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THYSANOPTERA -- Thrips Photos-1, Photos-2

Overview

A number species of thrips demonstrate predaceous behavior, and in previous years the entire order was considered to be essentially predaceous rather than phytophagous. However, although phytophagy is the predominant habit , some species partake of both types of food and others subsist only on insect food. Scolothrips sexmaculatus Perg. is recorded from many parts of North America as a predator of red mites. The eggs are preferred, although nymphs and adults also are attacked. The species has been observed to attack the citrus thrips extensively, and this tendency to prey on members of its own order was noted in several other instances (Clausen 1940/1962). C. V. Riley (cited by Clausen, 1940) recorded an undetermined species as being very effective in destroying the eggs of the plum curculio.

There is evidence that members of the order may be of some importance as predators of eggs of Lepidoptera, particularly those of small size, which have a membranous chorion. Aleurodothrips fasciapennis Frankl. is an important natural enemy of the citrus white fly, Dialeurodes citri Ashm., in Florida. Both nymphs and adults feed extensively on Aspidiotus destructor Sign. in various parts of the world (Taylor 1930). The eggs are laid in empty puparia or underneath scale covers and at times in scales from which parasitoids emerged (Clausen 1940/1962).

Description

These are tiny, slender insects with fringes on their wings, and therefore the scientific name, from the Greek thysanos (fringe) + pteron (wing)). Other common names for thrips include storm flies, thunderflies, thunderbugs, thunderblights, and corn lice. Thrips species feed on a large variety of sources, both plant and animal, by puncturing them and sucking up the contents. Many species are pests, because they feed on plants of commercial value. Some species of thrips feed on other insects or mites and are thus beneficial, while some feed on fungal spores or pollen. By 2011 about 5,006 species have been described. They are usually tiny (1 mm long or less) and are poor flyers, although they can be carried long distances by wind. Many species can increase in population size to form large swarms, making them a pest of humans.

Thrips are small hemimetabolic insects with a distinctive cigar-shaped body: elongate with transversely constricted bodies. They range in size from 0.5 to 14 millimetres (0.022 to 0.56 in) in length for the larger predatory thrips, but most thrips are approximately a millimeter 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 have two tarsal segments with aa bladder-like structure known as an arolium at the pretarsus. This structure can be everted, which enables them 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 or 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 silvery or scarring on the surface of the stems or leaves where the feeding occurs.

Two suborders are the Terebrantia, and the Tubulifera. These two suborders can be separated by morphological, behavioral, and developmental characteristics. The Tubulifera have a 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 hve the eponymous saw-like ovipositor on the anteapical abdominal segment, and they lay eggs singly within plant tissue, and have two "pupal" stages.

Thysanoptera was first described in 1744 as a genus Physapus by De Greer, and then renamed Thrips by Linnaeus in 1758. In 1836 Haliday advanced the genus to an order, renaming them Thysanoptera.

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

Thrips may have descended from a mycetophilic ancestor during the Mesozoic and many groups still feed upon and inadvertently redistribute fungal spores, but most research has focused on those species feeding on or in association with economically significant crops. Some thrips are predatory, but the majority are phytophagous insects feeding on pollen and the chloroplasts harvested from the outer layer of plant epidermal and mesophyll cells. These species are minute organisms that prefer to feed within the tightly packed apical buds of new growth. Feeding usually occurs along the main vein or ribs of leaves and petals.

Flower-feeding thrips can pollinate while feeding, but their most obvious behavior remains the damage they can cause 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 there remains varying degrees of fidelity to a described host. Family Thripidae is particularly notorious for members with broad host ranges, and the majority of pest thrips come from this family.

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. Some Phlaeothripidae form eusocial groups similar to ant-colonies, with reproductive queens and non-reproductive soldier castes. Many thrips form galls on plants when feeding or laying their eggs.

Life Cycle

The rate at which thrips move through their developmental cycle is very dependent upon environmental conditions, including the temperature and nutrient quality of their food source. Thrips begin their lives as extremely small eggs. It may take from as little as a day to several weeks before hatching. The females of the suborder Terebrantia are equipped with an ovipositor, which they use to cut slits in plant tissue and then insert their eggs, one per slit. Females of the suborder Tubulifera lack an ovipositor and lay their eggs singly or in small groups on the outside surface of plants. Thrips then pass through two wingless instars of nymph.

These hemimetabolous insects do not undergo complete metamorphosis but pass through a similar stage where they do not feed and are mostly immobile. Both suborders of thrips will first enter a short prepupal stage lasting a day at most, during which they will seek out dark crevices on plant, hiding in the tightly packed bud of flowers or bark - or drop off of the plant entirely, burrowing into leaf litter or loose soil. Some thrips will then make a pupal cell or cocoon. In Terebrantian thrips, a single pupal instar follows, whereas in the Tubulifera, two pupal stages will follow. During these stages, wing-buds and reproductive structures grow and mature into their adult forms.

All genera are haplodiploid organisms capable of parthenogenesis, with some favoring arrhenotoky and others displaying thelytoky, although it remains possible that the sex-determining bacterial endosymbiont Wolbachia may also play a role in the sex-ratios for some populations of thrips. Several normally bisexual species have become established in the United States with only members of a single sex present.

Mating may last from minutes to hours. Most female thrips have a preoviposition period lasting from a day to a week during which their eggs mature, and before which they cannot mate.

Many thrips are pests of commercial crops due to the damage caused by feeding on developing flowers or vegetables which causes discoloration, deformities, and reduced marketability of the crop. Thrips may also serve as vectors for plant diseases, such as Tospoviruses. Over 25 plant 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. Viruses 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.

A global dispersion in thrips species' range is not uncommon, as their small size 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, it is no surprise that thrips are among the fastest growing group of invasive species in the world. Examples include Scirtothrips dorsalis and Thrips palmi.

Flower feeding thrips are attracted to bright floral colors .esp. white, blue, or yellow., and will attempt to feed. It is ususal for some species, e.g., Frankliniella tritici and Limothrips cerealium, to inflict bites on humans under such conditions, although no species feed are blood feeders.

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References: Please refer to <biology.ref.htm>, [Additional references may be found at: MELVYL Library]

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