Evaluation of the insecticidal activity of natives strains of Bacillus thuringiensis against Spodoptera frugiperda (Smith)
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Abstract
A bioassay method was developed to determine in artificial diet the insecticidal activity ofBacillus thi,ringiensis native strains against first instar larvae of Spodoptera .frupperda. Bioassays were conducted at 25°C of temperature, 70% of relative humidity, and a photoperiod of 12 hours-light. Native and reference strains were analyzed by PCR technology and SDS-PAGE in order to select strains containing cry I insecticide crystal family genes with toxicity against Lepidopteran insects. Bacteria concentrations were expressed as total protein and were incorporated in the surface of the diet. The mortality percentage was evaluated daily for 120 hours. In preliminary studies, reference strains H13-1 van kurstaki and HD- 137 var. aizawav evaluated as a unique concentration of 150 jsg of total protein/ml permitted choose reference strain HD- 137 as a positive control for bioassays. Using this reference strain, native strains Bt-127, Bt-147, Bt-292, Bt-444 were evaluated. Native strain Bt-127 exhibited higher insecticidal activity than all other strains. This strain showed in first instar larvae 88% mortality, a lethal time 50 (LT50) of 24 hours, and a lethal concentration 50 (LC50) of 83 sg/ml. The reference strain HD-137 showed an insecticidal activity significantly lower than native strain Bt-127; results were represented by a mortality percentage of 79%, an LT50 of 48 hours, and an LC50 of 138 pg/ml. The insecticidal effect of the reference strain was followed by native strain Bt- 147. Native strains Bt-292 and Bt-444 showed insignificant mortality percentages.
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