População, danos e parasitoides larvais de spodoptera frugiperda (j. e smith) (lepidoptera: noctuidae) em milho bt e não-bt

Abstract

The occurrence of Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), the Fall armyworm, has led to increased adoption of Bt maize genotypes in Brazil. However, the field impacts on parasitoids of using these genotypes, especially those associated with the larval stage of this insect pest, are little known. The aim of this study was to evaluate two different Bt maize genotypes and its isoline (non-Bt) on Fall armyworm larvae population, its damage to maize plants and its larval parasitoids in early and late cropping season (sowing dates). The experiments were conducted in areas of Universidade Federal de Santa Maria (UFSM), Santa Maria, Rio Grande do Sul State, which were grown three maize areas in both early an late sowing, with the treatments: control (non-Bt), YieldGard®, expressing the Bt toxin Cry1Ab and Herculex®, which expresses the Bt toxin Cry1F, all comercial hybrids 30F53 and isolines among them. The area of each treatment was divided into 20 points, 36 m2 each, composed by 12 rows of 6 m, where 4 plants per point were randomly sampled, totaling 80 sampled plants per treatment. In early and late season, 14 maize plant surveys were made and afterwards transferred to the laboratory and evaluated for: number of S. frugiperda egg masses, total and percentage of collected larvae, in different size ranges (up to 0.5 , 0.6 to 1.0, 1.1 to 1.5, 1.6 to 2.0 and > 2.0 cm), total and average larvae per plant in different size ranges, percentage of plants without damage, with scratches and leaf holes on the whorl; percentage and mean number of parasitized larvae, number of emerged parasitoids and percentage of parasitism by the main parasitoids found. The results on percentage were compared among evaluation dates by t test between two proportions. For the other results t test for two independent samplings with Bootstrap resampling (10000 simulations). There was a greater number of egg masses in Bt Herculex® and Bt YieldGard® genotypes, in early and late sowing dates, respectively, with a predominance of up to 0.5 cm size larvae, especially in Bt genotypes, and higher larvae mortality by Bt Herculex® than Bt YieldGard®. Undamaged plants in the whorl predominate in Bt Herculex® area, in both sowing seasons, and the low incidence of whorl leaf wholes was predominant in both Bt genotypes in both sowing seasons. The predominance among larval parasiotids was of Campoletis flavicincta (Ashmead, 1890) (Hymenoptera: Ichneumonidae) and Chelonus insularis Cresson, 1865 (Hymenoptera: Braconidae), in early and late sowing seasons, respectively. Parasitized larvae occurred more frequently in late sowing season, for the three genotypes with higher parasitism in non-Bt maize and Bt YieldGard® in early and late seasons, respectively. The low amount of parasitoids found in Bt Herculex® was associated with the low larvae survivorship.