Technical Exploration For The Development Of Wettable Powder Formulation Of Beauveria Bassiana Conidia Against Plutella Xylostella

As a classic fungal biocontrol agent, the entomopathogenic fungus Beauveria bassiana has played an important role in control of insect pests infesting crops or forests worldwide. The present study sought to evaluate quantitatively interactions of B. bassiana SG8702 with sublethal application rates of profurite aminium, a chemical insecticide against lepidopteran pests in China, by means of a standard bioassay protocol and to optimize components as additives to a wettable B. bassiana formulation for better application and storage in ambient environment.Time and concentration dependent interactions of B. bassiana and profurite aminium. Six bioassays were conducted to evaluate interaction between B. bassiana SG8702 and profurite aminium, a chemical insecticide highly compatible with the fungal biocontrol agent, against diamondback moth Plutella xylostella. Second instar larvae were exposed to sprays of B. bassiana alone (assay 1) at the concentrations of 21-38, 157-232 and 822-1133 conidia/mm2 or together with the chemcial at the low application rates of 5, 10, 25, 50 and 100 a.i. ug/ml (assays 2-6). and then maintained at 25°C and 12:12 h light:dark photoperiod for daily monitoring mortalities for 8 days. Based on modeling the resultant time-concentration-mortality data sets, the fungal agent was highly virulent to P. xylostell with an LC50 decreasing from 269 conidia / mm2 on day 4 to 107 on day 8. Lower lethal concentrations or shorter median lethal times resulted from fungal sprays including the tested chemical rates, which never caused higher mortalities than the fungal treatments alone. The fungal action was significantly enhanced by inclusion of the chemical 25 μg/ml in the fungal sprays for 4-5 fold reduction of the LC50 from day 6 after spray, 50 ug/ml for 12-42 fold reduction from day 4 after spray, 100 μg/ml for 9-2548 fold reduction from day 3 after spray, respectively. These rates were equivalent to 5-20% of the chemical rate labeled for fieldapplication. The fungal and chemical interaction clarified above highlights the feasibility of combined formulation or application of B. bassiana and the chemical insecticide for P. xylostella control.Optimized components as additives to wettable formulation. Based on measuring wetting times of 14 wetting agents and assaying their compatibilities to B. bassiana conidia, dodecyl sodium sulfate was found wetting the conidia most rapidly but significantly impairing conidial viabilities. Since organic silicon compounds have been recommended in previous reports, 11 surfactants of such compounds were included in the study but none of them was found being desirable for wetting the conidia. Based on wetting efficiency and compatibility, the surfactant NP-10 was a satisfactory wetting agent for B. bassiana conidia because it took only 5.2 sec to well wet conidia when incorporated into the formulation at 3% and had insignificant influence on the conidial viability. The negative effects of 9 dispersants on conidial germination increased with their proportions in most of the cases although all enhanced the stability of conidial suspension. In comparison to a blank control, the dispersant NPJ was exceptional and had no significant impact on conidial germination at the application rates of 1-3%. Thus, this dispersant was chosen for use in the wettable formulation at a desirable proportion of 1 %. In a comparative assay of two UV protentants, ascorbic acid (vitamin C) at 0.5% protected B. bassiana conidia better from damage by UV radiation than sodium carboxymethyl cellulose.Storage and shelf-life of wettable formulation. For the wettable formulation of B. bassiana conidia including 3% NP-10 as wetting agent, 1% NPJ as dispersant and 0.5% ascorbic acid as UV protectant, a 6-month storage at 25°C in dark resulted in a conidial viability of 80%, which was much higher than the viability of 55% observed from the unformulated conidia stored at the same regime. At the time of 12 months after storage, the formulated and unformulated conidial had viabilities of -60 and 8.7%, respectivelyo This indicates that B. bassiand. conidia in the wettable formulation were more tolearable to ambient storage than those unformulated. Moreover, it was confirmed that a residue rice powder (ground from rice grains from which conidia were grown and harvested) could be used for improving ambient storage of the wettable formulation by partially replacing diatomite as carrier. During an 18-month storage of wettable formulations containing thesame additives but different proportions (0, 20, 40, 60, 80 and 100%) of the rice powder, a carrier consisting of 40% rice powder and 60% diatomite was found favoring protection of conidial viability in the formulation. When B. bassiana was formulated in conjunction with 2.5, 5 and 10% of profurite aminium, however, the conidial viabilities in the combined formulations stored at either 4 or 25°C were greatly affected during a 6-month period of storage. At the time of 3 months after storage at 4°C, incorporation of 2.5, 5 and 10% of the chemical into the formulation resulted in viabilities of 89.7, 82.4 and 80.3% (95.2% in CK); such viabilities dropped to 35, 30.2 and 19.3%( 94% in CK) by the end of the storage. The much worse storage occurred at 25°C with the viabilities leaving 43.6, 19.6 and 14.3% (92.3% in CK) 3 months after storage but becoming undetectable at the end of the following month. Due to the undesired long-term compatibility with B. bassiana, the chemical insecticide could not be formulated together with fungal biocontrol agent although their positive interactions on P. xylostell may support in situ combination for field application.