Citrus thrips are adapted to hot and dry environments and thus, they are less likely to have evolved natural tolerance to fungi, whereas, avocado thrips thrive in a very wet environment where exposure to fungi is more likely. The differences may be due to different habitat adaptations and the different origins of the two thrips species . We find it interesting that two congenerics have such widely different habitat preferences and this may explain differences in fungal tolerance. Differences were seen when citrus thrips and avocado thrips were placed on leaves of their associated host plants, then placed separately in sealed zip-lock bags where the moisture that condensed in the bags was lethal to citrus thrips but not to avocado thrips. Thus, it is possible that avocado thrips, due to their adaptation to living in cool and wet climates , have a higher tolerance to fungal pathogens, as they may encounter them more frequently than citrus thrips, which prefer a hot and drier climate . Many researchers have investigated alternatives to traditional insecticides such as biopesticides, i.e. natural or organismal methods of controlling pest populations. The utilization of entomopathogens against thrips is not a new concept; entomopathogenic fungi, such as, 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 much success,large plastic pots for plants whereas field trials have shown limited successes.
However, various strains of B. bassiana have been shown to effectively control western flower thrips on greenhouse ornamentals and peppers , and several reports indicated that F. occidentalis, Thrips palmi Karny and T. tabaci Lindeman were successfully controlled under field or laboratory conditions . In conclusion, both citrus and avocado thrips can be infected by B. bassiana but high doses may be required, especially for avocado thrips. These high doses are difficult to obtain outside the laboratory and application of such doses would be costly. We believe B. bassiana is not a sufficiently effective alternative to traditional insecticides to warrant further study with avocado thrips, particularly because the commercially available strain GHA gave poor control on avocado thrips, but it may have potential against citrus thrips in an integrated pest management program. Further studies are warranted to determine if GHA could be used in field control of citrus thrips. Citrus thrips, Scirtothrips citri , has been recognized as a major pest of California citrus since the 1890s and is also known to scar mango fruits . Historically, highbush varieties of blueberries could only be grown in regions too cold for citrus production . However, breeding efforts to cross the northern highbush blueberries with several other Vaccinium species led to the development of heat-tolerant highbush blueberry varieties . This has enabled the establishment of a blueberry industry in the San Joaquin Valley, a region where both citrus and citrus thrips flourish . The known host range of citrus thrips has broadened and in recent years, they have become a significant pest of blueberries planted in the San Joaquin Valley of California .
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. Repeated pesticide applications of the few effective and registered pesticides to reduce thrips populations pose a concern regarding pesticide resistance management, and this issue is relevant not only to the blueberry industry but also for the 108,665 ha of California citrus which has experienced repeated documented cases of pesticide resistance in citrus thrips populations . Currently, there are no integrated pest management plans available for control of citrus thrips in blueberry, probably due to the recent nature of this crop-pest association. With a limited number of pesticides available for thrips control and the frequency of insecticide resistance shown by thrips, populations should be monitored carefully, treatments limited to populations of economic concern, and applications timed 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. Understanding citrus thrips’ life history in the blueberry system to determine where and if susceptible stages could be exploited, is one of the first steps in the development of alternative methods to the use of traditional insecticides. In citrus, citrus thrips pupation occurs on the tree in cracks and in crevices, however, the majority of thrips drop as late second instars from trees to pupate in the upper layer of leaf litter below trees and move upward onto the plant after adult eclosion. Propupae and pupae are rarely seen, move only if disturbed, and do not feed.
