The labrum forms the front of the proboscis; the basal portions of the maxillae form the sides and the labium forms the rear . Thrips also exhibit unique metamorphosis, being neither truly hemimetabolous nor holometabolous. The first two instars have no external wings, are referred to as larvae, and are mobile but are relatively slow moving. In many cases, the wings develop internally during these two instars . In the Terebrantia, the third and fourth instars are inactive, unless disturbed, non-feeding, and some lack external wings. The third and fourth instars are referred to as the propupa, and pupa respectively, though they are not ‘true pupae’ and they are followed by the adult stage. Thrips range in color from translucent white or yellowish to dark brown or blackish depending on the species and life stage . The sexes of thrips are similar in appearance, though the male is often smaller and moves faster than the female. Parthenogenesis can occur in many species of thrips . When an ovipositor is present, phytophagous females ususally oviposit into planttissue . The eggs are typically oviposited under the cuticle of new leaves, large pots plastic stems and fruit where the larvae feed. In many species, one female may lay as many as 250 eggs. The late second instar of many species drop to the soil or leaf litter or lodge within plant crevices to pupate.
However, greenhouse thrips pupate openly on lower leaf surfaces while pupae and eggs of some gall-forming species occur on leaf surfaces but are enclosed within distorted plant tissues. Thrips have several generations per year, some having 8 or more. In some species, the life cycle from egg to adult may be completed in as short as two weeks when environmental conditions are optimal. Nearly 50% of the known species of thrips feed on fungi, about 40% feed on living tissues of dicotyledonous plants or grasses, and the rest exploit mosses, ferns, gymnosperms, and cycads or are predatory . Phytophagous thrips mainly feed upon the rapidly growing foliage or ‘flush’ as well as very small, developing fruits typically not larger than 5 cm in diameter. Because of the preference of thrips for immature fruit and flush foliage, external injury to commercial fruit and plants may be difficult to detect in the early stages of growth. Thrips feeding causes tiny scars on leaves and fruit, often referred to as stippling. Stippled leaves become distorted, colored, rolled, stunted and are often abscised by the plant. Avocado, citrus and greenhouse thrips cause silver to brown scabby scars on avocado and citrus fruit surfaces, but the damage is usually cosmetic . Evidence of thrips damage on grapes appears as dark scars surrounded by lighter colored ‘halos’ . Thrips damage may cause apples, nectarines, onion, pears,soybean, sugar pea pods, raspberries and tomato, to be deformed, scarred and scabbed . Some thrips detrimental to crops of economic importance feed on and over-winter in weed hosts and plant material left in growing fields and presumably move into crops when environmental conditions are appropriate .
Like many insects with incomplete metamorphosis, thrips adults and larvae compete for the same food resources. Phytophagous species are often broadly polyphagous, attacking a wide range of host plants representing a spectrum of agricultural crops and non-crop species. For example, western flower thrips, Frankliniella occidentalis , is an important and polyphagous greenhouse and field pest with vegetables and ornamental crops being the most important host plants. Western flower thrips cause direct damage on the plants and indirect damage as a vector of tomato spotted wilt and other viruses . Tomato spotted wilt virus is one of 14 tospoviruses that are known to infect crops. It infects a wide range of crop and non-crop hosts and causes economic losses worldwide that are estimated at $1 billion per year . Thrips tabaci Lindeman is an important pest of sweet peppers and also vectors a tospovirus . The melon thrips, Thrips palmi Karny, is a pest of over 60 economically important crops and bean thrips, Caliothrips fasciatus , is a pest of over 60 plant genera and approximately 30 economically important crops, although this number may be an overestimation . Because thrips populations can build up to injurious levels very quickly if left unchecked in native habitats and in crops where flush is present, colored sticky cards are often used to monitor for thrips levels . Other monitoring methods include visual inspection, a turpentine funnel wash, and shaking, sweep-net, or beat tray sampling . Visual inspection and sweep-netting can give some indication as to the presence of thrips, while beating or shaking the plant material and collecting the insects is also sufficient but more labor intensive with extraction efficiency being low. Because thrips can rapidly increase in numbers and move from field to field, it is often necessary to sample thrips frequently. Many thrips species are key pests of economically important crops and ornamentals, including avocado, citrus, cotton, cowpea, melon, onion, pecan, rose, strawberry and many ornamental flowers . Historically, chemical controls have been used to combat these pests.
