Bioatividade de óleos essenciais de Croton grewioides e seus compostos majoritários : toxicidade sobre Chrysodeixis includens e seletividade sobre o predador Podisus nigrispinus

Abstract

Bioinsecticides obtained from essential oils of plants have become an alternative in the control of insect pests, since they seem to be safe for the environment when compared to conventional organosynthetic insecticides. However, many approaches on this subject are carried out without taking into account the selective effects of these compounds on non-target organisms. Thus, the present study aimed to evaluate the lethal and sublethal effects of the essential oil of Croton grewioides (CGR126 and CGR112), and of its major compounds eugenol and methyl eugenol on the false-medideira caterpillar, Chrysodeixis includens, and on the predatory stink bug. Podisus nigrispinus. For this, laboratory experiments were conducted to evaluate the susceptibility of organisms through dose-mortality curves, where groups of twelve second-instar caterpillars of C. includens and groups of ten third-instar P. nigrispinus nymphs were used. The evaluations were performed 72 hours after the experiments were set up. To evaluate the survival curves and the lethal time required to kill 50% of the population (TL50), second instar caterpillars were submitted to the LD90 of the treatments obtained in the bioassays. Mortality data for C. includens and P. nigrispinus were subjected to Probit analysis. Survival analyzes were performed using Kaplan-Meier estimators using the Log Rank test. The essential oils of C. grewioides (CGR126 and CGR112), methyl eugenol and eugenol were toxic to caterpillars of C. includens and nymphs of P. nigrispinus. The doses needed to cause 50% mortality in each species ranged from 4.65 to 8.82 mg/g and 1.22 to 4.45 mg/g, respectively. Survival of C. includens caterpillars exposed to LD90 of treatments was significantly reduced over time. Eugenol was the most toxic compound for second instar caterpillars, followed by essential oil CGR126. The estimated lethal times for essential oils to cause mortality in 50% of the population of C. includens were less than 15 hours. There was selectivity of CGR126 essential oil at lethal doses above 90%. Although treatments showed little effect on P. nigrispinus development, body mass and reproductive parameters were negatively affected, with the exception of CGR126 essential oil. The essential oil of C. grewioides CGR126 may represent a promising source to be used in the synthesis of new insecticides, which are efficient against C. includens and at the same time are safer for the natural enemy P. nigrispinus.