Thrips

Thrips belong to the Phylum Arthropoda, Class Insecta, and Order Thysanoptera. They are minute (most are less than 1 mm long), slender insects with fringed wings and distinctive asymmetrical mouthparts. There are currently over 7,700 species-groups of recognized thrips, grouped into 780 extant and fifty-eight fossil genera. The traditional classification of the order recognizes nine families for extant species (plus five fossil families), with two subfamilies in the Phlaeothripidae (the only family in suborder Tubulifera) and four subfamilies in the Thripidae (one of eight families comprising suborder Terebrantia).

The earliest recorded mention of thrips is from the seventeenth century and was a sketch made in 1691 by an Italian Jesuit scholar, Philippo Bonanni (1628‒1723). In 1744, the Swedish entomologist Baron Charles De Geer (1720‒1788) described two species of thrips in the genus Physapus, and in 1746, the father of classification, Carl von Linné (a.k.a. Carolus Linnaeus, 1707‒1778), added a third species and named this group of insects as thrips. In 1836, the Irish entomologist Alexander Henry Haliday (1806‒1870) described forty-one species within eleven genera and proposed the ordinal name of Thysanoptera. The first monograph on the group was published in 1895 by the father of Thysanoptera studies, the Czechoslovakian entomologist Jindřich (Heinrich) Uzel (1868‒1946). Much of the more recent scientific information on thrips has been conducted by Laurence A. Mound of Canberra, Australia, recognized as a leading authority on the taxonomy, phylogeny, and biology of the thrips.

The fossil record shows that the earliest fossils of thrips date back to the Permian Period (298.9 to 252.2 million years ago). By the Early Cretaceous Period (146 to 100 million years ago), true thrips became very abundant. The extant family Merothripidae is probably basal to the order and most resembles these ancestral Thysanoptera.

Thrips are generally considered to be the sister group to the true bugs (Hemiptera). Recent preliminary analyses of thirty-seven species using three genes, as well as a phylogenies based on ribosomal DNA and three proteins, supported the monophyly of the two suborders, Tubulifera (one family) and Terebrantia (thirteen extant and extinct families). In Terebrantia, the family Melanothripidae (with sixty species) is thought to be sister to all other families, but other relationships remain unclear. In Tubulifera, the Phlaeothripidae (with 600 species) and its subfamily Idolothripinae are monophyletic. The two largest thrips subfamilies, Phlaeothripinae and Thripinae, are paraphyletic and need additional research to determine their phylogenetic structure.

The Tubulifera consists of a single family, Phlaeothripidae. Its members can be identified by: (1) their characteristic tube-shaped apical abdominal segment; (2) egg-laying atop the surface of leaves; and (3) three “pupal” stages. In this family, the males are often bigger than females, and a range of sizes may be found within certain populations. The largest recorded phlaeothripid species is about 14 mm (0.55 in.) long.

All females of the eight extant families of the Terebrantia: (1) possess the eponymous saw-like ovipositor on the anteapical abdominal segment, (2) lay eggs singly within plant tissue, and (3) have two “pupal” stages. In most Terebrantia, the males are smaller than females. The family Uzelothripidae, with a unique whip-like terminal antennal segment, was originally thought to contain a single species, Uzelopthrips scabrosus, known from Brazil, Australia, Singapore, and Angola. However, in 2012, a new extinct species, U. eocenicus, was described from the lowermost Eocene of France.

Thrips are small hemimetabolous insects with a unique elongated cigar-shaped, transversely constricted body. They range in length, for the largest predatory thrips, from 0.5 to 14 mm (0.02 to 0.55 in.), but most are only about 1 mm (0.04 in.) long. Thrips possess asymmetrical mouthparts, a unique characteristic of the group. Unlike the Hemiptera (true bugs), the right mandible of thrips is reduced and vestigial, and in some species it is completely absent. Those thrips that are capable of flight have two similar, strap-like pairs of wings with a fringe of bristles that, when at rest, are folded back over the body. Their flight is considered weak compared to other insects, and their feathery wings are unsuitable for conventional insect flight. However, thrips exploit an unusual mechanism of clap and fling to help create lift using an unsteady circulation pattern with fleeting vortices near their wings. The legs usually end in two tarsal segments at the pretarsus with a bladder-like structure known as an arolium. By way of hemolymph pressure, the arolium can be everted, permitting the thrip to walk on vertical surfaces. They also have compound eyes consisting of three ocelli (simple eyes) on the head and a small number of ommatidia.

