The development of molecular techniques, PCR in particular, during the last three decades has provided a variety of rapid, simple, sensitive and reliable tools, e.g., PCR-based typing methods, which has revolutionized the genetic understandings in the biological sciences especially when only minimal amounts of template DNA were available . PCR-based DNA technologies such as species-specific PCR , PCR restriction fragment length polymorphism , multiplex PCR , DNA sequencing and oligonucleotide array analyses are suitable to aid in the development of comprehensive identification methods to differentiate easily between various known species, assist in monitoring for invasive species, and establish and understand species complexes . These issues are of particular interest as the availability of trained taxonomic experts declines and long-term research strategies are required to address the deficiencies in existing taxonomic keys to deal with morphologically indistinct immature life stages, cryptic species and damaged specimens . A number of the most economically significant and global pests morphotaxonomic keys are now supported by molecular diagnostic technology, e.g., fruit flies , tussock moths , leafroller moths and some thrips . The genus Scitrothrips Shull, for example, black plastic plant pots currently includes approximately 100 species throughout the tropics and subtropics and roughly 10 species are economic pests of agricultural commodities such as avocado, citrus, cotton, mango, tea and vegetables .

For this genus alone, sufficient molecular data from the conserved 28S-D2 domain of the large subunit rRNA, the cytochrome c subunit I of mitochondrial DNA and internal transcribed spacer regions 1 and 2 of nuclear ribosomal DNA have been acquired to delineate some of the relationships within the genus but further investigation is required to understand the associations between citrus thrips present in citrus growing regions in North America where citrus is grown and citrus thrips are a major pests but in disparate ways. A personal communication from a collaborator in Florida supplied the idea to investigate the differences seen in citrus thrips in Florida and California. In Florida, citrus thrips are the most abundantly collected thrips on weeds but is not an agricultural pest in citrus, blueberries and mangos. However, in California citrus thrips is a pest of many agricultural crops especially the three listed crops. Thus, it seems prudent to determine if there are genetic differences between citrus thrips populations within North American citrus growing regions.History. Citrus thrips, Scirtothrips citri , is a plant-feeding pest most widely recognized for damage caused to citrus and mango fruits and has been recognized as a major pest of California citrus since the 1890s . Distribution. Citrus thrips appears native to southwestern North America and northwestern Mexico. One of its more common native host plants in this area prior to the introduction of citrus was likely laurel sumac, Malosma laurina Abrams . In the USA, citrus thrips are known from Arizona, California, and Florida , whereas in Mexico they are reported only from northern Mexico .

Adult citrus thrips are small, orange-yellow insects with the characteristic fringe wings of Thysanoptera. The females measure 0.60 – 0.90 mm in length while males are similar in appearance, but slightly shorter and narrower. The eggs measure about 0.20 mm and are oviposited under the cuticle of flush leaves, stems and small fruit. Eggs laid in the fall pass the winter and hatch about the time of the spring foliage flush of citrus. The first and second instar larvae are very active and feed on flush leaves and tender young fruit, typically seeking refuge under the sepals of the young fruit, resulting in a characteristic ring scar around the top of the fruit after it expands. The propupal and pupal stages do not feed or move much unless they are disturbed. These latter two stages complete development on the ground in litter beneath the tree or in crevices in the tree. Life History. Citrus thrips are multivoltine with 8 – 12 generations per year depending on climate . The life history of citrus thrips and a degree-day model has been developed by Rhodes et al. . This model explains the timing of citrus thrips events and is often used to schedule monitoring activities both in the field and laboratory. They reported a developmental threshold of 14.6°C, which to our knowledge, is the highest threshold for any insect species. Host Plants. Citrus thrips is primarily a pest of citrus in California particularly in the San Joaquin and Coachella valleys. They can have a broad host range, including, but not limited to, alfalfa, rose, grape, laurel, cotton, date, fir, Lucerne and various grasses, pecans and other ornamentals. Citrus thrips have been collected from over 55 different plant species . Their native host plant is hypothesized to be Quercus or more likely Malosma laurina .