The insecticides imidacloprid , abamectin , methomyl , spinosad , spinetoram and endosulfan include only a few of the many insecticides that have been or are currently used to chemically control thrips species . However, management of thrips with insecticides can be difficult,especially when they invade fields and crops when flush is present, which can be at multiple points during the year depending upon the crop. In addition, repeat sprays are often required as thrips populations continue to emigrate into crops from nearby plants in native habitats or other crops. Also, the egg and pupal stages of thrips are often protected from spray impact. The utility of chemical control can change rapidly because of resistance development, environmental contamination, non-target effects on beneficial insects and increased public awareness and concerns, resulting in restrictions or near elimination of several classes of insecticides. In association with integrated pest management, methods that minimize the use of broad-spectrum insecticides are being developed . For example, increased use of natural enemies, mating disruption with pheromones, use of sterile insects, and genetic engineering of plants are all part of the arsenal being developed to manage insect pests. Acaricides and insecticides are commonly used for pest suppression in agriculture, forestry and public health. Adverse effects of pesticide use can include the killing of non-target organisms, contamination of water supplies and persistence of unwanted residues on foods and animal feed. Insecticidal effects on non-target organisms have been a concern since the early 1960’s and resistance to one or more pesticides has evolved in populations of over 500 insect and mite species , rendering many of those pesticides ineffective against the resistant populations. For these and other reasons, supplementing or replacing pesticides with non-chemical or non-traditional control tactics, including biological control and biorational insecticides, is a goal in many crop and livestock production systems. Many researchers have investigated alternatives to conventional insecticides such as biorational insecticides or biopesticides, i.e. natural or organismal methods of controlling pest populations. The utilization of entomopathogens against thrips is not a new concept. For example, entomopathogenic nematodes of the families Steinernematidae and Heterorhabditidae are currently used as biological control agents for soil-inhabiting insect pests as these nematodes are lethal insect parasites and nematodes in conjunction with predatory mites have also been used in thrips control . Entomopathogenic fungi, such as Beauveria bassiana Vuillemin, Metarhizium anisopliae Sorokin , Neozygites parvispora Remaudière & Keller , Verticillium lecanii Viegas and Paecilomyces fumosoroseus Brown & Smith have also been used in laboratory and greenhouse trials with success , whereas field trials have shown limited successes. Various strains of B. bassiana have been shown to effectively control western flower thrips on greenhouse ornamentals and peppers , plant plastic pot and several reports indicate that F. occidentalis, Thrips palmi Karny and T. tabaci Lindeman were successfully controlled under field or laboratory conditions . Evaluation of acaricides and insecticides on non-target organisms is an essential component of any IPM program and of particular interest to California avocado growers. California alone grows 95% of the United States avocados on more than 2,500 hectares of land and roughly 99% of this land is infested with avocado thrips . Several species of predaceous insects and mites feed upon avocado thrips and these natural enemies include brown and green lacewing larvae, several predaceous thrips and several Aeolothrips spp. and the native predaceous mite Euseius hibisci . McMurtry and Croft classifly the feeding behavior of predatory Phytosiidae into four groups and Group IV comprises the genus Euseius, members of which can subsist on pollen in the absence of prey with minimal reduction in fitness. Species of Euseius are the most common phytoseiids on both citrus and avocado. Euseius hibisci is known from Santa Barbara County in California to the sate of Oaxaca in southern Mexico .
It mainly has a coastal distribution in California and is the dominant phytoseiid on avocados . Euseius hibisci is common and abundant in avocado orchards year round, is an important generalist predator, and feeds on pollen and leaf exudates in the absence of prey . The most studied member of this genus is probably Euseius tularensis Congdon and not nearly as much is known about E. hibisci with regards to pesticide exposure. In fact, E. tularensis was ‘discovered’ and described as a new species different from E. hibisci based on finding several populations of the former that showed higher tolerance to pesticides . Several studies have indicated the relevance of E. hibisci as effective bio-control agents of spider mites and thrips on some crops and although E. hibisci is not a specialized predator, it potentially aids in enhancing control of many different pest mites and thrips . Evaluating the effects of registered pesticides for avocado thrips management in avocados on Euseius hibisci is worthwhile research especially as thrips pressure increases and growers rely more on pesticides for management. Developments in molecular biology have produced transgenic crops such as cotton, soybeans and corn, which express the Bacillus thruingiensis endotoxin to protect the plant from primary pests. As a result, the use of insecticides in transgenic crops has declined . Bacillus thuringiensis are gram-positive spore-forming bacteria with entomopathogenic properties. Bt produces insecticidal proteins during the sporulation phase as parasporal crystals. These crystals are primarily comprised of one or more proteins, i.e. Crystal and Cytolitic toxins, also called δ-endotoxins. Cry proteins are parasporal inclusion proteinsfrom Bt that exhibit experimentally verifiable toxic effects to a target organism or have significant sequence similarity to a known Cry protein . Similarly, Cyt proteins are parasporal inclusion proteins from Bt that exhibits hemolytic activity or has obvious sequence similarity to a known Cyt protein. These toxins are highly specific to their target insect, are innocuous to humans, vertebrates and plants, and are completely biodegradable. A major threat to the use of Bt is the appearance of insect resistance, which has been documented in the field with lepidopteran insects . However, no resistance has been observed in the field to date in mosquito species controlled with Bti . The lack of resistance to Bti is due to the presence of the Cyt1Aa protein in the crystal . It was demonstrated that Cyt1Aa protein synergizes Cry11Aa toxicity by functioning as a receptor molecule ; Cyt1Aa can also extend activity to Cry11Aa within insects that do not posses binding receptors, again by functioning as the receptor . Therefore, Bt is a viable alternative for the control of insect pests in agriculture and of important human disease vectors . Cyt1Aa and Cry11Aa would be uncommon Bt protein pairings for agricultural pests to encounter, but because synergism has been shown repeatedly, they are worthy of investigation against the Thysanoptera. Molecular biology has afforded many researchers the ability to distinguish between seemingly morphologically identical and difficult to identify organisms . Larval thrips are often confused for other insects, such as Collembola , and adults as Staphylinid beetles ; it is often the case that identification of the larvae is impossible without the presence of adults. The majority of thrips are host-plant specific, but some economically important species are polyphagous and many species are predatory and therefore beneficial for management of immature scale, whiteflies, and mites . Predatory thrips may be mistaken for pestiferous thrips but the use of genetic markers represents a valuable addition or alternative to traditional phenotypic methods of species recognition.