ENTOMOPATHOGENICITY OF Beauveria bassiana (BALS.) VUILLEMIN TO CERTAIN LARVAL INSTARS OF THREE CORN BORERS UNDER LABORATORY CONDITIONS Metwally, M. M.

Laboratory experiments were conducted in 2008 at biological control department, plant protection research institute Sakha Kafr El-Sheikh station to study the efficacy of the fungus Beauveria bassiana (Bals.). An isolate collected from Sesamia cretica larvae in Kafr El-Sheikh region and cultured on potato dextrose agar medium (PDA) at 25 °C spore suspensions at concentrations of 1x10^7, 2.5x1067, 5x10^7, 7.5x10^7 and 1x10^8 spore/ml were prepared in distilled water containing 0.1% Tween 80. The larvae were individually treated by dipping method in the fungus suspension and bioassayed against the third larval instar of each species, Ostrinia nubilalis (Hbn.), Sesamia cretica (Led.) and Chilo agamemnon (Bles.) incubated at 25+1°C and 75+5 % RH). All tested concentrations induced different mortalities. For O. nubilalis L3, mortality ranged between 5-100 % was reached at 4-10 days with the all tested concentrations LC50 value was 1.2127x107 after 12 days while LC_90 reached 9.7625x10^7 spore/ml, respectively, meanwhile LT50 values reached 12.7, 10.5, 8.6, 8.2 and 7.6 days with the tested concentrations of 1x10^7, 2.5x10^7, 5x10^7, 7.5x10^7 and 1x10^8 spore/ml, respectively. As for S. cretica L3, the highest mortality (75-90%) was achieved at 12-14 days post treatment. LC_50 value was 8.5037x10^7 while LC_90 was 15.05x10^7 spore/ml at the same mentioned period meanwhile, LT_50 values reached 17.4, 14.8, 11.0, 10.7 and 8.5 days by all tested concentrations. In case of C. agamemnon L3 the highest mortality reached 80-90 % after 12-14 days of treatment. LC50 value was 1.5884x10^7, while LC_90 was 9.8329x10^7 spore/ml., at 14 days post treatment, LT_50 values were 15.5, 11.8, 8.8, 8.6 and 8.1 days for the concentrations of 1x10^7, 2.5x10^7, 5x10^7, 7.5x10^7 and 1x10^8 spore/ml, respectively.