Depending on the species, thirty percent of thrips feed mostly on green leafy tissues of plants by puncturing and drawing the contents. They prefer tender parts of plants, such as buds, flowers, and new leaves. Their left mandible is used for a short time to cut into the 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 digestive 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. A few others are predators, and many are pests of commercially important crops. Also, about forty percent of thrip species feed only on fungus in leaf-litter or on dead twigs and inadvertently redistribute fungal spores. Some thrips live among leaf litter or on dead wood and are important members of the ecosystem, as their diet is often supplemented with pollen and the chloroplasts harvested from the outer layer of plant epidermal and mesophyll cells. As well as feeding on plant tissues, the common blossom thrips (Thripidae) feeds on pollen grains and on the eggs of mites. When larval thrips supplement their diet in this approach, its developmental time and mortality is actually reduced, and adult females that consume mite eggs increase their fecundity and life expectancy.

Two species of thrips in Brazil, Aulacothrips tenuis and A. levinotus, are ectoparasites on aetalionid and membracid plant-hoppers (Hemiptera). Another, Mirothrips arbiter, has been found within paper wasp (Vespidae) nests in Brazil. For example, the eggs of vespid hosts including Mischocyttarus atramentarius, M. cassununga, and Polistes versicolor are eaten by the thrips. In addition, thrips are predators for various stages of the life of codling moths (Tortricidae).

Thrips are the most important insect pests of seedling cotton in the mid-southern United States. The tobacco thrips, Frankliniella fusca (Thripidae), are the dominant species found on cotton in the Mid-South, often composing more than ninety percent of all thrips collected on seedling cotton. When cotton fields are left untreated with pesticides, injury by thrips can lead to stunted growth, delayed maturity, reduced stands, and loss in yield. The most noticeable contribution that thrips make to their ecosystem is the damage they inflict during feeding. In particular, those in the family Thripidae are notorious for members with broad host ranges, and the majority of pest thrips come from this family. For example, Thrips tabaci damages crops of cotton, onions, potatoes, and tobacco.

Some species of thrips create galls (curls, horns, pouches, rolls, rosettes, or folds) in leaf tissue. These alter and expand tissues, causing alteration to leaf blades. Many of these thrips form the galls when feeding or during reproduction when laying their eggs. Most of these species occur in the tropics and sub-tropics, and the structures of the galls are diagnostic of the species involved. In Australia, radiation of thrip species seems to have taken place on Acacia trees; some of these species produce galls in the petioles, sometimes fixing two leaf stalks together, while other species live in every available cranny in the bark.

Only one percent of thrip species are significant pests and less than 0.2 percent transmit viruses to plants. However, a few species can serve as vectors for over twenty viruses that cause plant disease, especially the Orthotospoviruses or Tospoviruses. These enveloped viruses are considered among some of the most damaging of emerging plant pathogens around the world, with those vector species having an outsized impact on human agriculture. Virus members include the tomato spotted wilt virus and the impatiens necrotic spot viruses. The former has caused losses in tomatoes in southern states, including Alabama, Arkansas, Florida, Georgia, Mississippi, and Tennessee. The latter is a serious disease of greenhouse flower crops and certain vegetables and weeds. The western flower thrip, Frankliniella occidentalis, has spread it so much that it now has a cosmopolitan distribution and is the primary vector of plant diseases caused by tospoviruses. 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. In addition, thrips are known to enter houses and infest objects such as bedding, furniture, and even computer monitors.

Their small size and penchant toward enclosed places makes thrips difficult to detect by phytosanitary inspection, whereas their eggs, laid inside plant tissue, are well-protected from pesticide sprays. When this is coupled with the increasing globalization of trade and the growth of greenhouse agriculture, thrips are among the fastest growing group of invasive species in the world. Examples of invasive thrips include F. occidentalis, Thrips palmi, and T. simplex.

As irritants to humans, flower-feeding thrips are customarily attracted to bright floral colors (such as white, blue, and particularly yellow), and will land on them and attempt to feed. It is not unusual for some species, including Frankliniella tritici and Limothrips cerealium, to “bite” humans under such situations. Although no thrips have been reported to feed on blood, and no recognized animal disease is transmitted by thrips, some skin irritation has been described in humans. Thrips develop resistance to insecticides easily, and this makes thrips ideal as models for testing the effectiveness of new pesticides and methods. For example, an insecticidal soap spray has been shown to be effective against thrips. It is commercially available or can be made of certain types of household soap. In Japan, scientists have reported that significant reductions in larval and adult melon thrips (T. palmi) occurred when plants were illuminated with red light. Biocontrol agents of adults and larvae include anthocorid bugs (Hemiptera) of genus Orius, and phytoseiid mites. Biological insecticides such as the fungi Beauveria bassiana and Verticillium lecanii can kill thrips at all life-cycle stages.

