Desenvolvimento de formulações de Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV): efeito da radiação ultravioleta, do armazenamento e do substrato alimentar
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Data
2019-10-23
Autores
Paiva, Carlos Eduardo Costa
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Universidade Federal do Espírito Santo
Resumo
Rising is the industry's interest in the manufacture of microbial insecticides, including Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV) -based products that are effective in controlling the fall armyworm, Spodoptera frugiperda J. E. Smith (Lepidoptera: Noctuidae). However, limiting factors for the production of baculovirusbased bioinsecticides are the high production cost and rapid inactivation by ultraviolet (UV) radiation, which reduces the persistence of viruses in the field and, consequently, the efficiency of the products. Moreover, the efficiency of these products is influenced by the food substrate and most of the time studies focus on determining the efficiency of a product on the pest when applied to the preferred pest crop, but in the case of polyphagous pests such as S. frugiperda, considered the main maize (Zea mays L.) crop in Brazil and which feeds on several other plant species, including cassava (Manihot esculenta Crantz), there are no studies on the effects of SfMNPV baculovirus application in cassava leaves on the mortality of S. frugiperda caterpillars. The works were divided into three types of bioassays, with different objectives: in the first, the objective was the formulation of products based on SfMNPV and the evaluation of the pathogenicity and virulence of these formulations on S. frugiperda caterpillars, and storage for 12 months in an uncontrolled environment, ie without temperature, humidity and photoperiod control; in the second, the effects of ultraviolet radiation (UV) on the quality of these formulations were studied; and, third, the feasibility of applying the SfMNPV virus to control cassava leaf-fed S. frugiperda caterpillars. In the first study nine formulations of SfMNPV were evaluated, six as wettable powder (WP) and three as emulsifiable oil (EO), which were compared with each other and with a commercial product. The inert materials tested were: kaolinite; commercial clay (clay soil); maize starch; coloral (annatto dye); commercial clay + corn starch (5%) + coloral (5%); corn meal; and corn oil. In the case of EO formulations, all were made with corn oil (50% of the formulation), but there was a treatment in which only corn oil was added, a treatment in which annatto essential oil was added (5% of the formulation) and another in which annatto and copaiba essential oils were added (5% and 0.5% of the formulation, respectively). Semiannual efficiency evaluations of baculovirus batches, applied on corn leaves as food substrate, were carried out to control S. frugiperda. Mortality, survival time, amount of virus present in the formulated during storage, percentage of original activity remaining (AOR) and efficiency of each formulated were evaluated. In the second study, all products mentioned in the first trial plus a viral suspension prepared with the unformulated virus (semi-purified SfMNPV isolate 6) were exposed to UV radiation (254 nm) for 30 minutes immediately after application to leaves of corn and offered to second instar S. frugiperda caterpillars (four days old). In the third study viral suspensions of the semi-purified SfMNPV isolate 6 and of the commercial formulation (CartuchoVit®) with different concentrations (2 x 105 , 2 x 106 and 2 x 107 OB/mL) were applied to the food substrates (maize leaves or cassava) which were then offered to second instar S. frugiperda caterpillars. In the first trial it was found that soon after manufacture, products formulated as PM caused the highest mortality (between 93 and 98%), especially commercial clay, which is the cheapest inert (R$ 0.01 per dose for one hectare), and with the exception of the commercial xii product which is also a WP and caused 86% mortality. However, virulence declined for all products, mainly after 12 months of storage. Considering the mortality variables at time 0 (soon after formulation), after storage for six and 12 months, applying a constant dose, or adjusting the dose to obtain a viral suspension containing 2.00 x 106 OB/mL, the virus loss observed in this period, the original remaining activity (AOR) and the virus efficiency during storage, the kaolinite formulated product presented the best performance, compared to the other formulated ones. In the second trial all products caused similar mortality of second instar S. frugiperda caterpillars in the absence of UV radiation exposure, ranging from 80 to 97%. Exposure to UV radiation (254 nm) for 30 minutes reduced virulence in all treatments, however the baculovirus formulation helped preserve virulence, especially those based on clay and/or annatto, which maintained the percentages remaining original activity (AOR) ranging from 67 to 70%, while the unformulated virus had 5% AOR. Results from the third trial indicated that mortality was higher when the caterpillars fed on SfMNPV-treated cassava leaves, considering the food substrates, cassava leaves and corn. Therefore, kaolinite is the most recommended inert material for the large-scale production of a SfMNPV-based bioinsecticide for storage at room temperature and should not exceed six months in these circumstances. However, commercial clay baculovirus (WP) formulation is also a viable alternative for large-scale production of SfMNPV, mainly due to its low cost and increased virus protection against the deleterious effects of UV radiation. Also, as there was no detrimental effect on the virulence of entomopathogenic virus based on the food substrate, it is possible to use SfMNPV as an alternative for management of S. frugiperda in areas that grow cassava and are infested with this pest species
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Baculovírus , Entomopatógeno , Fotoprotetores , Manihot esculenta , Spodoptera , Virulência , Pathogenicity , Spodoptera , Entomopathogen , Virulence