Interação de inseticidas e controle biológico natural na redução dos danos de Spodoptera frugiperda (J.E. SMITH, 1797) (Lepidoptera: Noctuidae) na cultura do milho (Zea mays).
Figueiredo, Maria de Lourdes Corrêa
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All the experiments were conducted inside of the experimental area of Embrapa Milho e Sorgo (Embrapa National Maize and Sorghum Research Center) in Sete Lagoas, MG, Brazil with the objective of evaluating the role of natural enemies and insecticides on the damages of Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae) in maize crop. In the first experiment the occurrence of natural enemies associated to eggs and larvae of S. frugiperda using artificial infestation with laboratory egg masses was evaluated, accomplished fifteen days after the plant emergency (one egg mass/m2). At a 2-day interval, beginning two days after infestation, sampling of 20 plants/plot were accomplished, separating egg masses and larvae of the pest, which were maintained in artificial diet, in the laboratory. The natural enemies presented in the experiment were the predators Doru luteipes Scudder (Dermaptera: Forficulidae) and Orius sp. (Heteroptera: Anthocoridae) and the parasitoids Chelonus insularis (Cresson) (Hymenoptera: Braconidae), representing 91.07% of the collected species, Eiphosoma laphygmae Costa Lima (Hymenoptera: Ichneumonidae) Exasticolus fuscicornis (Cameron) (Hymenoptera: Braconidae), Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae), Campoletis flavicincta (Ashmead) (Hymenoptera: Ichneumonidae) and Archytas incertus (Macquart) (Diptera: Tachinidae). Parasitoids and predators had significant effect on the suppression of the population of S. frugiperda. The number of collected larvae that was 364 at the first sampling period fell down to only 2.6 larvae, at the sampling accomplished 16 days after the infestation. Using the exclusion method (with cage), protecting the pest since egg stage against its natural enemies, it was demonstrated that the larger the pest protection period, the greater the damage. The visual damage scale reached 4.01 (maximum of 5) when the protection period was 16 days. When the protection period was minimum (two days), the average damage scale was only 0.93. As a consequence of the differences in the provoked damage, there was reduction of 52.73% in the production of dry matter and 45.51% in the production of grain yield. In another experiment, exposing the pest to its natural enemies, starting from the eggs, and then, protecting by the cages for different periods, the leaf damage in the plots in that the pest was protected by the largest period was also the highest (3.43). In relative terms, there were reductions of 54.2% in the dry matter production and of 20% in the grain yield. Experiments to evaluate the impact of different groups of insecticides and the possible additional contribution of natural control agents on the suppression of S. frugiperda were also developed, using also egg masses of the pest. The insecticides were applied at 2-day intervals, beginning two days after the infestation and finishing 16 days after. Before spraying, the natural parasitism varied from 27.7% to 54.0%. After the application of the insecticide Match the parasitism was 9.7% (evaluation 24 h after the application) and 1.5% (72 h after the application). Those values were, for the insecticide Fury, 23.37 and 18.23% respectively. For the insecticide Lorsban, the index was 27.3 and 28.2%. Higher index of parasitism was verified when the insecticide microbial Baculovirus spodoptera was applied, respectively, 41.3 and 33.7%. In a general way, all the products protect the plant against the pest acting by themselves or together with other mortality factors. Among such factors, the presence of the predator D. luteipes can be mentioned, especially on plots sprayed with B. spodoptera and Match. For the insecticide Fury, the population of the predator was lower in the 24-h evaluation period than in the 72-h period after application. With relation to the parasitoids, soon after the application of the chemical products, parasitism was much lower than the parasitism observed before spraying. Actually, the number of living larvae also decreased. That reduction can be due to the direct action of insecticides and/or the effect of natural enemies. For instance, the decrease of collected larvae coincided with to growing presence of the predator D. luteipes on plots, before and after the application, especially on those plots where the B. spodoptera and Match were used. The reduction in the number of larvae brought, as a consequence, a significant reduction in the damage caused by the pest. There was no significant difference in grain yields from plots where B. spodoptera, Match and Fury were used. In the case of the insecticide Lorsban, there were some variations in grain yields, probably due to your immediate effect on the larvae and/or killing or repelling the predator D. luteipes. New pest infestation could occur later. Significant occurrence of natural enemies was observed in the experimental area. The density of the natural enemies was enough to give an effective control of the pest. It was concluded that such natural enemies could complete the action of certain insecticides, especially those of high level of selectivity. Considering that the natural enemy observed in the area is associated to eggs and first instar larvae of the pest, it is important in integrated pest management program that a control measure is used starting 12 days after the verification of egg masses in the field, for the control of remaining larvae, and only if their density reaches the level of economical damage.