Efficacy of selected Beauveria bassiana (Bols.) Vuill. isolates in combination with a resistant cotton variety (PSB-Ct 9) against the cotton boll worms, Helicoverpa armigera (Hubner) (Lepidoptera:Noctuidae)
2004
Soetopo, D.
The goal of this study is to identify the most virulent among the 12 Beauveria bassiana isolates from the NCPC [National Crop Protection Center, College, Laguna, Philippines] collection and the possibility of using the best isolate in combination with the recommended resistant cotton variety for the control of Helicoverpa armigera. This study was conducted at the University of the Philippines Los Banos [College, Laguna, Philippines] from July 2002 until January 2004. In the laboratory, the tested isolates included BbEd2, BbEd5, BbEd6, BbEd7, BbEd8, BbEd9, BbEd10, Bb4a, Bb3a and the two standard isolates Bb1 and Fb4. Bb4a was the most virulent with LT50 of 9.44 plus or minus 0.55 days at 1 x 10E8 conidia per ml, LC50 of 3.94 x 10E9, 3.08 x 10E8 and 5.22 x 10E6 conidia per ml at 8, 12 and 16 days, respectively, and LC90 of 4.35 x 10E12, 3.13 x 10E12 and 6.20 x 10E9 conidia per ml at 8, 12, and 16 days, respectively. The effective dose range were 1 x 10E10, 5 x 10E9 and 1 x 10E9 conidia per ml or equal to 1 x 10E13, 5 x 10E12, and 1 x 10E12 conidia per l. Bb4a also showed the best performance in the screen house test and caused more than 90% mortality of H. armigera larvae at all conidia concentrations. All tested isolates were able to survive two months and still maintained their ability to infect the larvae, though their virulence decreased by more than 50%. In the field trials, that plots with a cotton resistant variety PSB-Ct9 and treated with the best isolate Bb4a, significantly had lower bollworm larvae infestation than standard and control, but there was no interaction between variety and isolate. The population of the bollworm, however, was quite low during the field trials.
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