Thrips

Thrips (order Thysanoptera) are minute (mostly long or less), slender insects with fringed wings and unique asymmetrical mouthparts. Different thrips species feed mostly on plants by puncturing and sucking up the contents, although a few are predators. Entomologists have described approximately 6,000 species. They fly only weakly and their feathery wings are unsuitable for conventional flight; instead, thrips exploit an unusual mechanism, clap and fling, to create lift using an unsteady circulation pattern with transient vortices near the wings.

Many thrips species are pests of commercially important crops. A few species serve as vectors for over 20 viruses that cause plant disease, especially the Tospoviruses. Some species of thrips are beneficial as pollinators or as predators of other insects or mites. In the right conditions, such as in greenhouses, many species can exponentially increase in population size and form large swarms because of a lack of natural predators coupled with their ability to reproduce asexually, making them destructive to crops. In addition to damaging plants, thrips may invade houses and infest household objects such as furniture, bedding and computer monitors – in the latter case by forcing their way in between the LCD and its glass covering.
Their identification to species by standard morphological characteristics is often challenging.

Etymology

The first recorded mention of thrips is from the 17th century and a sketch was made by Philippo Bonanni, a Catholic priest, in 1691. Swedish entomologist Baron Charles De Geer described two species in the genus ''Physapus'' in 1744 and Linnaeus in 1746 added a third species and named this group of insects ''Thrips''. In 1836 the Irish entomologist Alexander Henry Haliday described 41 species in 11 genera and proposed the order name of Thysanoptera. The first monograph on the group was published in 1895 by Heinrich Uzel who is considered the father of Thysanoptera studies.

The generic and English name ''thrips'' is a direct transliteration of the ancient Greek , ''thrips'', meaning "woodworm". Like some other animal names such as ''sheep'', ''deer'', and ''moose'', in English the word thrips is both the singular and plural forms, so there may be many thrips or a single thrips. Other common names for thrips include thunderflies, thunderbugs, storm flies, thunderblights, storm bugs, corn fleas, corn flies, corn lice, freckle bugs, harvest bugs, and physopods. The older group name "physopoda" is with reference to the bladder like tips to the tarsi of the legs. The name of the order Thysanoptera is constructed from the ancient Greek words , ''thysanos'', "tassel or fringe", and , ''pteron'', "wing", for the insects' fringed wings.

Morphology

Thrips are small hemimetabolic insects with a distinctive cigar-shaped body plan. They are elongated with transversely constricted bodies. They range in size from in length for the larger predatory thrips, but most thrips are about 1 mm in length. Flight-capable thrips have two similar, strap-like pairs of wings with a fringe of bristles. The wings are folded back over the body at rest. Their legs usually end in two tarsal segments with a bladder-like structure known as an "arolium" at the pretarsus. This structure can be everted by means of hemolymph pressure, enabling the insect to walk on vertical surfaces. They have compound eyes consisting of a small number of ommatidia and three ocelli or simple eyes on the head.

Thrips have asymmetrical mouthparts unique to the group. Unlike the Hemiptera (true bugs), the right mandible of thrips is reduced and vestigial – and in some species completely absent.
The left mandible is used briefly to cut into the food plant; saliva is injected and the maxillary stylets, which form a tube, are then inserted and the semi-digested food pumped from ruptured cells. This process leaves cells destroyed or collapsed, and a distinctive silvery or bronze scarring on the surfaces of the stems or leaves where the thrips have fed.

Thysanoptera is divided into two suborders, Terebrantia and Tubulifera; these can be distinguished by morphological, behavioral, and developmental characteristics. Tubulifera consists of a single family, Phlaeothripidae; members can be identified by their characteristic tube-shaped apical abdominal segment, egg-laying atop the surface of leaves, and three "pupal" stages. In the Phlaeothripidae, the males are often larger than females and a range of sizes may be found within a population. The largest recorded phlaeothripid species is about 14 mm long. Females of the eight families of the Terebrantia all possess the eponymous saw-like (see terebra) ovipositor on the anteapical abdominal segment, lay eggs singly within plant tissue, and have two "pupal" stages. In most Terebrantia, the males are smaller than females. The family Uzelothripidae has a single species and it is unique in having a whip-like terminal antennal segment.

