Thrips - Thysanoptera

Thrips (Order Thysanoptera) are tiny, slender insects with fringed wings (thus the scientific name, from the Greek thysanos (fringe) + pteron (wing)). Other common names for thrips include thunderflies, thunderbugs, storm flies, thunderblights, storm bugs, corn flies and corn lice. Thrips species feed on a large variety of plants and animals by puncturing them and sucking up the contents. A large number of thrips species are considered pests, because they feed on plants with commercial value. Some species of thrips feed on other insects or mites and are considered beneficial, while some feed on fungal spores or pollen. So far around 5,000 species have been described. Thrips are generally tiny (1 mm long or less) and are not good flyers, although they can be carried long distances by the wind. In the right conditions, many species can exponentially increase in population size and form large swarms, making them an irritation to humans.

Like the words sheep, deer or moose, the word thrips is used for both the singular and plural forms, so there may be many thrips or a single thrips. The word thrips is from the Greek, meaning "wood louse".

Morphology and classification

Characteristics

They are small hemimetabolic insects with a distinctive cigar-shaped bauplan. They are elongated with transversely constricted bodies. They range in size from 0.5 to 14 millimetres (0.020 to 0.551 in) 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 ciliated fringe - from which the order derives its name. 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.

Thrips have asymmetrical mouthparts that are also unique to the group. Unlike the Hemiptera, the right mandible of thrips is reduced and vestigial – and in some species completely absent. The left mandible is larger, and forms a narrow stylet used to pierce the cell wall of tissues. Some species may then inject digestive enzymes as the maxillary stylets and hypopharynx are inserted into the opening to drain cellular fluids. This process leaves a distinctive silvery or bronze scarring on the surfaces of the stems or leaves where the thrips feed.

Thysanoptera is divided into two suborders: Terebrantia, and Tubulifera; these can be distinguished by morphological, behavioral, and developmental characteristics. Members of Tubulifera can be identified by their characteristic tube-shaped apical abdominal segment, egg-laying atop the surface of leaves, and three "pupal" stages. Females of the eight families of the Terebrantia all possess the eponymous saw-like ovipositor on the anteapical abdominal segment, lay eggs singly within plant tissue, and have two "pupal" stages.

Evolution and systematics

The Thysanoptera were first described in 1744 as a genus Physapus by De Geer, and then renamed Thrips by Linnaeus in 1758. In 1836 Haliday promoted the genus to the taxonomic rank of order, renaming them Thysanoptera. There are currently over six thousand species of thrips recognized, grouped into 776 extant genera and fifty-eight fossil genera.

The earliest fossils of thrips date back to Permian (Permothrips longipennis Martynov, 1935). By the Early Cretaceous, true thrips became much more abundant. The extant family Merothripidae most resemble these ancestral Thysanoptera, and are probably basal to the order.

Many thrips are pests of commercial crops due to the damage caused by feeding on developing flowers or vegetables, causing discoloration, deformities, and reduced marketability of the crop. Thrips may also serve as vectors for plant diseases, such as Tospoviruses. Over 20 plant-infecting viruses are known to be transmitted by thrips. These enveloped viruses are considered among some of the most damaging of emerging plant pathogens around the world. Virus members include the tomato spotted wilt virus and the impatiens necrotic spot viruses. The western flower thrips, Frankliniella occidentalis, now has a worldwide distribution and is considered the primary vector of plant diseases caused by Tospoviruses.

This global explosion in thrips species' range is not uncommon, as their small sizes and predisposition towards enclosed places makes them difficult to detect by phytosanitary inspection. When coupled with the increasing globalization of trade and the growth of greenhouse agriculture, thrips, unsurprisingly, are among the fastest growing group of invasive species in the world. Examples include Scirtothrips dorsalis and Thrips palmi.

Flower-feeding thrips are routinely attracted to bright floral colors (including white, blue, and especially yellow), and will land and attempt to feed. It is not uncommon for some species (e.g., Frankliniella tritici and Limothrips cerealium) to "bite" humans under such circumstances. Although no species feed on blood and no known animal disease is transmitted by thrips, some skin irritation has been described.

Management

Due to their small sizes and high rates of reproduction, thrips are difficult to control using classical biological control. All predators must be small and slender enough to penetrate the crevices where thrips hide while feeding, and then prey extensively on eggs and larvae. Only two families of parasitoid Hymenoptera are known to parasitize eggs and larvae, the Eulophidae and the Trichogrammatidae. Other biocontrol agents of adults and larvae include aphid wasps, anthocorid bugs of genus Orius, and phytoseiid mites. For this reason, many growers are occasionally forced to make limited use of pesticides to control thrips populations in the field and in greenhouses. Another effective strategy for pest thrips are biological insecticides, including Beauveria bassiana or Verticillium lecanii. These demonstrate a clear effect on eggs, larvae and adults of thrips.