Laboratory And Field Evaluation On The Interactive Effects Of An Oil-Based Emulsifiable Formulation Of Beauveria Bassiana Conidia And Sublethal Rates Of Imidacloprid Against Sucking Insect Pests

The entomopathogenic fungus, Beauveria bassiana (Balsamo) Vuillemin, is a classic fungal biocontrol agent and has been developed into a variety of commercial formulations for insect control in the world. Technical problems involved in ambient storage, slow action, and unstable field efficacy have been for a long run obstacles to commercial development and practical use of B. bassiana-based mycoinsecticides for insect control. Based on previous studies on B. bassiana SG8702 in our laboratory, the present study was performed to determine the shelf-life of an oil-based formulation of B. bassiana conidia at ambient temperature, to search for more efficient emulsifier to ease field spray of the formulation, and to quantitatively evaluate separate or interactive effects of the emulsifiable conidial formulation and sublethal imidacloprid application rates on the control of sucking insect pests such as aphids, planthoppers and leafhoppers under laboratory and field conditions. Our objectives were to facilitate technical progress in formulation and integration of B. bassiana into insect pest management systems.Ambient storage of oil-based conidia] formulation and its optimal emulsifier for field spray. During an 18-month period of storage at ambient temperatures in dark, the oil-based B. bassiana formulation with or without additives (suspension stabilizer, UV protectant) was very stable during the first 7 months. On average, 95% of the conidia were viable at the end of 7 months, followed by >75% at the end of 10 months. Thereafter, however, the viability of the conidia in the formulation containing the additives decreased rapidly and was not detected at the end of 16 months. In contrast, the formulation not containing the additives had conidial viabilities of 57%, 51%, and 42% on months 16, 17, and 18, respectively. Apparently, the shelf-lives of the two formulations at ambient temperatures satisfied a general requirement for commercial development of mycoinsecticides. A novel emulsifier AE-3 was found to best emulsify 1000-fold aqueous dilution of the oil-based conidial formulation for field spray at a proportion of around 5% of the formulation. However, the biological compatibility of the emulsifier to B. bassiana conidia was less satisfactory although it was much better than other emulsifiers tested. Inclusion of the emulsifier in the conidial formulation allowed for only 45-day shelf-life of its storage at ambient temperature.To take both emulsification and shelf life into account, the emulsifier should not been included in the conidial formulation until aqueous dilutions were made prior to field sprays.Synergistic interaction between B, bassiana and imidacloprid. Interactive effects of B. bassiana SG8702 and imidacloprid were assayed on Nilaparvata lugens under laboratory conditions by spraying suspensions of unformulated conidia (assay 1) and aqueous dilutions of its emulsifiable formulation alone (assay 2) or together with imidacloprid at the sublethal application rates of a.i. 0.5, 1.0 and 2.0 g/mL (assays 3-5). Each assay included 5 conidial concentrations (no. conidia/mm2) plus a control, and replicated 3 times with 30-40 third instar brachypterous nymphs per replicate. It was found that an emulsifable, mineral oil-based liquid suited to formulate B. bassiana by enhancing fungal activity but not significantly increased background mortality of N. lugens. Synergistic effects of both agents or formulations were determined by estimating relative potencies (paired ratios of LC50 estimates on days 5-12 after spray) of assay 2 over assay 1 (1.2-3.6), assay 3 over assay 2 (1.3-2.0), assay 4 over assay 2 (6.2-8.5), assay 5 over assay 2 (22.7-72.4), assay 4 over assay 3 (3.7-5.6), assay 5 over assay 3 (16.0-35.6), and assay 5 over assay 4 (3.0-8.4). The results demonstrated in detail that a time-dose-mortality modeling method was not only mathematically but al50 biologically robust to evaluate interactive effects of B. bassiana and imidacloprid on N. lugens. Enhanced fungal formulations, compared to their counter...