Evolution

The earliest fossils of thrips date back to the Permian ('' Permothrips longipennis''). By the Early Cretaceous, true thrips became much more abundant. The extant family Merothripidae most resembles these ancestral Thysanoptera, and is probably basal to the order. There are currently over six thousand species of thrips recognized, grouped into 777 extant and sixty fossil genera.

Phylogeny

Thrips are generally considered to be the sister group to Hemiptera (bugs).

The phylogeny of thrips families has been little studied. A preliminary analysis in 2013 of 37 species using 3 genes, as well as a phylogeny based on ribosomal DNA and three proteins in 2012, supports the monophyly of the two suborders, Tubulifera and Terebrantia. In Terebrantia, Melanothripidae may be sister to all other families, but other relationships remain unclear. In Tubulifera, the Phlaeothripidae and its subfamily Idolothripinae are monophyletic. The two largest thrips subfamilies, Phlaeothripinae and Thripinae, are paraphyletic and need further work to determine their structure. The internal relationships from these analyses are shown in the cladogram.

Taxonomy

The following families are currently (2013) recognized:

* Suborder Terebrantia
:* Adiheterothripidae (11 genera)
:* Aeolothripidae (29 genera) – banded thrips and broad-winged thrips
:* Fauriellidae (four genera)
:* † Hemithripidae (one fossil genus, '' Hemithrips'' with 15 species)
:* Heterothripidae (seven genera, restricted to the New World)
:* † Jezzinothripidae (included by some authors in Merothripidae)
:* † Karataothripidae (one fossil species, '' Karataothrips jurassicus'')
:* Melanthripidae (six genera of flower feeders)
:* Merothripidae (five genera, mostly Neotropical and feeding on dry-wood fungi) – large-legged thrips
:* † Scudderothripidae (included by some authors in Stenurothripidae)
:* Thripidae (292 genera in four subfamilies, flower living) – common thrips
:* † Triassothripidae (two fossil genera)
:* Uzelothripidae (one species, '' Uzelothrips scabrosus'')
* Suborder Tubulifera
:* Phlaeothripidae (447 genera in two subfamilies, fungal hyphae and spore feeders)

The identification of thrips to species is challenging as types are maintained as slide preparations of varying quality over time. There is also considerable variability leading to many species being misidentified. Molecular sequence based approaches have increasingly been applied to their identification.

Biology

Feeding

Thrips are believed to have descended from a fungus-feeding ancestor during the Mesozoic, and many groups still feed upon and inadvertently redistribute fungal spores. These live among leaf litter or on dead wood and are important members of the ecosystem, their diet often being supplemented with pollen. Other species are primitively eusocial and form plant galls and still others are predatory on mites and other thrips. Two species of ''Aulacothrips'', ''A. tenuis'' and ''A. levinotus'', have been found to be ectoparasites on aetalionid and membracid plant-hoppers in Brazil.

''Mirothrips arbiter'' has been found in paper wasp nests in Brazil. The eggs of the hosts including ''Mischocyttarus atramentarius'', ''Mischocyttarus cassununga'' and ''Polistes versicolor'' are eaten by the thrips. Thrips, especially in the family Aeolothripidae, are also predators, and are considered beneficial in the management of pests like the codling moths.

Most research has focused on thrips species that feed on economically significant crops. Some species are predatory, but most of them feed on pollen and the chloroplasts harvested from the outer layer of plant epidermal and mesophyll cells. They prefer tender parts of the plant, such as buds, flowers and new leaves. Besides feeding on plant tissues, the common blossom thrips feeds on pollen grains and on the eggs of mites. When the larva supplements its diet in this way, its development time and mortality is reduced, and adult females that consume mite eggs increase their fecundity and longevity.

