Controle de lepidópteros-praga nas culturas de soja e milho com Metarhizium rileyi (Farlow) (Hypocreales: Clavicipitaceae)

Abstract

The objective of this study was to evaluate the susceptibility of Chrysodeixis includens (Walker, 1857) (Lepidoptera: Noctuidae) and Spodoptera frugiperda (Smith, 1797) (Lepidoptera: Noctuidae) to isolates of Metarhizium rileyi (Farlow) (Hypocreales: Clavicipitaceae) and the application impact of this pathogen in the corn crop on S. frugiperda and natural enemies. Laboratory bioassays were carried and three isolates [CG1312 (C. includens as original host), CG381 (S. frugiperda as original host) and a new wild isolate (C. includens as original host)] were tested against larvae of C. includens and S. frugiperda, in third instar age, by treatment of surfaces with conidial suspension. To evaluate the M. rileyi efficacy in the control of S. frugiperda in field conditions, the following treatments were evaluated: applications of M. rileyi fungus, applications of commercial chemical insecticides and a negative control without the use of any agent against caterpillars. The experiments were conducted in the maize crop seasons of 2017 and 2018 (February-June), after soybean harvest. Through molecular characterization by ISSR (Inter Simple Sequence Repeat) the application influence of M. rileyi on the abundance and genetic diversity of M. rileyi isolates that infect S. frugiperda was verified. The possible impacts of the control with M. rileyi on the community of natural enemies, were evaluated through biweekly samples using pitfall traps, installed in the center of each plot. With the results of the laboratory bioassays it was possible to verify that the three isolates of M. rileyi caused similar mortality rates for C. includens (53-58%) and S. frugiperda (74-84%), therefore, there was no influence of the original hosts in the pathogenicity of the isolates evaluated. When evaluating the efficiency of M. rileyi in the field, we verified in the first year that there was no significant difference in the number of S. frugiperda caterpillars infected by M. rileyi among the treatments. However, we verified that after the sprays carried out in the second year, the number of caterpillars killed by M. rileyi was higher in plots treated with the fungus. We also verified through the molecular characterization by ISSR that although the presence of native isolates had occurred in all treatments during the two evaluated crop seasons (2017 and 2018), in 2018, in the plots treated with the fungus, a greater number of caterpillars infected with strains belonging to the genetic group of the introduced strain. In addition, the application of the M. rileyi isolate did not show a significant impact on the community of natural enemies in the corn crop.