thrips

Thrips Fossil range: 299–0 Ma PreЄ Є O S D C P T J K Pg N Permian - Recent
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Subclass:	Pterygota
Superorder:	Exopterygota
Order:	Thysanoptera Haliday, 1836
Families
Terebrantia Adiheterothripidae Aeolothripidae Fauriellidae † Hemithripidae Heterothripidae † Jezzinothripidae † Karataothripidae Melanthripidae Merothripidae † Scudderothripidae Stenurothripidae Thripidae † Triassothripidae Uzelothripidae Tubulifera Phlaeothripidae

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 the Greek, meaning wood louse.^[1]

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 0.5 to 14 millimetres (0.020 to 0.55 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 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.^[2][3]

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 ^[4]. Some species may then inject digestive enzymes as the maxillary stylets and hypopharynx are inserted into the opening to drain cellular fluids^[5][6]. This process leaves a distinctive silvery or bronze scarring on the surface of the stems or leaves where the thrips feed ^[7].

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^[8]. The extant family Merothripidae most resemble these ancestral Thysanoptera, and are probably basal to the order.^[9]

The following families are currently (2006) recognized:

• Suborder Terebrantia

• 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)

• Suborder Tubulifera

• Phlaeothripidae (447 genera in two subfamilies)

Ecology

Scanning electron micrograph of a thrips (Thysanoptera), showing fine structure, the compound eyes, wing construction, and setae.

Natural History

Thrips are believed to have descended from a mycetophilic ancestor during the Mesozoic^[8], 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 ^[7][10]. 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^[11][12].

Flower-feeding thrips may be responsible for pollination while feeding^[13][14], 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 ^[10][15]. Family Thripidae is particularly notorious for members with broad host ranges, and the majority of pest thrips come from this family ^[16][17].

While poorly documented, chemical communication is believed to be important to the group^[18]. Anal secretions are produced in the hindgut^[19], and released along the posterior setae as predator deterrents^[11][19][20]. Some Phlaeothripidae form eusocial groups similar to ant-colonies, with reproductive queens and non-reproductive soldier castes^[21]. 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 highly dependent upon environmental conditions, including the temperature and nutrient quality of their food source. Thrips begin their lives as eggs. These are extremely small (about 0.2 mm long) and kidney-shaped. 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.

As hemimetabolous insects, the Thysanoptera do not actually undergo complete metamorphosis but pass through a similar stage in which 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 construct 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 will grow and mature into their adult forms.

All described genera of thrips are haplodiploid organisms capable of parthenogenesis, with some favoring arrhenotoky and others displaying thelytoky^[11][12], although it remains possible that the sex-determining bacterial endosymbiont Wolbachia^[22] may also play a role in defining sex-ratios for some populations of thrips ^[15]. Several normally bisexual species have become established in the United States with only members of a single sex present^[11][12][23].

When mating occurs, it 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.

Human Impact

Ponticulothrips diospyrosi on finger for scale.

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^[24]. 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.^[25]

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.

Thysanoptera diagram

References Cited

External links

Wikispecies has information related to: Thysanoptera

• Thrips of the World checklist
• Thrips images from the "Pests and Diseases Image Library (PaDIL)" of Australia
• University of California Pest Management Guidelines for Thrips
• University of California Thrips Identification
• CISR: Center for Invasive Species Research Fact Sheet on Avocado Thrips
• CISR: Center for Invasive Species Research Fact Sheet on Western Flower Thrips
• CISR: Center for Invasive Species Research Fact Sheet on Myoporum Thrips

on the UF / IFAS Featured Creatures Web site

• Gynaikothrips ficorum, Cuban laurel thrips (Phlaeothripidae)
• Heliothrips haemorrhoidalis , greenhouse thrips (Thripidae)
• Selenothrips rubrocinctus, redbanded thrips (Thripidae)
• Thrips palmi, melon thrips (Thripidae)
• Thrips simplex, gladiolus thrips (Thripidae)