Document Type : Research Article
Authors
Faculty of Plant Production, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Gorgan, Iran
Abstract
Introduction: The granary weevil, Sitophilus granarius (L.), is one of the most destructive insect pests of stored grains worldwide. The larvae feed inside the grain kernels. Larvae and adults feed from grain kernels such as wheat, barley, buckwheat, rice, maize, millet, oat, and rye. This pest causes heavy damages especially to wheat and barley kernels. Due to the negative side effects of chemical pesticides on human health and environment, efficacy of substitution compounds with acceptable insecticidal effects have been evaluated extensively during past two decades. Diatomaceous earths and plant essential oils are amongst the most important substitution compounds that having delayed effects and instability, respectively, and cause problems during their application. Using the two insecticidal agents in combination is a method for solving this problem. Plant essential oils are odorous and volatile compounds that present only in 10% of plant species. Myrtle (Myrtus communis L.) essential oil has shown significant effects on different human and plant pests and pathogens, including insecticidal effects. Diatomaceous earths, in the other hand, are inert dusts that absorb insect cuticle lipids and as a result, disrupt the waterproofing nature of the integument. Treated insects with diatomaceous earths finally go to death due to losing their body water content. These materials are not toxic to mammals that is an important privilege in comparison to conventional insecticides. Due to the economic importance of the granary weevil in Iran and the limitations of the application of diatomaceous earths and plant essential oils, in the present research insecticidal efficacy of the diatomaceous earth Sayan® in combination with myrtle essential oil was evaluated against adults of this pest.
Materials and Methods: GC-MS analysis carried out using a Varian CP-3800 gas chromatograph equipped with a Varian Saturn 2200 ion trap mass spectrometer (Helium as carrier gas; current rate 1:100 ml/min.; pressure 10 psi, and injection volume of 0.1 μl; with VF-5ms capillary column, 30 m × 0.32 mm, and 0.25 μm inner diameter; injector and detector temperatures 220 and 300°C, respectively). The granary weevil adults were initially obtained from a colony in Entomology laboratory, Plant Protection Department, Gorgan University of Agricultural Sciences and Natural Resources and reared on wheat kernels (Gonbad cultivar) for three generations (26 ± 2 °C, 65 ± 5% R.H., and L:D 16:8). Seven-day old adults of forth generation were used in bioassays. All experiments carried out on kernels of Gonbad cultivar under above mentioned conditions. In myrtle essential oil dose-setting bioassays, concentration causing 20 and 80% mortality (μl/l air) were determined. In each Petri dish (7 cm in diameter; 50 mL), a filter paper (Whatman no. 1; 2 cm in diameter) was attached to the inner part of the lid as evaporation surface. The two halves of petri dishes were sealed with parafilm to prevent essential oil loss. Experiments were repeated four times and adults mortalities were counted after 24 h. Adults that did not move their legs and antennae after a gentle poking with a fine pin were considered as dead individuals. After doing and collecting data via final bioassays, LC10, LC20, and LC50 values were estimated by using probit analysis (1267, 1790, and 3451 μL/L air, respectively). In combination effects experiments, five dose rates of 0.125, 0.25, 0.5, 1.0 and 1.5 g Sayan®/kg grain in combination with myrtle essential oil at four concentrations of LC0, LC10, LC20, and LC50 (equal to 0.0, 1267.0, 1790.0 and 3451.0 µL/L air, respectively) were assessed. Adults’ mortalities were recorded after 24, 48, and 72 h of exposure. To determine progeny production at F1 and F2 generations, dead and alive adults were counted and removed from treatment containers after the 72 hand containers were left at the same conditions for 45 and 90 days more. After these periods, emerged adults were counted and recorded as dead or alive. Probit analyses were done using PoloPlus ver. 2.0 software. Mortality data were analyzed using one-way ANOVA as factorial (dose rate × essential oil concentration). Means were separated using Tukey-HSD test at α= 0.01. Synergistic and/or antagonistic interactions were determined using Tallarida (2000) method and co-toxicity factor was proposed by Mansour et al. (1966).
Results and Discussion: In myrtle essential oil, 46 compounds were identified that 1, 8- cineole (eucalyptol) (18.029%), Isoterpinolene (14.704%), and α-Fenchene (13.135%) showed the highest amounts. Application of Sayan® alone did not result in notable mortality even after 72 h, so that at the highest dose of 1.5 g/kg it was 1.03%. Application of Sayan® in combination with myrtle essential oil increased the mortality of adults significantly, so that after 48 and especially 72 h, it was highly notable and increased from 12.25% using LC10-0.125 g/kg after 48 h up to 92.50% using the LC50-1.5 g/kg. After 72 h, mortalities of adults using the combination of LC50 with 0.5, 1.0, and 1.5 g/kg were 98.25, 100, and 100%, respectively. Interaction between Sayan® in combination with myrtle essential oil, especially after 48 and 72 h, was mainly synergistic. After 45 days, progeny production using the combination of 0.125, 0.25, and 0.5 g/kg of Sayan® with LC10, LC20, and LC50 of essential oil was very low (max. 7 adults), but their mortality was 100%. Combination of 1.0 and 1.5 g/kg of Sayan® with different concentrations of essential oil resulted to no progeny production. Compared to the combination treatments, percentage mortality of progenies using the Sayan® alone was much lower. The same trend was observed for progeny production after 90 days.
Conclusion: Results of this study indicated that application of the diatomaceous earth Sayan® in combination with the myrtle essential oil significantly increased the insecticidal efficacy of Sayan® and remedied its delayed effect (it decreased the exposure interval form about more than 14 days down to about 72 h) (p < 0.0001). This indicates that due to the synergistic interactions, application of the two insecticide agents in combination can remedy the delayed effects of Sayan® and instability of myrtle essential oil that could be considered in control of the granary weevil.
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