Effects of adjuvant and conidial concentration on the efficacy of Beauveria bassiana for the control of the two spotted spider mite, Tetranychus urticae

Abstract

Greenhouse experiments were conducted on various crops (cucumber, tomato, eggplant, green bean) to ascertain the effects of Break-thru (polyether-polymethylsiloxane-copolymer, a silicone surfactant) and an oil emulsion, on Beauveria bassiana (Balsamo) Vuillemin (Bb) applications for the control of the two spotted spider mite, Tetranychus urticae Koch. The objectives were to compare a) the efficacy of Bb control when applied in aqueous Break-thru or an oil emulsion; b) the effects of various concentrations of Bb conidia, as affected by each surfactant; and c) the effects of Break-thru on the activity of the fungus. Conidia were suspended either in an aqueous Break-thru or an emulsifiable formulation at different conidial concentrations (1.05 x 10(6), 2.1 x 10(6) and 4.2 x 10(6) conidia ml(-1)) and sprayed onto leaves 2 weeks after artificial pest inoculation. Two sprays were performed, with an interval of one week from one spray to another, and T. urticae population counts (both motile and egg stages) were made on plant leaves 7 days after each spray. Bb conidia in Break-thru((R)) were more efficacious than conidia in emulsifiable formulation. With the highest rate of conidia (4.2 x 10(6) conidia ml(-1)), mortality of adult mites ranged from 60 +/- 4.2 (mean +/- SE) to 85.7 +/- 4.3% in the Break-thru suspension and 39.4 +/- 7 to 61.3 +/- 6% in the oil emulsion. Leaf damage index was also substantially reduced from 70% in the unsprayed control to 40% by the application of Bb conidia at the highest rate with Break-thru. Break-thru can be combined with Bb in the integrated management of T. urticae and Isolate R444 is a promising candidate for the control of the pest.