Prospecção de microrganismos para o controle de insetos-praga nas culturas da soja e do algodão

Abstract

The occurrence of insect pests in crops of high economic value directly affects the yield of plants and grains. This scenario led to the mass investigation of chemical products that overcome these adversities and provide control potential. Nonetheless, over the years, there has been an intense exploration of this type of strategy, resulting in high production costs, generation of waste harmful to the environment, and resistance of target insects. The adoption of alternative practices, such as the formulation and production of products of microbial origin, emerges as an encouraging tool compared to control alternatives, indicating a sustainability bias, and allowing a reduction in the risks of human and animal contamination. The objective of this study was to perform a search for microorganisms with bioinsecticide effects, through bioscreening, for the management of insect pests in soybean and cotton crops. Initially, dead insects were collected in production areas where invasive species control was not performed. Subsequently, the samples obtained were superficially sterilized with 70% ethanol, 0.5% NaOCl solution, and sterilized water in three consecutive washes, for 3 minutes each. Immediately, the insects were placed in Petri dishes with 25 mL of Potato Dextrose Agar (BDA) solution to check whether or not there was microbial growth. Furthermore, the Petri dishes were placed in an incubator at 25 ºC for one week. Subsequently, the isolated microorganisms were subjected to the submerged fermentation process, at 28 ºC and 120 rotations per minute (rpm), for seven days. The fermented product was filtered using a vacuum pump and subjected to successive centrifuges at 3,200 x g for 10 minutes. The biological broths were applied to insect pests to test potential toxicity. The insect pests used as a reference for control were: Euschistus heros (neotropical brown stink bug), Anticarsia gemmatalis (soybean caterpillar), Helicoverpa armigera (old world bollworm), Chrysodeixis includens (soybean looper caterpillar), Spodoptera frugiperda (caterpillar-cartridge), Spodoptera eridania (Southern armyworm), Helicoverpa zea (corn earworm), Spodoptera cosmioides (black armyworm), and Elasmopalpus lignosellus (lesser cornstalk borer). A total of 163 microorganisms were obtained from the bioscreening strategy. A total of 50 microorganisms were- pre-selected based on mortality rates (%) of insect pests. Afterwards, the best results for each insect pest species were obtained through a two-parameter log-logistic model with binomial distribution, based on mortality data. Based on the model and a previous classification at the genus level, eight potential microorganisms were obtained, five of which were classified as fungi (FT4.1.1, A6.1, OL1, C7, and MI5) and three as bacteria (BR7, P1, and BR3.2). Furthermore, dilutions of the fermented broths of each microorganism were conducted (n ×10-5, n ×10-6, n ×10-7, and n ×10-8 spores mL-1). Mortality was maximum (100%) for H. zea and E. heros. Other encouraging results were indicated in the control of A. gemmatalis and C. includens (up to 87.5%) and E. lignosellus (up to approximately 83.5%). For H. armigera, mortality reached 75%. Fungal isolate A6.1 was identified based on the ITS sequence as Talaromyces piceae. The fungi C7, CL1, and M15 were also identified as T. piceae, but based on the sequence of the 28S ribosomal gene. Similarly, isolate FT4.1.14 was identified as T. piceae based on the beta-tubulin gene sequence. Among the bacteria, based on sequencing of the 16S ribosomal gene, isolate BR3.2 was identified as Lysinibacillus fusiformis, isolate BR7 as Paenibacillus ottowii, and isolate P1 as Clostridium sphenoides. It was considered that the results obtained were relevant to the scientific community as they are part of a line of research at the frontier of knowledge and, especially, are interesting for companies that are operating in this field in the agricultural scenario.