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, 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,nursery pots 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. Species of Euseius are the most common phytoseiids on both citrus and avocado. Euseius hibisci is known from Santa Barbara County in California to the sate of Oaxaca in southern Mexico . It mainly has a coastal distribution in California and is the dominant phytoseiid on avocados . Euseius hibisci is common and abundant in avocado orchards year round,large pots plastic is an important generalist predator and feeds on pollen and leaf exudates in the absence of prey . The most studied member of this genus is probably Euseius tularensis Congdon and not nearly as much is known about E. hibisci with regards to pesticide exposure. In fact, E. tularensis was ‘discovered’ and described as a new species different from E. hibisci based on finding several populations of the former that showed a high tolerance to pesticides . Several studies have indicated the relevance of E. hibisci as effective bio-control agents of spider mites and thrips on some crops and although E. hibisci is not a specialized predator, it potentially aids in enhancing the control of many different pest mites and thrips . Assessment of acaricides and insecticides on non-target organisms is an essential component of any IPM program and is of particular interest to California avocado growers. This study evaluates the non-target impacts of the four pesticides currently recommended for avocado thrips management on Euseius hibisci. Avocado trees for all of these studies were located at the Agricultural Operations Facility at the University of California, Riverside. All avocado leaves used in these studies were fully expanded, mature leaves but not ‘hardened’ off based on leaf flexibility and color . Leaves selected for bio-assays were hand flagged with color-coded flagging tape the day prior to pesticide application. The four pesticides used in these studies were abamectin , fenpropathrin , spinetoram and sabadilla . All pesticides were applied at their maximum per ha label use rate using a dilution rate of 2,805 L/ha .
A water only control was also applied. Pesticides were mixed the morning of the application and administered using a hand sprayer . Each flagged leaf was located, the flag was marked with a number and the leaf sprayed one time each on the axial and abaxial sides with a light spray that ensured the whole leaf area was covered. The leaves remained on the avocado trees to weather naturally in the field until they were picked on the day laboratory bio-assays were conducted. An initial pesticide application and bio-assay were conducted beginning November 9, 2009 to determine a number of factors for the subsequent spring pesticide application and bio-assay. Late April early May is typically when growers would be treating for avocado thrips in California avocados . Therefore pesticides for the field trial were applied May 4, 2010 and based on the preliminary study, bio-assays were conducted, 1, 3, 7, 14, 21, 28, 49, 70 90, 111 and 132 d post pesticide application. From each treatment group five replicate leaves were randomly selected on the morning of each bio-assay. If the flagging tape tied to a leaf did not bear a hand written number , it was not included in the study. The leaf petioles were placed into 2.4 ml glass jars filled with deionized water such that the leaf surfaces were not in contact with the water at any time, and transported to the laboratory. A 2.5 cm hole was punched out of the center of each leaf and placed abaxial side up on a wet, white felt covered sponge in a plastic rectangular Tupperware container with no lid. Each of the five replicate leaf discs was randomly arranged in the traywith a disc near each corner and one disc in the center. The discs were surrounded with strips of wet white felt to provide a wet border around each disc to keep the mites on the disc. Four to five strands of CelluCotton were placed in the middle of each leaf disc and topped with 1/16 of a clear plastic cover slip . A very small amount of ice plant pollen was carefully placed on the piece of cover slip with a fine tipped paintbrush ensuring no pollen came into contact with the previously treated leaf surface. Once each of the Tupperware trays was set up with the five replicated leaves, a minimum of 20 mature female E. hibisci was added to each disc. This was conducted systematically by dipping the tip of the paintbrush into DI water, lightly tapping the brush to remove excess water; then while viewing under a microscope, mature females were selected by lightly and carefully touching the tip of the brush to their dorsal surface, removing the mite from the colony tray and placing it onto the leaf disc such that the mites would grab the disc and pull themselves away from the paintbrush tip. The trays were then placed in an environmental chamber at 24°C, 50% RH, and long day light conditions . The trays were checked daily for 5 days noting the number of live and dead mites and any mites drowned in the wet felt. Mite mortality was recorded and assessed by lack of movement upon light probing with a fine tipped paintbrush. Drowned mites were recorded as mites stuck in the wet felt, either moving or not moving, and were not ‘rescued’ from the felt if found alive struggling in the felt . Data were analyzed by day in two ways: % of mites found trapped in the felt and % dead mites, excluding those trapped in the felt .