Many hypothesize that because thrips feed with a punch and suck method, rather than direct chewing and mastication of leaf tissues, they do not receive toxic amounts of the Bt proteins . Alternatively, they may not possess the proper binding receptors for the Bt proteins tested to date and thus, no pore can be formed in the midgut lining and the Bt proteins are excreted . The literature indicates the latter hypothesis is more likely based on findings from life table parameters where development, fecundity, and adult longevity or relative abundance are not significantly different from thrips reared on Bt positive versus Bt negative corn, cotton, or potato plants. The aforementioned studies were not specifically looking at Bt effects on thrips nor were the Bti toxins tested here involved in previous studies involving thrips. The combinations of proteins used in thisstudy were, to date, unique pairings with thrips. It is indeed possible that there are no Bt endotoxins currently available that cause mortality to Thysanoptera. The LC50 with strain GHA was 8.61 x 104 conidia/ ml and was two orders of magnitude lower than for the other five B. bassiana strains tested . GHA also gave the only statistically valid dose-response values in probit analysis, and provided the only data that fit the probit model. The other B. bassiana strains failed to provide a linear relationship based on their p-values , i.e. the probit regression lines were of poor quality, except for GHA.Therefore, data were evaluated based on line slopes as is commonly seen in the scientific literature with other biological agents where data lines are not straight and do not fit the model .
Strains 1741ss, SFBb1, S44ss, and NI1ss showed a flat dose-response between concentrations, did not fit the model,flower harvest buckets and LC50’sranged from 2.7 x 106 – 9.6 x 108 . Assessment of Beauveria strain while adjusting for concentration, in both Logrank and Wilcoxon tests showed that strain and concentration had a highly significant effect on the infection rate. Multiple comparisons for the Logrank test to assess the strain effect while adjusting for the concentration differences showed that strains 1741ss, S44ss, 3769ss, and NI1ss infection rates were not distinct from one another. Strain GHA and SFBb1 had infection rates different from each other as well, and GHA had the fastest infection rate and SFBb1 showed the slowest kill rate . The Survival Distribution Function analysis coupled with the probit analysis clearly shows that GHA would be the best strain choice for citrus thrips control. Results with avocado thrips. The LC50 for strain GHA was 2.2 x 106 conidia / ml and was similar to that obtained with the other five B. bassiana strains tested . Again, because a strong linear response was not observed, the performance between strains was rated based upon the LC50 and relative linearity of the response. Based on overlap of confidence intervals, there were no significant differences between any of the strain LC50’s or LC95’s . Assessment of Beauveria strains while adjusting for the concentration, using both Log-rank and Wilcoxon analysis showed that strain did not have an effect on the infection rate. The multiple comparisons for the Log-rank test to assess the strain effect while adjusting for the concentration differences showed infection rates for all 5 strains were not distinct from one another . The Survival Distribution Function analysis coupled with probit analysis indicated there was no one best strain to select for avocado thrips management.
Citrus thrips were more susceptible to Beauveria than avocado thrips; citrus thrips LC values were much lower for the most active strain, GHA, indicating that significantly lower dosages of strain GHA were required to infect and kill citrus thrips compared with avocado thrips. The overall survival analysis results showed a similar pattern to the results of the probit analysis; GHA had the fastest infection rate and SFBb1 had the slowest rate . Infection rates for the other threestrain’s fit in between the rates for GHA and SFBb1, and 1741ss, S44ss, 3769ss, and NI1ss infection rates were not separable. This low dosage association and having the fastest infection rate suggest GHA is the best candidate for field-testing among the strains examined. Except for the worst performing strain, SFBb1, the performance of all of the strains with avocado thrips were similar. The LC50 value for citrus thrips was 8.6 x 104 conidia/ml, which may suggest economical feasibility in some cases, e.g., for use on organic products. The maximum recommended field application rate is 5.0 x 1012 conidia/ha. Therefore, 8.6 x 1011 conidia/ha of GHA is needed based on the estimated LC50 of 86 conidia/µl and this amount is reasonable to obtain in a field setting. Conducting the same analysis for avocado thrips control using GHA, with an LC50 of 2.2 x 106 , 2.2 x 1013 conidia/ha would be required. This is 4.4 times greater than the standard field use rate of GHA. We hypothesize that differences in susceptibility between citrus and avocado thrips may be due to the different habitats in which they evolved. 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 anisopliaeSorokin , Neozygites parvispora Remaudière & Keller , Verticillium lecaniiViegas , and Paecilomyces fumosoroseusBrown & Smith have also been used in laboratory and greenhouse trials with much success, 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,round flower buckets 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, high bush varieties of blueberries could only be grown in regions too cold for citrus production . However, breeding efforts to cross the northern high bush blueberries with several other Vaccinium species led to the development of heat-tolerant high bush 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 bassianaVuillemin 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. Citrus thrips were collected in Riverside County, Riverside, CA from wild laurel sumac, Malsoma laurina , a suspected major host for this species before citrus was introduced into the state . Thrips were collected via aspiration the morning of the bioassay and held in 15-dram plastic aspiration vials with a copper mesh screened lid. A small sumac leaf, just large enough to fit in the vial, was included to allow the insects to settle on the leaf and feed. In experiments where late second instar thrips were needed, i.e. thrips that were close to pupation, selected thrips were large and had darkened in color. Their abdomens appeared plump and the overall color of the thrips was a deep yellow with almost no opalescence. When adult females were used, selected females were of unknown age. 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.