Document Type : Research Article
Department of Plant Protection, Faculty of Agriculture, University of Zabol
Department of Plant Protection, Faculty of Agriculture, University of Guilan
Introduction: Today, the control of Rhynchophorus ferrugineus Olivier (Coleoptera: Curculionidae) as quarantine and destructive pest of date plantation due to the inner parts of the tree trunk is limited to chemical control that indiscriminate application of different types of pesticides such as imidacloprid has caused the resistance of this insect. In this study, the lethal effect of botanical compounds including garlic essential oil and its secondary metabolites (diallyl disulfide, diallyl trisulfide) and eucalyptus essential oil and its secondary metabolites (1,8-cineole, aromadendrene) on enzymatic activity (general esterases, glutathione S-transferase, acetylcholinesterase in red palm weevil were studied and compared with imidacloprid (commercial form and technical substance).
Materials and Methods: Adults (Male and female) of R. ferrugineus (red palm weevil) were collected from infected date palm plantations in Saravan (Iran) and transferred to the laboratory for propagation (25±3°C, 60±5% relative humidity, 12:12-h light: dark cycle). Bioassay tests were performed on larvae of the same age (2nd instar). The toxic effects of all compounds were investigated separately and in binary mixtures. The bioassay experiment was performed using a topical-fumigant method in three replications (10 larvae per replicate) in a completely randomized design. Two μl of different lethal concentrations (LCs) of chemicals were poured on the anterior part of the 2nd instar larval thorax and they were transferred to 8 cm Petri dishes. The mortalities were recorded 24 hours after treatment. Lethal concentrations of LC25 and LC50 were calculated using SPSS software version 21. Then, binary mixtures of LC25 and LC50 concentrations (LC25+LC25, LC50+LC25, LC50+LC50) of the studied compounds were performed to investigate the additive, synergistic, and antagonistic effects with a similar bioassay method. Enzymatic assays were performed using conventional methods. The effect of these binary mixtures, as well as LC25 and LC50 values of the individual status of each toxic compound on the activity of the mentioned enzymes, were evaluated 24 hours after treatment. Lethal concentrations (25 and 50%) and inhibition concentration of 50% of acetylcholinesterase (IC50) activity were calculated using the probit model and SPSS (v. 21). Scatter diagrams and regression lines between different concentrations of chemicals for inhibition of acetylcholinesterase were calculated with Sigma Plot software version 12.3. Also, the comparison between lethal concentrations was performed using the ratio of lethal concentrations and 95% confidence interval. In addition, the mean comparison between the data obtained from biochemical experiments with SPSS software (v. 21) and the Tukey test was performed at a 5% level.
Results and Discussion: The LC25 and LC50 values of garlic essential oil, diallyl disulfide, diallyl trisulfide were calculated as "9.23 and 23.61", "2.33 and 4.64 ","2.75 and 5.01 "µL mL-1; for eucalyptus essential oil, 1,8-cineole, aromadendrene were as "12.46 and 33.41", "4.26 and 7.83", "3.68 and 7.84 " µL mL-1 and for commercial form and technical substance imidacloprid were as" 0.012 and 0.025 "and" 0.009 and 0.004 µL mL-1, respectively. Results showed that the binary mixtures of LC50+LC50 including "diallyl trisulfide+imidaclopride technical substance", "diallyl trisulfide+aromadendrene","diallyl trisulfide+1,8-cineole","diallyl disulfide+technical substance imidacloprid","diallyl disulfide+aromadendrene","diallyl disulfide+1,8-cineole" had synergistic effects. The results showed a significant increase in general esterases and glutathione S-transferase activity in the larvae treated with the individual status and binary mixtures. A significant decrease in acetylcholinesterase activity was observed in all treatments. Results showed that the lowest and highest concentrations of the studied toxic compounds for 50% inhibition of acetylcholinesterase activity were obtained by 0.328 mg ml-1 of "diallyl trisulfide+1,8-cineole" and 4.485 mg ml-1 of garlic essential oil, respectively. In addition, the results showed that the highest (80.30%) and lowest (6.50%) levels of acetylcholinesterase inhibition were obtained by 2 μl ml-1 of "diallyl trisulfide+1,8-cineole" and 0.1 μl ml-1 of the commercial form of imidacloprid.
Conclusion: Despite the better control of red palm weevil after treatment with imidacloprid compare to botanical insecticides (essential oils and secondary metabolites), however, the resistance to this pesticide has been demonstrated because of long-term exposure. Therefore, according to the results of the synergistic effects of secondary metabolites together or even with imidacloprid, a decrease in the activity of detoxifying enzymes, as well as acetylcholinesterase, was observed, which may indicate the key point that these enzymes have not been able to eliminate these binary mixtures from the hemolymph of red palm weevil, so they could play an effective role in the management of this pest. Therefore, it can be hoped that plant essential oils can control red palm weevil in palm plantation alone or in binary mixture together or with other conventional pesticides. Further studies are needed to make a more confident decision in this regard. One of the problems of plant essential oils as well as their secondary metabolites is the low stability of these compounds in the environment, so one of the issues that can be paid more attention to is increasing their stability in the environment and how to release them in the environment, which can be done with various formulations such as nanoformulations, in particular, assume to solve this problem. Therefore, the study of the stability and formulation of plant essential oils and their secondary metabolites in the environment is a topic that should be considered in future research to be able to implement the potential ability of these compounds in agricultural pest management in practice.