In terms of pollination, some flower-feeding thrips pollinate the flowers they are feeding on, which has led some researchers to suspect that they may have been among the first insects to evolve a pollinating relationship with their host plants. Examples include chili or yellow tea thrips, Scirtothrips dorsalis, which carry pollen of commercially important chili peppers, and Thrips setipennis, the lone pollinator of Wilkiea huegeliana of the rainforests of eastern Australia and an obligate pollinator for other Australian rainforest plant species, including Myrsine howittiana and M. variabilis. Those in the genus Cycadothrips are specialist pollinators of cycads, the flowers of which are adapted for pollination by small insects. Thrips are likewise the primary pollinators of heathers in the family Ericaceae, found most commonly in acid and infertile growing conditions, and play a significant role in the pollination of pointleaf manzanita, Arctostaphylos pungens of Mexico and the southwestern United States.

Unfortunately, the social behavior of thrips has been inadequately documented, but chemical communication is believed to be important to the group. In the thrips’ hindgut, anal secretions are produced and released along the posterior setae as deterrents to predation. In Australia, aggregations of male common blossom thrips (Frankliniella schultzei) have been observed on the petals of Hibiscus rosa-sinensis and Gossypium hirsutum. Here, females are attracted to these groups, so it seems probable that males were producing pheromones. In the phlaeothripids that feed on fungi, males compete to protect and mate with females, followed by defending the egg-mass. Males fight by flicking their rivals away with their abdomen and may kill with their foretarsal teeth. Smaller males may slip in to mate while the larger males are busy fighting. In the families Merothripidae (20 species) and in the Aeolothripidae (200 species), males are again polymorphic with large and small forms, and probably also compete for mates, so this approach may well be ancestral among the Thysanoptera. Some of the gall-forming Phlaeothripidae, such as genera Kladothrips and Oncothrips, form eusocial groups similar to ant (Hymenoptera) colonies, with nonreproductive soldier castes and reproductive queens.

For reproduction, females, with their ovipositor, place slits in plant tissues, and insert their eggs, one per slit. Those in the suborder Tubulifera lay their eggs singly or in small groups on the outside surfaces of plants. Thrips are hemimetabolous (egg-nymph-adult stage), metamorphosing gradually to the adult form. The first two instars, called larvae or nymphs, are similar to small wingless adults without genitalia; these feed on plant tissue. In the Terebrantia, the third and fourth instars, and in the Tubulifera also a fifth instar, are non-feeding resting stages similar to pupae: in these stages, the body’s organs are reshaped, and wing-buds and genitalia are formed. The adult stage (winged and wingless forms) can be accomplished within about eight to fifteen days; adults can live for around forty-five days. Thrips can survive the winter as adults or through egg or pupal diapause.

Thrips are haplodiploid with haploid males (from unfertilized eggs, as in Hymenoptera) and diploid females proficient in parthenogenesis (reproducing without fertilization). Many species also use arrhenotoky (unfertilized eggs develop into males), while a few use thelytoky (females from unfertilized eggs). The sex-determining gram-negative bacterial endosymbiont Wolbachia is an influence that affects the reproductive mode. Several normally bisexual species have become established in the United States with only females present. Under the right conditions, such as in greenhouses, many species can exponentially increase in populations and form large swarms due to the lack of natural predators coupled with their ability to reproduce asexually, making them a possible irritation to humans.

The taxonomic identification of thrips to specific level using old-school standard morphological characters is often difficult. Also, type specimens have been maintained in museum collections as slide preparations that vary in quality and often will degrade over time depending on the mounting medium. There is also considerable variability in thrips, leading to many species being misidentified.

In Arkansas, little is known about the thrips that occur in the state, as no comprehensive survey has been done. However, most research involves thrips infesting plants in the state. A survey of thrips infesting cotton seedlings was conducted, as was another that involved thrips on cowpeas in western Arkansas. Another study of thrips on spring-planted snap beans was also conducted. One common thrip species reported from the state is Microcephalothrips abdominalis (Thripidae).

For additional information:
Ananthakrishnan, T. N. “Biosystematics of Thysanoptera.” Annual Review of Entomology 24 (1979): 159–183.

