Thysanoptera

(Thrips)

Class Insecta

Order Thysanoptera

Number of families 9

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," referring 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.

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 populations 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 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 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.

Species accounts

List of Species

Western flower thrips

Frankliniella occidentalis

family

Thripidae

taxonomy

Euthrips occidentalis Pergande, 1895, California, United States.

other common names

None known.

physical characteristics

About 0.04 in (1 mm) long; color yellow to dark brown.

distribution

Throughout temperate parts of the world, but originally western United States.

habitat

Flowers and leaves of many plants.

behavior

Individuals commonly fly for hundreds of yards when their host plant is disturbed, but long-distance transport is due to the horticultural trade in plants. Males sometimes compete for territories on a leaf, but only when the population density is low.

feeding ecology and diet

Feeds on pollen, also flower and young leaf tissues; sometimes predatory on mites.

reproductive biology

Usually bisexual, but males develop from eggs that have not been fertilized.

conservation status

Not threatened; a serious pest that requires control throughout much of the world.

significance to humans

This is one of the most important horticultural pests in the world, causing serious damage to flower crops, tomatoes, capsicums and cucumbers, as well as stone fruits and table grapes. As well as direct feeding damage, the thrips infects many plants with viruses.

Ectoparasitic thrips

Aulacothrips dictyotus

family

Heterothripidae

taxonomy

Aulacothrips dictyotus Hood, 1952, Brazil.

other common names

None known.

physical characteristics

About 0.06 in (1.5 mm) long; dark brown, with third and fourth antennal segments larger than remaining seven segments and both bearing a greatly elongate, convoluted sensory area.

distribution

Southern Brazil.

habitat

Under the wings on the upper surface of the abdomen of a plant-feeding treehopper, Aetalion, (Aetalionidae) unlike the flower-feeding habit of all other Heterothripidae.

behavior

The host insect lives in colonies, thus making it easier for the larval thrips to transfer between individuals when these molt. The bugs become disturbed when the adult thrips walk over them.

feeding ecology and diet

All life stages, from the youngest larvae to pupae and adults, live under the wings of the host insect, presumably sucking its blood.

reproductive biology

Possibly parthenogenetic; the male is not known. Eggs have not been observed, but newly emerged first instar larvae occur on the abdomen of their host, together with later instar larvae and pupae. Each pupa is enclosed in a transparent cocoon fixed to the surface of the host's abdomen.

conservation status

Not listed by the IUCN, although currently reported from only two localities.

significance to humans

None known.

Crowned thrips

Ecacanthothrips tibialis

family

Phlaeothripidae

taxonomy

Idolothrips tibialis Ashmead, 1905, Philippines.

other common names

None known.

physical characteristics

About 0.08–0.16 in (2–4 mm) long; third antennal segment bears crown of at least 10 large sensoria, (most of the 3,500 members of the family have only one to three small sensoria on this segment). Front legs or large males are greatly enlarged and bear a stout tooth on the tarsus; smallest males and females have slender front legs. The last abdominal segment is tubular (as in all Phlaeothripidae); forewings do not have veins bearing setae.

distribution

Old World tropics, from Africa to northern Australia.

habitat

On dead twigs and branches.

behavior

Males exhibit great variation in size and are known to be sub-social or even truly social. Males fight other males to defend either a single female or a single egg mass to which several females will progressively contribute. Smallest males may attempt to sneak mate while their larger siblings are fighting. Small males have a nutritional advantage in that they require less food to achieve maturity; thus there is a balance between feeding and mating advantages.

feeding ecology and diet

Adults and larvae feed on fungal hyphae growing on freshly dead wood.

reproductive biology

Eggs are laid on the surface of dead twigs among the fungus on which adults and larvae feed.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Australian acacia gall thrips

Oncothrips waterhousei

family

Phlaeothripidae

taxonomy

Oncothrips waterhousei Mound and Crespi, 1995, Australia.