Scirtothrips citri has broadened its known host range and become a significant pest of a relatively new crop to California, blueberries . Thrips feeding on blueberry during the middle and late portions of the season cause distorted,discolored, and stunted flush growth and poor development of fruiting wood for the subsequent crop . Thrips pressure of this magnitude, coupled with repeated pesticide applications of the few effective and registered pesticides, poses a concern regarding pesticide resistance management. Currently, there are no integrated pest management plans available for control of citrus thrips in blueberry. This is primarily due to the recent nature of this crop-pest association. Historically, low-bush varieties of blueberries could only be grown in regions too cold for citrus production. However, the development of heat-tolerant high-bush varieties, which has enabled the development of a blueberry industry in the San Joaquin Valley , has also caused blueberries to be grown in a region where citrus and citrus thrips flourish. This issue is relevant not only to the blueberry industry, but also for the 108,665 hectares of California citrus , which has experienced repeated documented cases of pesticide resistance in citrus thrips populations . It is also important to note that not all varieties of high-bush blueberries are fed on equally by citrus thrips; i.e. there is a distinct varietal preference for some hybrids with similar parentage .History. Avocado thrips, Scirtothrips perseae Nakahara, is a relatively new pest of avocados in California. This species was first noticed in California in June 1996 damaging fruit and foliage in two distant avocadogroves, one each in Irvine, Orange County and Oxnard, Ventura County, CA. By July 1997, infestations of S. perseae were spread throughout avocado groves in Ventura and Orange counties . Distribution. Scirtothrips perseae is native to Mexico and Guatemala and is now present in most avocado growing regions in southern California from San Luis Obispo Country south to San Diego County . Description. Female avocado thrips lay eggs hidden inside the underside of leaves, in young fruit and stems . The first instar is white to pale yellow while the second instar is larger, more robust, and bright yellow . Avocado thrips larvae are typically found along major veins on the underside of younger leaves and anywhere on the surface of young fruit . Although some pupation occurs on the tree in cracks and in crevices, black plastic garden pots about three fourths of avocado thrips second instars drop from trees to pupate in the upper layer of dry, un-decomposed leaf litter . Propupae and pupae are rarely seen and they do not feed and move little unless disturbed. Adults are 0.7 mm long and have the typical fringed-tipped wings. Adults are orange-yellow with distinct, thin, brown bands between segments of their abdomen and three small red dots on top of the head . Life History. Adult avocado thrips resemble citrus thrips to the untrained eye and to an even lesser degree, western flower thrips, which occur on, but do not damage, avocado and citrus. Avocado thrips develop well under cool, humid temperatures . Populations typically begin increasing in late winter and spring, when avocado thrips feed on young leaves and fruit. Population abundance peaks in late spring and early summer, when most fruit are young and after the growth flush when hardening of leaves induces thrips to move from foliage to feed on young fruit. Populations are suppressed by warm, dry conditions, but this weather usually occurs later in the season, when most fruit are larger and no longer susceptible to damage by thrips. Scirtothrips perseae can have 6 or more generations a year.

Egg to adult development occurs in about 20 to 30 days when temperatures average 18 to 24°C . Hoddle reported avocado thrips developmental biology and created a developmental degree-day model listing a developmental threshold of 6.9°C, which to our knowledge is the lowest threshold for any insect species. Monitoring temperatures and using degree-day calculations can predict actual development time. Foliar feeding is usually unimportant, except when very high populations cause premature leaf drop . Host Plants. Avocado thrips adults can feed on over 11 plant species, however, larvae have only been found on avocados in the field in California suggesting that S. perseae has a restricted host range . Although it has little effect on tree health, avocado thrips feed directly on immature fruit , and obvious feeding scars cause severe downgrading or culling damaged fruit . Moreover, severe scarring when fruit are young can slow and stunt fruit growth. As fruit grow, early feeding by avocado thrips becomes apparent as scabby or leathery brown scars that expand across the skin and is sometimes referred to as “alligator skin” . Avocado thrips damage is affected by practices that increase or decrease the abundance of succulent foliage during set and growth of young fruit. Thrips move to young fruit when leaves harden after the growth flush has finished and the most damage occurs when fruit are 5.1 to 15.2 mm long . Although Hass fruit are susceptible to feeding until they reach about 51 mm in length, thrips feeding rarely causes scars on fruit larger than about 19.1 mm. This scarring on young fruit may not become obvious until fruit enlarge. In severe cases, all fruit on a tree can have their entire fruit surface scarred by avocado thrips, causing some packinghouses to sell such fruit with the box marked “papacado.” The California Avocado Commission estimated a $50 million dollar crop lost in the 2006 due to avocado thrips scarring and the costs of control .Monitoring methods. Sticky card and beating tray sampling are research methods used for these two insects but are rarely used by growers or pest control advisors . Both PCAs and researchers monitor citrus thrips by counting the percent of fruit infested with immature thrips and the number of immature thrips per fruit is also indicative of the severity of the infestation. Thresholds in use in the San Joaquin Valley are 20% of Valencia oranges or 10% of navel oranges infested with immature thrips until the fruit reaches 20 mm in diameter or more. Thresholds are halved if Euseius tularensis levels are less than 0.2 per leaf . Avocado thripsare monitored by counting the number of immature thrips per leaf prior to fruit set or the number of thrips per fruit. No firm economic threshold has yet been developed for avocado thrips but PCAs typically treat at 3-5 immature thrips per leaf prior to bloom in San Diego County due to restrictions on use of abamectin during bloom. Control by natural enemies. The major documented citrus thrips predator is the phytoseiid mite, E. tularensis , although Jones and Morse questioned the importance of this predator. Avocado thrips are frequently preyed upon by Franklinothrips orizabensis Johansen and Chrysoperla carnea and is parasitized by the larval parasitoid Ceranisus menes . Franklinothrips vespiformis , black hunter thrips , and several banded-wing thrips also feed on avocado thrips . In many years, natural enemies are unable to suppress avocado and citrus thrips populations below economic thresholds and chemical control is needed to reduce fruit scarring. Control with pesticides. By the time damage is noticed on ripening fruit, the thrips that caused the injury are often absent from the fruit. A variety of pesticides are registered for thrips control in different cropping systems . After a number of years of use, pesticides like dimethoate , formetante hydrochloride , cyfluthrin , and fenpropathrin resulted in failures in citrus thrips control in some regions, along with an increase in resistance confirmed with both laboratory and field bioassays. Also, these materials are detrimental to natural enemies such as Aphytis melinus DeBach and other biological control agents important to citruspest control.