Buckman, Rebecca S., Laurence A. Mound, and Michael F. Whiting. “Phylogeny of Thrips (Insecta: Thysanoptera) Based on Five Molecular Loci.” Systematic Entomology 38 (2012): 123–133.

Burris, E., C. Allen, R. Bagwell, D. Cook, B. Freeman, G. Herzog, G. Lentz, R. Leonard, and J. Reed. Thrips (Thysanoptera: Thripidae), a Multi-State Survey: Summary of Observations for Arkansas, Alabama, Georgia, Louisiana, Mississippi, and Tennessee. Baton Rouge: Louisiana State University Agricultural Center Research Information Sheet 103, 2000.

Cavalleri, Adriano, André R. De Souza, Fábio Prezoto, and Laurence A. Mound. “Egg Predation Within the Nests of Social Wasps: A New Genus and Species of Phlaeothripidae, and Evolutionary Consequences of Thysanoptera Invasive Behaviour.” Biological Journal of the Linnean Society 109 (2013): 332–341.

Chisholm, I. F., and T. Lewis. “A New Look at Thrips (Thysanoptera) Mouthparts, Their Action and Effects of Feeding on Plant Tissue.” Bulletin of Entomological Research 74 (2009): 663–675.

Cook, Donald R., Charles T. Allen, Eugene Burris, Barry L. Freeman, Gary A. Herzog, Gary L. Lentz, B. Roger Leonard and Jack T. Reed. “A Survey of Thrips (Thysanoptera) Species Infesting Cotton Seedlings in Alabama, Arkansas, Georgia, Louisiana, Mississippi, and Tennessee.” Journal of Entomological Science 38 (2003): 669‒681.

Diffie, S., G. B. Edward, and Lawrence A. Mound. Thysanoptera of Southeastern U.S.A.: A Checklist for Florida and Georgia.” Zootaxa 1787 (2008): 45–62.

Fedor, Peter J., Martina Doricova, Pavol Prokop, and Laurence A. Mound. “Heinrich Uzel, the Father of Thysanoptera Studies.” Zootaxa 2645 (2010): 55–63.

Grimaldi, D., A. Shmakov, and N. Fraser. “Mesozoic Thrips and Early Evolution of the Order Thysanoptera (Insecta).” Journal of Paleontology 78 (2004): 941–952.

Heming, B. S. “Functional Morphology of the Thysanopteran Pretarsus.” Canadian Journal of Zoology 49 (1971): 91–108.

Milne, M., and G. H. Walter. “The Significance of Prey in the Diet of the Phytophagous Thrips, Frankliniella schultzei.” Ecological Entomology 22 (1997): 74–81.

Milne, M., G. H. Walter, and J. R. Milne. “Mating Aggregations and Mating Success in the Flower Thrips, Frankliniella schultzei (Thysanoptera: Thripidae), and a Possible Role for Pheromones.” Journal of Insect Behavior 15 (2002): 351–368.

Morse, Joseph G., and Mark S. Hoddle. “Invasion Biology of Thrips.” Annual Review of Entomology 51 (2006): 67–89.

Mound, Lawrence A. “Homologies and Host-Plant Specificity: Recurrent Problems in the Study of Thrips.” Florida Entomologist 96 (2013): 318–322.

———. “Order Thysanoptera Haliday, 1836.” Zootaxa 3703 (2013): 49–50.

———. “Thysanoptera: Diversity and Interactions.” Annual Review of Entomology 50 (2005): 247–269.

Stannard, Lewis J., Jr. “A Synopsis of Some Ant-Mimicking Thrips, with Special Reference to the American Fauna (Thysanoptera: Phlaeothripidae: Idolothripinae).” Journal of the Kansas Entomological Society 49 (1976): 492‒508.

Sweeden, M. B., and P. J. McLeod. “Abundance of Thrips (Thysanoptera: Thripidae) on Spring-Planted Snap Beans.” Journal of Entomological Science 31 (1996): 72‒75.

———. “Seasonal Occurrence of Thrips (Thysanoptera) on Cowpeas in Western Arkansas and Northeast Oklahoma.” Journal of Entomological Science 28 (1993): 427‒432.

Whitcomb, W. H., and K. Bell. “Predaceous Insects, Spiders, and Mites of Arkansas Cotton Fields.” University of Arkansas Agricultural Experiment Station Bulletin 690 (1964): 1‒84.

Chris T. McAllister
Eastern Oklahoma State College