Respostas induzidas de defesa por Beauveria bassiana, Metarhizium anisopliae e jasmonato de metila a Spodoptera frugiperda (Lepidoptera: Noctuidae) em soja

Abstract

Several lepidopteran pests can reduce soybean productivity through defoliation, such as the fall armyworm Spodoptera frugiperda (J. E. Smith). The use of resistance elicitors is a viable alternative in pest management, as they act on plant metabolism, inducing resistance. There is evidence that entomopathogenic fungi, in addition to causing direct effects on target insects, can also induce resistance in plants; however, few studies have evaluated these effects through foliar application in soybeans. This study evaluated the defense effects induced by foliar application of Beauveria bassiana and Metarhizium anisopliae, in comparison with methyl jasmonate, against S. frugiperda. The experiment was conducted in the field, greenhouse, and laboratory with the following treatments: control (water); B. bassiana; M. anisopliae; and methyl jasmonate (MeJA), with two foliar sprays at the beginning of vegetative development. In the field, productivity was evaluated (number of pods, pod weight, grain weight, and 100-grain weight) to verify if the products stimulated production. In the greenhouse, soybean plant growth was evaluated in the presence and absence of herbivory by S. frugiperda, recording plant height, diameter, chlorophyll content, and shoot and root dry mass, in addition to the stress markers hydrogen peroxide (H2O2), malondialdehyde (MDA), and the activities of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). In the laboratory, the biological performance of S. frugiperda was evaluated on soybean leaf disks, and fungal recovery from the plants was assessed to investigate possible endophytic action. In the field experiment, no significant differences were observed in productivity parameters or in the H2O2 and MDA stress markers. Regarding the biological performance of S. frugiperda, larvae fed with leaf disks treated with M. anisopliae showed reduced weight gain, with no differences in leaf consumption. In the greenhouse, no differences were detected in soybean plant growth parameters in the presence or absence of herbivory. In the laboratory bioassay, the highest leaf consumption by larvae occurred on disks treated with B. bassiana, with no differences in weight gain. M. anisopliae caused higher production of H2O2, while M. anisopliae and B. bassiana showed lower levels of lipid peroxidation (MDA), indicating reduced oxidative stress with the fungi. Regarding enzyme activities in soybean plants subjected to herbivory by S. frugiperda, treatment with M. anisopliae induced higher SOD activity under herbivory, while B. bassiana stimulated higher CAT and APX activities, explaining the reduced oxidative stress with the fungi. The presence of entomopathogenic fungi could not be detected from leaf fragments collected from treated plants, showing no evidence of endophytic action. The results suggest that elicitors can trigger induced defense responses in soybean plants against S. frugiperda, mediated by the accumulation of reactive oxygen species and the activities of antioxidant enzymes.