A personal communication from a collaborator in Florida supplied the idea to investigate the differences seen in citrus thrips in Florida andCalifornia. 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. 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 Quercusor 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,black plastic plant pots bulk 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 . 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, about threefourths 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 . 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 .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 . 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. The major documented citrus thrips predator is the phytoseiid mite, E. tularensis ,procona system 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. 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. Since it was registered in 1998, spinosad has been the main material used for control of citrus thrips and a related and more effective material, spinetoram , was registered late in 2007 and will soon replace spinosad once MRL issues are resolved with export countries. Resistance to sabadilla has been shown with avocado thrips and a similar pattern of resistance development with abamectin is of concern due to the persistence of this material in leaf tissue. To date, citrus thrips resistance to spinosad has not been documented but there is concern that resistance to it or spinetoram may appear soon. With a limited number of pesticides available for control and the frequency of resistance shown by thrips such as citrus thrips, it is wise to monitor population levels carefully, limit treatments to population levels of concern, and time treatments optimally . Appropriate cultural practices and conservation of natural enemies should be practiced in concert with the use of pesticides only on an as-needed basis. Thus, the search continues for effective biological and chemical controls useful in citrus and avocado thrips management. For both species of thrips, some pupation occurs on the tree in cracks and in crevices, however, about three-fourths of avocado thrips drop as late second instars from trees to pupate in the upper layer of dry leaf litter . Propupae and pupae are rarely seen, move only if disturbed, and do not feed.

This phenomenon of dropping down to the leaf-litter or soil surface for pupation may create the ideal interface for control using the entomopathogenic fungi B. bassiana. Adding coarse organic mulch beneath trees and maintaining a mulch layer may reduce survival of thrips that drop from trees to pupate below the tree, especially in avocados, because this is common practice by many growers as a method of Phytophthora management. The effectiveness of mulching to control thrips is uncertain and labor costs of adding mulch may not be justified solely for thrips control. However, applying coarse organic material such as composted yard waste beneath trees may help control weeds, and thrips reduction might be an additional benefit, particularly for blueberries. The deep mulch layer that is standard practice with blueberry culture in the San Joaquin Valley may also provide an ideal habitat for B. bassiana. It is possible that as citrus thrips are adapted to and evolved in a hot, dry climate, they may be more susceptible to B. bassiana, whereas avocado thrips has adapted to and evolved in a wet and cool climate and may be less susceptible to or even tolerant to B. bassiana. There is increasing pressure in the United States to move away from broadspectrum insecticides and focus on alternative methods of control, e.g., geneticallymodified crop plants expressing Bt toxins, use of entomopathogens, biorational insecticides. Implementation of such methods on avocado and citrus are difficult due to the relatively primitive methods available for thrips sampling, which are labor intensive and rely on experienced and intuitive pest control advisors. The goal of the workdescribed here is to examine alternatives to traditional insecticides such as Bt proteins and entomopathogenic fungi to control avocado and citrus thrips, with the ultimate target of utilizing entomopathogens to aid in field control, evaluate the insecticides registered for avocado thrips management on the native predaceous mite Euseius hibisci, assess citrus thrips oviposition on blueberry varieties, and determine whether citrus thrips is actually a complex of species. Citrus thrips, Scirtothrips citri , is a plant-feeding pest most widely recognized for the damage it causes to citrus and mango fruits and has been recognized as a major pest of California citrus since the 1890s . Recently, its known host range has broadened and they have become a significant pest of a relatively new crop planted in the San Joaquin Valley of California, high bush blueberries . Citrus thrips feed on blueberry foliage during the middle and late portions of the season causing distorted, discolored, and stunted flush growth and poor development of fruiting wood required to obtain the subsequent crop . High numbers of thrips on blueberries , coupled with repeated pesticide applications of the few effective and registered pesticides, poses a concern regarding pesticide resistance management .