Seleção e caracterização molecular de cepas de Bacillus thuringiensis visando o controle de noctuídeos-praga

Abstract

Several species of noctuids are important pests of the soybean crop, among them Helicoverpa armigera, Spodoptera cosmioides and Spodoptera eridania. Control strategies become important due to the economic damage caused by these species. Among the most sustainable ways to control outbreaks of these and other pests is the use of biopesticides and genetically modified plants from the bacterium Bacillus thuringiensis (Bt). The search for Bt strains with toxic activity against more than one agricultural pest is of extreme importance for Integrated Pest Management. Therefore, this study aimed to select and characterize strains of Bacillus thuringiensis with toxic activity for Helicoverpa armigera, Spodoptera cosmioides and Spodoptera eridania. Fifty strains of Bacillus thuringiensis were used in selective bioassays for Helicoverpa armigera, and four of these were tested on Spodoptera cosmioides and Spodoptera eridania. Twelve strains of Bacillus thuringiensis were characterized by PCR for the detection of some cry, cyt and vip genes. Bioassays were also performed to identify the production of β-exotoxins and the presence of Vips proteins. In addition, LC 50 and CL 90 were determined in Spodoptera eridania caterpillars for strains 237, 1368, 1521, 1814 and 3299. Strain 237 was the most efficient for the control of Helicoverpa armigera causing mortality of 95.6% while strains 1368, 1521, 1814 and 3299 showed toxic activity for Spodoptera eridania. However, no strain was effective against Spodoptera cosmioides. The presence of β-exotoxin was detected only in the supernatant of strain 237. As five strains tested for LC 50 were similar to HD1 control. However, as strains 1368, 1521 and 1814 presented lower CL90 for Spodoptera eridania caterpillars. The molecular characterization of the strains using specific primers for cry, cyt and vip genes demonstrated great genetic diversity among the 12 strains tested that present some activity for Helicoverpa armigera. Through bioassays with the supernatants of the four strains tested in Spodoptera eridania it was possible to detect an activity of Vips proteins corroborating with the results of the molecular characterization, which demonstrated a presence of some vip genes. The results obtained prove that the strain 237 is active against Helicoverpa armigera, but due to its ability to synthesize β-exotoxin may not be used for the formulation of biopesticides, being useful in programs of genetic transformation of plants. As strains 1368, 1521, 1814 and 3299, even without toxicity to Helicoverpa armigera, were toxic to Spodoptera eridania and did not produce β-exotoxin, being indicated both for the production of new formulations of biopesticides and for the transformation of Bt plants.