Pollination

Some flower-feeding thrips pollinate the flowers they are feeding on, and some authors suspect that they may have been among the first insects to evolve a pollinating relationship with their host plants. '' Scirtothrips dorsalis'' carries pollen of commercially important chili peppers. Darwin found that thrips could not be kept out by any netting when he conducted experiments by keeping away larger pollinators.
''Thrips setipennis'' is the sole pollinator of '' Wilkiea huegeliana'', a small, unisexual annually flowering tree or shrub in the rainforests of eastern Australia. ''T. setipennis'' serves as an obligate pollinator for other Australian rainforest plant species, including '' Myrsine howittiana'' and '' M. variabilis.'' The genus '' Cycadothrips'' is a specialist pollinator of cycads, the cones of which are adapted for pollination by small insects. Thrips are likewise the primary pollinators of heathers in the family '' Ericaceae'', and play a significant role in the pollination of pointleaf manzanita. Electron microscopy has shown thrips carrying pollen grains adhering to their backs, and their fringed wings are perfectly capable of allowing them to fly from plant to plant.

Damage to plants

Thrips can cause damage during feeding. This impact may fall across a broad selection of prey items, as there is considerable breadth in host affinity across the order, and even within a species, varying degrees of fidelity to a host. Family Thripidae in particular is notorious for members with broad host ranges, and the majority of pest thrips come from this family. For example, '' Thrips tabaci'' damages crops of onions, potatoes, tobacco, and cotton.

Some species of thrips create galls, almost always in leaf tissue. These may occur as curls, rolls or folds, or as alterations to the expansion of tissues causing distortion to leaf blades. More complex examples cause rosettes, pouches and horns. Most of these species occur in the tropics and sub-tropics, and the structures of the galls are diagnostic of the species involved. A radiation of thrips species seems to have taken place on ''Acacia'' trees in Australia; some of these species cause galls in the petioles, sometimes fixing two leaf stalks together, while other species live in every available crevice in the bark. In '' Casuarina'' in the same country, some species have invaded stems, creating long-lasting woody galls.

Social behaviour

While poorly documented, chemical communication is believed to be important to the group. Anal secretions are produced in the hindgut, and released along the posterior setae as predator deterrents In Australia, aggregations of male common blossom thrips have been observed on the petals of ''Hibiscus rosa-sinensis'' and '' Gossypium hirsutum''; females were attracted to these groups so it seems likely that the males were producing pheromones.

In the phlaeothripids that feed on fungi, males compete to protect and mate with females, and then defend the egg-mass. Males fight by flicking their rivals away with their abdomen, and may kill with their foretarsal teeth. Small males may sneak in to mate while the larger males are busy fighting. In the Merothripidae and in the Aeolothripidae, males are again polymorphic with large and small forms, and probably also compete for mates, so the strategy may well be ancestral among the Thysanoptera.

Many thrips form galls on plants when feeding or laying their eggs. Some of the gall-forming Phlaeothripidae, such as genera '' Kladothrips'' and '' Oncothrips'', form eusocial groups similar to ant colonies, with reproductive queens and nonreproductive soldier castes.

Flight

Most insects create lift by the stiff-winged mechanism of insect flight with steady state aerodynamics; this creates a leading edge vortex continuously as the wing moves. The feathery wings of thrips, however, generate lift by clap and fling, a mechanism discovered by the Danish zoologist Torkel Weis-Fogh in 1973. In the clap part of the cycle, the wings approach each other over the insect's back, creating a circulation of air which sets up vortices and generates useful forces on the wings. The leading edges of the wings touch, and the wings rotate around their leading edges, bringing them together in the "clap". The wings close, expelling air from between them, giving more useful thrust. The wings rotate around their trailing edges to begin the "fling", creating useful forces. The leading edges move apart, making air rush in between them and setting up new vortices, generating more force on the wings. The trailing edge vortices, however, cancel each other out with opposing flows. Weis-Fogh suggested that this cancellation might help the circulation of air to grow more rapidly, by shutting down the Wagner effect which would otherwise counteract the growth of the circulation.

Clap and fling