Efeito do polissacarídeo ulvana sobre a lagarta-do-cartucho, "Spodoptera frugiperda" (Smith, 1797) (Lepidoptera: Noctuidae) e o percevejo barriga-verde, "Dichelops melacanthus" (Dallas, 1851) (Hemiptera: Pentatomidae)

Abstract

The fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), is considered the most important pest of maize crops, because their damage can cause great losses in the production of this cereal. Moreover, in recent years it was verified the attack of sucking insects in the initial phase of corn growth, especially the green belly stink bug, Dichelops melacanthus (Hemiptera: Pentatomidae). There are some strategies available to manage these pest species; however, the application of synthetic insecticides are the most used, causing damage to the environment and human health. In this sense, the use of natural compounds becomes increasingly important due to their characteristics as biodegradability and low toxicity to non-target organisms. Some studies have been conducted in order to detect new sources of insecticidal compounds, including screening of sea algae as Ulva, which synthetizes the polysaccharide ulvan as a secondary metabolite. Thus, the objective of this study was to evaluate the toxicity of the polysaccharide ulvan on the fall armyworm and green belly stink bugs. The assessments ware performed through laboratory bioassays, using three treatments: T1 - polysaccharide ulvan; T2 - Azamax® insecticide (positive control) and T3 - deionized water (negative control). For the bioassay with the fall armyworm, the treatments were mixed in the artificial diet of larvae at concentrations of 5.000 mg kg-1 . The variables evaluated were mortality and larval weight at seven days after exposure. On the other hand, bioassay with the green belly stinkbugs was performed by means of spraying of treatments in corn plants, followed by insect inoculation. The mortality of exposed insects was performed after seven days, while the damage was evaluated after 22 days of the inicial infestation. The polysaccharide ulvan promoted only 6.25% of larval mortality after seventh day of exposure in treated diet without difference with control. Moreover, there was no significant difference between the ulvan and the control in relation to the weight of the caterpillars at the seventh days of exposure. Treatment with the Azamax® insecticide resulted in 100% larval mortality at the fifth day of exposure. Although polysaccharide ulvan resulted in bed bug mortalities close to 70%, the same did not differ significantly of negative and positive control. In light of these results, it’s possible to conclude that the polysaccharide ulvan has not promising acute toxicity against fall armyworm and green belly stink bug. However, new studies are necessary in order to evaluate the induced resistance properties of this natural compound.