Mortalidade de Spodoptera frugiperda (LEpidoptera: Noctuidae) exposta ao algodão Bt e à misturas compatíveis de produtos fitossanitários

Abstract

The challenges in controlling Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) have intensified due to the adaptability of this pest, its wide range of host plants, and its ability to develop resistance to active ingredients and to plants with the Bacillus thuringiensis (Bt) technology. In this context, the aim of this study was to evaluate the mortality of S. frugiperda on Bt cotton cultivars, test the compatibility of mixtures of phytosanitary products, and evaluate the mortality of larvae exposed to these mixtures. The experiments were conducted at the applied entomology laboratory (Laboratório de Entomologia Aplicada - LEA) of the Universidade Federal do Ceará (UFC) in partnership with commercial cotton producers in the state of Mato Grosso, and were divided into three bioassays. The first bioassay used leaves from three cotton cultivars (two Bt and one susceptible). 1st and 3rd instar larvae were fed on vegetative and reproductive parts within a climate-controlled room (25 ± 2 ºC, RH 75±10%, and 12-h photoperiod); mortality was evaluated at 72, 96, and 120 h after feeding. The trials were set up in a completely randomized design (CRD), with 10 replications (three larvae each) when using vegetative/leaf parts, and four replications (five larvae each) for the test with reproductive/flower bud structures. In the second bioassay, the compatibility of four mixtures of phytosanitary products (five replications per treatment) was evaluated, involving insecticides, acaricides, and fungicides, along with a control treatment (distilled water + adjuvant). Scores from 1 to 5 were attributed in the evaluation, according to the degree of stability of the mixtures. In the final bioassay, the efficiency of compatible mixtures and the action of individual products were evaluated. The treatments were distributed on the surface of an artificial diet, and provided to 2nd and 4th instar larvae. The trial was set up in a CRD with 10 replications (five larvae each). Larval mortality was evaluated at 24, 48, and 72 hours after exposure. No significant difference in larval mortality was found when comparing the two Bt cotton cultivars tested, but differences were found between instars in each treatment. 1st instar larvae exhibited higher mortality than 3rd instar larvae, except in the control. All the mixtures were physically compatible and effective (synergistic and additive effects), causing over 90% mortality in 2nd and 4th instar larvae. Therefore, results showed higher susceptibility of early instar larvae to Bt cotton, and the mixtures tested targeting S. frugiperda were found to be viable.