Efeitos letais e subletais de SfMNPV (Spodoptera frugiperda múltiplos nucleopoliedrovírus) sobre a lagarta do cartucho em soja Bt e não Bt

Abstract

Brazil is currently the largest producer of soy (Glycine max (L.) Merril) in the world. However, in recent years it has been suffering from the attack of the caterpillar Spodoptera frugiperda (J.E. Smith), considered the main pest of the maize crop (Zea mays L.). In soybean culture, this fact is mainly justified by the low susceptibility of the pest to Cry1Ac protein expressed in Bt soybean and also the evolution of resistance to different synthetic insecticides. The adoption of nucleopolyhedrovirus (SfMNPV - Baculoviridae: Alphabaculovirus) for the management of the species may be an alternative for use in soybean culture. Therefore, this study aimed to evaluate, under laboratory conditions, the lethal and sublethal effects of SfMNPV on S. frugiperda neonates on Bt soy expressing protein Cry1Ac and non Bt soy (isoline). After spraying with different concentrations of SfMNPV on plants grown in a greenhouse, the following biological parameters were evaluated: survival at ten days of age, duration of the period from larva to pupae, larval and pupal weight, fecundity and fertility. Furthermore, the fertility life table was calculated. Results indicated that S. frugiperda neonates did not survive infection on Bt and non Bt soybeans sprayed with the recommended field dose of SfMNPV (7.50 × 109 occlusion bodies/mL (OBs) of SfMNPV). In contrast, S. frugiperda neonates infected with the lowest concentrations of SfMNPV [(7.50 × 108 OBs/mL - 10% of field dose, 1.88 × 109 OBs /mL - 25% of field dose and 3.75 × 109 OBs/mL - 50% of field dose)] in Bt and non Bt soybeans were able to complete the biological cycle to adulthood. However, caterpillars surviving on Bt and non Bt soybeans, when infected with concentrations of 25% and 50% of the field dose, showed longer developmental time (larva to pupae), lower larval and pupal weight, in addition to reductions in fecundity and fertility. Sublethal effects in the larval stage negatively affected the fertility life table parameters with a lower reproductive rate and lower population growth of the species in Bt and non Bt soybeans. The results found in the present work revealed that, in addition to mortality, the bioinsecticide containing SfMNPV also causes significant sublethal toxicities in S. frugiperda neonates, which demonstrates a synergistic effect between SfMNPV and Bt soy expressing the Cry1Ac protein. When used at concentrations recommended by the manufacturer, it was concluded that the use of the SfMNPV virus proved to be a promising alternative in the management of S. frugiperda neonates, and its use in integrated pest management programs is opportune.