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Thrips
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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, and corn lice. Thrips species feed on a large variety of sources both plant and animal by puncturing them and sucking up the contents.
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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, and corn lice. Thrips species feed on a large variety of sources both plant and animal 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 explode in population and swarm everywhere, making them an irritation to humans.
Like the words sheep or moose, the word thrips is used for both the singular and plural forms. So while there may be many thrips there can also be a solitary thrips. The word thrips is from Greek, meaning wood louse.
Morphology and Classification
Characteristics
They are small hemimetabolic insects with a distinctive cigar-shaped bauplan: elongate with transversely constricted bodies. They range in size from half a millimeter to fourteen millimeters in length for the larger predatory thrips, but most thrips are approximately a millimeter in length. Flight-capable groups will bear two similar pairs of wings with the ciliated fringe from which the order derives its name. Their legs usually end in two tarsal segments with an 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.
The asymmetrical thrips mouthparts are also unique to the group. Unlike the Hemiptera, the right mandible is vestigial, and in some species completely absent. The left mandible may be relatively larger, and is 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 surface of the stems or leaves where the thrips feed .
The Thysanoptera are divided into two suborders: the Terebrantia, and the Tubulifera. These two suborders can be distinguished by morphological, behavioral, and developmental characteristics. The 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 Greer, and then renamed Thrips by Linnaeus in 1758. In 1836 Haliday promoted the genus to the taxonomic rank of order, renaming them Thysanoptera.
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.
The following families are currently (2006) recognized:
- Adiheterothripidae Shumsher, 1946 (11 genera)
- Aeolothripidae Uzel, 1895 (29 genera) - banded thrips and broad-winged thrips
- Fauriellidae Priesner, 1949 (4 genera)
- †Hemithripidae Bagnall, 1923 (1 fossil genus, Hemithrips with 15 species)
- Heterothripidae Bagnall, 1912 (7 genera)
- † Jezzinothripidae zur Strassen, 1973 (included by some authors in Merothripidae)
- †Karataothripidae Sharov, 1972 (1 fossil species, Karataothrips jurassicus)
- Melanthripidae Bagnall, 1913 (6 genera)
- Merothripidae Hood, 1914 (5 genera) - large-legged thrips
- † Scudderothripidae zur Strassen, 1973 (included by some authors in Stenurothripidae)
- Thripidae Stevens, 1829 (292 genera in four subfamilies) - common thrips
- † Triassothripidae Grimaldi & Shmakov, 2004 (2 fossil genera)
- Uzelothripidae Hood, 1952 (1 species, Uzelothrips scabrosus)
Ecology
Natural History
Thrips are believed to 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 small cryptophilic 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 may be responsible for pollination while feeding, but their most obvious contribution to their ecosystem 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 .
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.
To survive the winter temperatures most thrips species over-winter as either adults or as pupae under ground litter. A typical flower thrips generation time will be from 7 to 22 days depending on the temperature. The eggs are about 0.2 mm long and reniform (kidney shaped), and may take on average 3 days to hatch. Thrips have 2 larval stages then go through a prepupal and a pupae stage, with the adults taking 1 to 4 days to reach sexual maturity. In the two suborders, the females of the suborder Terebrantia are equipped with an ovipositor which they use to cut slits into plant tissue into which they insert their eggs, one per slit, while females of the suborder Tubulifera lack an ovipositor and lay their eggs singly or in small groups on the outside surface of plants.
All described genera of thrips 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 defining 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.
Human Impact
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 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, has a worldwide distribution and is considered the primary vector of plant diseases caused by Tospoviruses.
Flower feeding thrips are routinely attracted to bright floral colors (esp. white, blue, or yellow), and will land and attempt to feed. It is common for some species (e.g., Frankliniella tritici and Limothrips cerealium) to "bite" humans under such circumstances, though no species feed on blood; such biting does not result in any known disease transmission but skin irritations are known to occur.
Management
Due to their small size, cryptophilic behavior, and high rate of reproduction, thrips are difficult to control using classical biological control. All predators must be small and slender enough to penetrate the crevices that thrips hide in 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.
External links
on the UF / IFAS Featured Creatures Web site
- (Phlaeothripidae)
- (Thripidae)
- (Thripidae)
- (Thripidae)
- (Thripidae)
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