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. Avocado thrips are 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 , 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,square pots 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 bio-assays. Also, these materials are detrimental to natural enemies such as Aphytis melinus DeBach and other biological control agents important to citrus pest 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. Abamectin is the main material used for avocado thrips control with occasionally rotation with sabadilla . 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 work described 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, highbush 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 . 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. Avocado thrips, Scirtothrips perseae Nakahara, is a relatively new pest of avocados in California. It appeared in the state in 1996, and, at the time, was a species new to science . By 1998, crop damage reduced industry revenues by 12% . Avocado thrips adults can feed on over 11 plant species’, however, larvae have been found only on avocados in the field in both California and Mexico, suggesting that S. perseae has a highly 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 and culling of damaged fruit . With a limited number of pesticides available for thrips control and the propensity with which economically important thrips develop insecticide resistance, it is wise to monitor population levels carefully,large plastic pots limit treatments to population levels of economic 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, continuing the search for effective biological and chemical controls useful in citrus and avocado thrips management is important. For both species of thrips, some pupation occurs on the tree in cracks and in crevices’, however, the majority of both species drop as late second instars from trees to pupate in the upper layer of the leaf litter under trees . Propupae and pupae are rarely seen, move only if disturbed, and do not feed. Thus, pupation in the upper layers of the soil surface may create the ideal interface for control using the entomopathogenic fungi Beauveria bassiana . Coarse organic mulch beneath trees and the maintenance of a mulch layer, a common practice by many growers as a method of Phytophthora spp. management in avocados , may reduce survival of thrips that drop from trees to pupate below the tree. The effectiveness of mulching to control thrips is uncertain and labor costs are required to add mulch may not be justified solely for thrips control. There is increasing pressure in the U.S. to move away from broad-spectrum insecticides and focus on alternative methods of control, e.g., genetically modified crop plants expressing Bacillus thuringiensis toxins , use of entomopathogens, and similar approaches. Applications of B. bassiana have been reported to decrease populations of thrips in greenhouse cucumbers, chrysanthemums, gerbera daisies, roses, and carnations .

Microbial insecticides containing δ-endotoxins from Bt have been used as alternatives to conventional chemical insecticides for almost 70 years . 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. From a practical perspective, Cry proteins are parasporal inclusion proteins from 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 exhibit 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, are regarded as environmentally friendly, are completely biodegradable, and show little adverse effect on non-target species . The Cyt proteins are significantly different both in their structure and their biological activities from the Cryproteins. However, Cyt proteins have shown toxicity to non-dipterous insects . In fact, Cyt proteins in some cases can extend activity to other Bacillus spp. for mosquitoes that lack the proper receptor . Many studies with thrips involving Bt proteins have typically evaluated Cry toxins in transgenic crops targeted mainly toward lepidopterous pests and there are no published studies we know of representing the impact of Cyt proteins on thrips. Due to the synergism seen between these two Bt proteins and the method of thrips feeding, commonly described as ‘punch and suck’ , whereby leaf tissue is macerated prior to ingestion, we hypothesized that Cry or Cyt proteins could potentially be useful against thrips pests. The goal of this investigation was to determine if Cry or Cyt proteins or B. bassiana could be used effectively to manage citrus and avocado thrips. Field management of both thrips species is the ultimate goal with these biopesticides but field studies are laborious and expensive. Thus, we evaluated these materials in the laboratory to determine which were sufficiently efficacious to warrant follow-up field studies. Leaves of both avocado and citrus for all bio-assays were chosen in observably identical states; young and soft but fully expanded leaves were used as these are the type on which both species of thrips prefer to feed and large leaves were needed to fit in the Munger cell bio-assay units that confined the thrips on treated leaves . Briefly, Munger cells were constructed by using a Plexiglas sandwich; the middle cell layer was drilled with 3.2-cm diameter bit to provide a circular test arena .