biology

Sex determinism in thrips is haplodiploid, that is, males have half the number of chromosomes of females and develop from unfertilized eggs. Thrips metamorphosis is unique, with two larval instars followed by two (or even three) nonfeeding pupal instars. In fungus-feeding Phlaeothripidae, males are commonly larger than females, but in gall thrips and most Terebrantia, males are smaller than females. Many flowerliving species have only a single generation in a year, but most species breed whenever suitable conditions exist. Pest thrips often breed more or less continuously, with a new generation developing every three weeks. In arid parts of Australia, a considerable number of phlaeothripine species construct a domicile in which to breed by gluing or sewing together pairs of leaves.

Conservation status

Thrips faunal diversity is dependent on conserving the diversity of the native flora. Thus large areas of Australia or

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North America that are intensively disturbed commonly have few or no native thrips species. No Thysanoptera are included on the IUCN Red List.

Significance to humans

Thrips are commonly considered pest insects, although fewer than 10% of known species have been recorded as causing crop damage. When in high population numbers, some

Order: Thysanoptera

species may sometimes bite humans by probing the skin with their mouthparts, and adult thigmotactic behavior can result in thrips triggering smoke detectors when entering these for shelter during massed flights in late summer. Pest species are usually highly adaptable insects that can feed on a wide range of plants under varying conditions, whereas most thrips are relatively host and habitat specific. Ten species of Thripidae, including the western flower thrips, are known to infect plants with virus diseases known as tospoviruses, and worldwide such thrips are among the most serious of insect pests.

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1. Crowned thrips (Ecacanthothrips tibialis); 2. Western flower thrips (Frankliniella occidentalis). (Illustration by John Megahan)

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