For two weeks before through two weeks after the applications of B. bassiana , counts were taken of thrips levels on plants twice per week. Beat samples were taken by beating random canes of flush foliage such that the thrips would fall onto a 12 x 12 cm black acrylic beat tray. The numbers of thrips on the beat trays were counted quickly in the field. The counts were taken twice per week from each of the 10 central “data plants” from the ‘Star’ variety of each of the 24 test plots. The new green flush growth was measured on three dates to record the amount of growth since the beginning of the fungal treatment applications to determine if there were differences based on the treatments and amount of water applied to the different plots. Measurements were made of the average cm of new shoot growth over the 6-week trial period. Due to the complex nature of the experimental design, i.e. treatments nested in a 5-way ANOVA , beat count data were analyzed using PROC MIXED and means were separated using Tukey’s test . Figure 3-2 shows the location of late second instar citrus thrips at death in the greenhouse study as well as those that located pupal refuges on the plant. Based on where they dropped off the plant, data indicated that more than 92% of the thrips would have pupated off the plant, likely in the soil near the base of the plant. Numbers did not vary significantly by location over the seven sample dates of this study; therefore data were pooled . A key result was that the proportion of second instar thrips crawling down the base of the plant was higher than the proportion dropping off the plant at distances measured past the base the plant . The four emergence cages placed under the field blueberry plants in each cardinal direction provided a means of sampling late second instar thrips moving towards the soil to pupae versus adults emerging out of the soil following pupation .
Total numbers of thrips collected were pooled for the four traps in each direction at each respective location to determine which cardinal direction showed the most activity,french flower bucket and therefore was the most appropriate location to sample for citrus thrips in the field trial. Emergence cage data were summarized in two ways; the number of thrips moving off the plant to pupate in the leaf litter and the number of thrips emerging from the leaf . Data from the nested ANOVA generated p-values for direction , as well as distance grouping from the base of the plant . The cage closest to the base of the plant had significantly higher numbers of thrips emerging from the soil . Numbers of thrips trapped from the eastern cardinal direction were significantly higher for both mean numbers of thrips moving to and from the soil , indicating that for the field trial, emergence cages should be placed directly next to the base of the plant on the eastern side to sample the location that would have the most thrips activity. Not only did the second instar thrips not avoid the GHA colonized millet seed, they were observed actively walking through it. Mortality was 100% in millet seed treatments across all trials compared to the untreated checks, which ranged from 0% to 8% mortality across these trials. Of the proportion of thrips not finding pupal refuge on the plants with colonized seed , 100% infection was seen with each of the different quantities of seed, i.e. each of 0.5, 1, or 2 seeds/cm2 was a sufficient density to infect and kill all late second instar thrips in the greenhouse study. No thrips were infected in the control treatment. There were insufficient data to conduct a 3-way ANOVA because all recovered thrips were infected with the fungus.
Because all three densities tested were effective, we chose to utilize the most economical density in the field trial, i.e. 0.5 seeds/cm2 . In the split-plot design model, the whole plot factor was water and the split-plot factor was fungus treatment in a type three analysis of variance . Water, time and treatment were the main effects in the full model. Thrips levels measured on pupation traps at 3 days after treatment were lowest with colonized millet seed, intermediate with Mycotrol O ® , and highest in the untreated control . Additionally, there was fewer thrips counted in the colonized millet seed treatment than in Mycotrol O® treated plots . However, at time two , thrips levels with Mycotrol O® were no longer significantly reduced in relation to the control . While thrips levels measured using pupation traps were significantly less than observed in the control in all plots , thrips levels on plants measured using beat samples did not show a significant decrease , although comparing data with no overhead water that with overhead water, there appeared to be fewer numbers of thrips in the overhead sprinkler plots . The measurements from the new green flush growth in the overhead sprinkler treatments showed that those plants had longer growth than those without overhead sprinklers , but thrips numbers were not significantly lower on those plants. At none of the times when foliar beat counts were taken were there significant differences in thrips numbers across any of the three treatments . The ultimate goal of this work was to determine if the GHA strain of Beauveria bassiana could be used effectively as an alternative to traditional insecticides in commercial blueberries in California. Laboratory and greenhouse trials with Beauveria bassiana have shown variable success in controlling thrips and several other insect species , whereas field trials have shown limited overall success, but very few field trials included Thysanoptera .