other common names

None known.

physical characteristics

Winged and wingless morphs of both sexes are about 0.06 in (1.5 mm) long. Wingless adults mainly yellow, with thorax and forelegs enlarged, antennae short. Winged adults are dark brown with slender forelegs and thorax, long slender antennae.

distribution

Widespread across the arid zone of Australia.

habitat

Gall-inducing on the phyllodes (leaves) of several Acacia species.

behavior

Wingless adults of both sexes function as soldiers to defend their gall from invading kleptoparasitic thrips and also moth larvae.

feeding ecology and diet

When a female feeds on a young phyllode, this is induced to form a small pouch that rapidly encloses the insect.

reproductive biology

Female lays eggs within the hollow gall soon after becoming enclosed. These hatch, and larvae of this first small generation develop into wingless soldiers. A second generation is then produced in which the adults are fully winged and disperse to found new galls.

conservation status

Not listed by the IUCN. Presumably threatened by progressive destruction by farmers of host plants to feed livestock, particularly during years of drought.

significance to humans

None known.

Australian cycad thrips

Cycadothrips albrechti

family

Aeolothripidae

taxonomy

Cycadothrips albrechti Mound and Terry, 2001, Australia.

other common names

None known.

physical characteristics

About 0.08 in (2 mm) long, color varies from golden yellow to light brown. Forewings are broad, as in other members of the family, but third antennal segment bears two unusually inflated sensoria. Large males have pair of stout, thornlike setae near tip of abdomen; these setae scarcely developed in small males.

distribution

Central Australia, near Alice Springs.

habitat

Breeds in the male cones of the cycad Macrozamia macdonnellii.

behavior

Breeds in a particularly hot and arid area, has only been seen to fly late in the afternoon when the humidity rises. At that time, both sexes fly in swarms from the male cones on which they have fed and produced larvae, and each adult carries about 20 pollen grains on its body. Concurrently, a powerful odor is given off by any mature female cone, and this attracts the flying thrips, and induces them to crawl inside with their pollen load.

feeding ecology and diet

Adults and larvae suck the contents from the pollen grains of M. macdonnellii; populations have been estimated at 50,000 adults in a single cone.

reproductive biology

Variation in size of males and the powerful setae near the tip of their abdomen suggests some sort of male competitive behavior in mating, but this has not been observed. Larvae develop only in male cones, and subsequently fall to the ground to pupate around the base of the cycad.

conservation status

Not listed by the IUCN. The only known pollinator of this species of cycad, a plant that grows only in a restricted area of central Australia. In their mutual adaptation, the future existence of both is threatened by human activity.

significance to humans

None known.

Resources

Books

Crespi, B. J., and L. A. Mound. "Ecology and Evolution of Social Behaviour Among Australian Gall Thrips and Their Allies." In Evolution of Social Behaviour in Insects and Arachnids, edited by J. Choe and B. J. Crespi. Cambridge, UK: Cambridge University Press, 1997.

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

Mound, L. A. "Thysanoptera." In Zoological Catalogue of Australia. Vol. 26, Psocoptera, Phthiraptera, Thysanoptera, edited by A. Wells. Melbourne: CSIRO Publishing, 1996.

Periodicals

Izzo, T. J., S. M. J. Pinent, and L. A. Mound. "Aulacothrips dictyotus (Heterothripidae), the First Ectoparasitic Thrips (Thysanoptera)." Florida Entomologist 85 (2002): 281–283.

Mound, L. A., and R. Marullo. "The Thrips of Central and South America: An Introduction." Memoirs on Entomology, International 6 (1996): 1–488.

Mound, L. A., and I. Terry. "Pollination of the Central Australian Cycad, Macrozamia macdonnellii, by a New Species of Basal Clade Thrips (Thysanoptera)." International Journal of Plant Sciences 162 (2001): 147–154.

Other

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

Laurence A. Mound, DSc