Pupation in the upper layers of the soil surface may create the ideal interface for control using the entomopathogenic fungus Beauveria bassiana Vuillemin due to this vertical movement of the citrus thrips. However, blueberry plants have much different plant architecture than citrus trees and citrus thrips pupation behavior has yet to be studied on blueberries. In the U.S., pressure is increasing to move away from broad-spectrum insecticides and focus on alternative methods of control. Earlier work with B. bassiana determined that the commercially available strain, GHA , was the most effective of six strains tested in laboratory trials against citrus thrips . The goal of this study was to determine if this strain of B. bassiana could be utilized effectively against citrus thrips in California blueberry production. To achieve this objective, several factors of importance to fungal efficacy were evaluated before commencement of our field trial: 1) location of citrus thrips pupation in commercial blueberry plantings, 2) field sampling locations and methods, 3) fungal formulation and timing of application, and 4) density of product used and method of thrips infection. We then conducted a field trial evaluating the potential utility of the GHA strain of Beauveria bassiana in commercial blueberries for citrus thrips management as a possible alternative to the use of traditional insecticides. Because of the complex arrangement and number of blueberry canes arising from the rhizome of commercial blueberry plants, we first evaluated movement of second instar citrus thrips on potted single cane blueberry plants in the laboratory. Known numbers of late second instar citrus thrips were released onto the leaves of potted blueberry plants in the lab. Paper sprayed with Tangle Trap sticky coating was placed a) at the base of the plants with a ring of sticky tape around the base of the stem and partially on the stem of the plant to capture any insects crawling down, and b) extending from the base of the plant horizontally outward above the pot surface to ensure complete coverage of the area covered by the plant canopy . This experiment was replicated on a single potted plant over time on 7 dates . Data were analyzed using Fisher’s exact test using SAS 9.2 . At our planned field trial site that would later be used in the B. bassiana trial, pupation emergence cages were used to sample insects moving off foliage towards pupation sites and later emerging out of the soil after pupation. Cages were made from Schedule 40 white PVC pipe with a diameter of 10.2 cm with cages cut to a height of 5.1 cm. The cage was then topped with a double-sided sticky card cut to fit, which was fixed into place with two elastic bands. Four lines of four cages were pushed into the soil to a depth of approximately 1 cm immediately adjacent to each other at the base of a blueberry plant and oriented in a cardinal plane to determine which direction showed the most thrips activity. The four adjacent cages in a particular plane were used to assess thrips movement in the understory of the blueberry plant in each directional. The study was replicated on 5 plants on a single date and conducted just prior to the commencement of the field trial. Data were analyzed with a nested ANOVA using SAS 9.2. In a greenhouse trial, Mycotrol O ® was applied directly to the soil surface as raw spores and compared to the same product colonized onto millet seed,plant pots with drainage also using soil application. Millet seed colonization used the Stanghellini and El-Hamalawi method as described below. The colonized millet seed, when allowed to imbibe water and incubate in the laboratory, can support 1.0 x 106 conidia/seed . Based on Stanghellini et al. with modification, we held the GHA colonized millet seed in containers such that the seed mat was at a depth of no greater than 2.54 cm.
The seeds were wet with the consistency of very thin slurry and were gently stirred three times per day for four days to ensure they imbibed water properly so that mycelial growth and sporulation would occur. Sporulation was confirmed by slide mounting random sections of mycelia and checking for condia formation under the microscope. Once spores were initially observed, the seed was held an additional three days so that sporulation could continue before use of the colonized seed in the field study. Mycotrol O® was applied in the maximum recommended field rate for high thrips levels of 2.84 L of material in 378.5 L of water. The colonized millet seed was tested in the greenhouse to determine if late second instar citrus thrips would become infected if they crawled over or through the seed when it was placed at the base of a laurel sumac seedling. A single small laurel sumac seedling, about ~10 cm tall, was placed into each of ten, 9.5 x 9.5 x 18 cm styrene cages with 6 cm diam air holes on all four sides that were covered with ultra fine mesh screening . Small holes were made in the bottom of the container and covered with pebbles to allow for drainage, then soil was added to a depth of 7.62 cm and the top of the container was covered with a removable lid. The base of each plant was completely surrounded by either B. bassiana colonized millet seed or with uncolonized seed . A minimum of 20 late second instar thrips were released onto the leaves of each plant, and were left until enough time had passed for the thrips to molt to the propupal stage. The seedling was then cut at the soil line and examined for pupating thrips; the removable lid of the cage was sprayed with Tangle Trap sticky coating to collect any emerging adults after 5 days so infection could be measured. The study was replicated on 5 dates . Data were analyzed using 1-way ANOVA with time as a factor and means were separated using Tukey’s Least Significant Difference test using SAS 9.2. To determine the optimum number of colonized millet seeds needed for close to 100% infection when thrips were seeking pupal refuges off the plant, varying amounts of colonized seed were evaluated in a greenhouse trial based on the size of the seed once it had imbibed water and sporulation had occurred. After water inhibition, nine seeds completely filled one square cm of soil surface. There was a 0.5 cm buffer area around all sides of the cage, which was kept clear of seed to provide a 9 x 9 cm grid of seed on the soil surface below the plant. All but two leaves were plucked from the seedling. Small holes were made in the bottom of the container, which was covered with pebbles to allow for drainage. The 9 x 9 cm2 grid was created from wire screen and differing amounts of sporulating seed or seed alone were placed on the light imprint made from the wire screen on the soil surface. Two replicate seedlings per treatment were set up per date in a complete block design . Plants were watered every third day. A minimum of 20 late second instar thrips were placed onto the leaves of the plant, and were left until enough degree-days had passed for the thrips to molt to the propupal stage, typically about 5 days. The seedling was then cut at the soil line and examined for pupating thrips; the removable lid was sprayed with Tangle Trap sticky coating to collect any emerging adults after another 5 days.