This is mainly due to the fact that climatic conditions in the laboratory and greenhouse situations are stable and often more humid than the ambient field environment in arid areas like most of California. Unfavorable environmental surroundings, including low humidity, high temperature and intense solar radiation are commonly referred to as the principal constraints to the field performance of B. bassiana . Raw fungal spores are prone to desiccation and death if, when sprayed, they do not contact a host immediately . The microclimate around the spore is thought to be primarily responsible for maintaining spore integrity ; temperature, sunlight and ultraviolet light affect spore integrity but humidity, especially the immediate local humidity around the spore, dictates the spore’s persistence and germination, particularly when ambient temperatures are high . Perhaps it is for these reasons,bucket flower that the water saturated seed was able to provide a suitable microclimate in which strain GHA could better sporulate and persist in comparison with the Mycotrol O® soil drench. Determining methods of applying the GHA strain of B. bassiana so as to optimize field efficacy was one of the more interesting parts of this work. We took advantage of the observation that late second instar larval citrus thrips did not avoid the colonized seed and were able to infect themselves by either walking through or over the colonized seed. Following the Stanghellini and El-Hamalawi protocols proved to be an effective method of applying and sustaining strain GHA in the field. Whereas this system is experimental, it provided a more persistent level of citrus thrips control than did the soil application of raw spores. The foliar beat samples taken before, during and just after the trial did not show significant differences in thrips numbers across any of the treatments, but this could be due to the citrus thrips emigrating and immigrating out of and into the study area. The plots were 27.4 m in length by approximately 5 rows wide and while the samples were taken from the middle plants of the middle row in these plots, flushing blueberry plants were surrounding the area. The new, green, flush growth was significantly longer in the blocks that had overhead sprinklers, which intuitively is not unexpected, because there was substantially more water available to the plants; this is likely not a positive from a citrus thrips management perspective. If there is an abundance of new plant growth in some areas versus others, thrips likely will move into the parts of the field with more flush, sustaining elevated populations at a time when the other plants becoming less suitable, as their leaves toughen between flushing periods. Currently, there is no integrated pest management program in place for citrus thrips pests of blueberries in California. The development of economic injury levels, economic thresholds and the optimal timing and rotation of registered insecticides are all essential portions of an IPM program and this information will form the basis of whether or not the application of B. bassiana in any form, e.g., raw spores or colonized seed, would be an effective alternative to rotate with the use of traditional insecticides.
There is the possibility of mixing entomopathogenic fungi and insecticide applications and several studies showed a synergistic relationship between the use of insecticides and fungi . Possible synergism of strain GHA and insecticides registered for citrus thrips management in blueberries may be worthy of future study. The costs of various registered pesticides used for citrus thrips management in commercial blueberries in California, including product and application costs but excluding purchase of mechanized equipment, ranges from $80 – $138/ ha . The cost of Mycotrol O® , not including application costs, ranges roughly from $50 – $120/ L and as mentioned previously, the maximum application rate per ha is approximately 6.5 L of formulated product. The cost of Mycotrol O® at the maximum application rate therefore would be approximately $325 – $780/ ha. Biopesticides, such as entomopathogenic fungus, are often higher in price than insecticides because they cost more to produce, are not in widespread use, and thus, are not produced on as large a scale as traditional insecticides. The fermentation process, i.e. submerged liquid fermentation or solid state fermentation for production of aerial conidia , propagation requirements and storage and shelf life are all important considerations and steps for mass production of entomopathogenic fungi and their successful use. Fungal strain sporulation failure under mass production settings is often the limiting factor to strain availability and usage and it is currently not well understood why this occurs. Our results suggest that B. bassiana strain GHA can be utilized against citrus thrips on blueberries. We showed that over 0-3 days post-treatment, mean thrips numbers were decreased by 50% in both fungal treatment plots, i.e. with both Mycotrol O ® and colonized seed. While this reduction is significant, it may or may not be economically competitive with traditional options. The cost associated with such an application of Mycotrol O® , once registered in blueberries in California, would be nearly triple the cost of a current insecticide treatment . Blueberries are a high value crop, estimated at $10 – $17/ kg for low and high fair-market price, respectively. This information, when coupled with the need for insecticide resistance management, indicates that utilizing entomopathogenic fungi could be worthwhile for insecticide resistance management of citrus thrips, as there are repeated documented cases of pesticide resistance in citrus thrips populations . Employing entomopathogenic fungi is costly and based on our data, not as effective as a current insecticide application . Because Mycotrol O® is an organic formulation , its utilization might be of interest to organic growers as an alternative to traditional insecticides.Avocado thrips, Scirtothrips perseae Nakahara, is the most serious arthropod pest attacking avocados in California . They were first noticed in the state in June 1996 damaging fruit and foliage in two distant avocado groves, one each in Irvine, Orange County and Oxnard, Ventura County, CA. By July 1997, infestations of S. perseae had spread throughout avocado groves in Ventura and Orange counties . California grows 95% of U.S. avocados on more than 2,500 hectares of land and most of this land is infested with avocado thrips . To date, there are four registered pesticides recommended for avocado thrips management: abamectin, fenpropathrin, spinetoram and sabadilla. Several species of predaceous insects and mites feed upon avocado thrips and these natural enemies include brown and green lacewing larvae, several predaceous thrips and several Aeolothrips spp.) and the native predaceous mite Euseius hibisci . McMurtry and Croft classified the feeding behavior of predatory Phytoseiidae into four groups and Group IV comprises the genus Euseius, members of which can subsist on pollen in the absence of prey with minimal reduction in fitness.