Virulencia de tres cepas locales de Bacillus thuringiensis (Berliner) sobre el gusano cogollero del maíz, bajo condiciones in vitro.

Abstract

The objective of the research was to characterize morphologically and evaluate the mortality caused by three local strains isolated from two agricultural soils and one from pine forest soil, possibly of Bacillus thuringiensis (Berliner) against Spodoptera frugiperda (J. E. Smith), under outdoor conditions. vitro. The fall armyworm is a primary pest that causes great damage and economic losses to Poaceae crop producers, mainly corn and dicotyledonous plants. Likewise, the acute toxicity (LC50) and TL50 of the three strains were evaluated at concentrations of 1x104 , 1x106 , 1x108 , 1x1010, 1x1012 CFU/ml and an absolute control on neonatal larvae. A randomized design was used with six treatments, five repetitions, repetition with five neonate larvae for each of the three strains. Mortality was evaluated every 12 hours until the total death of the larvae or the emergence of adults. The data were analyzed by means of SPSS Software and Probit Tests to calculate LC50, transforming the concentrations to log10 and the mortalities to Probit, and with the accumulated mortalities and the time of death, the TL50 was found, by means of Regression Analysis. The strains isolated from agricultural soils and the strain isolated from pine forest soils presented characteristics similar to bacilli of the genus Bacillus spp., presenting cream-colored colonies, irregular and rounded edges, with the appearance of a fried egg. Based on the morphology and the inclusion of the crystal next to the spores in the three strains studied, it was concluded with high possibilities that they are strains of B. thuringiensis. All concentrations of the three strains caused mortalities greater than 92% in neonatal larvae of S. frugiperda. The LC50 values obtained allow us to affirm that the three strains tested exhibit an insecticidal mode of action and may be a promising resource for the control of S. frugiperda populations in the field. The 1x108 CFU/ml concentration of the three bioassay strains showed very similar Median Lethal Times; thus, the TL50, were 56; 61 and 57 hours, respectively for strains 1, strain 2 and strain 3; which is why the three strains are presented as an excellent alternative as a biological controller of S. frugiperda, which should be included in future research work, since they kill 50% of the population between the second and third day of their application.