Insecticidal activity of Varronia curassavica Jacq essential oil. (Boraginaceae) on larvae of Aedes aegypti (Diptera: Culicidae)
Description
The experimental design adopted was completely randomized, represented by oil concentrations at 0.125; 0.0625; 0.0312; and 0.0156%, with negative (distilled water) and positive (commercial dosage of 0.001g/L pyriproxyfen) controls, with each treatment having a volume of 25mL and four repetitions with 20 larvae each. Dimethyl sulfoxide (DMSO) at 2% was used as the oil emulsifier. Larvae were removed from the tray using a Pasteur pipette and placed in disposable 50mL plastic cups. After exposure periods of 24, 48, and 72 hours to the treatments, mortality was observed, considering dead those that did not react to the mechanical stimulus of a fine brush. The data obtained were subjected to one-way analysis of variance (ANOVA) and Dunnett's post hoc test at the 0.05 level. Linear regression was also performed, with data presented through scatter plots, with determination coefficients (R²), applying linear regression. GraphPad Prism 5.0 software was used. Treatment efficiency was evaluated using Abbott's formula (1925), which takes into account the number of dead larvae and the number of live individuals in each treatment. Lethal concentrations (LC) LC10 (minimum), LC50 (average), and LC90 (maximum) of the oil on the larvae were also evaluated. The essential oil obtained from the crushing and hydrodistillation of the dried aerial parts (at room temperature) of the plant showed 1.6% of yield. In the phytochemical analysis, it was possible to quantify 37 constituents, of which 34 were identified, all belonging to the sesquiterpene group. The major compound was (E)-Caryophyllene, with an average content of 21.44%, and the secondary constituents were Bicyclogermacrene and δ-Cadinene, with average contents of 19.31% and 9.40%, respectively. At a concentration of 0.125%, without considering the relationship with pyriproxyfen, there was 100% larval mortality after 24 hours of exposure. At a concentration of 0.0625%, mortality ranged from 77.5% at 24 hours to 96.3% at 48 hours and 100% at 72 hours, while the concentration of 0.0312% resulted in 63.7% mortality at 24 hours and 90% and 95% at 48 and 72 hours, respectively. For the lowest concentration (0.0156%), mortality was 37.5%, 51.2%, and 25.5% at 24, 48, and 72 hours, respectively. V. curassavica essential oil (VCEO) showed varied larvicidal activity at different concentrations over exposure periods of 24, 48, and 72 hours. Figure 1 shows the R² (the relationship between time and mortality) in linear regression at each concentration over the three test periods, demonstrating a positive correlation, where the variables are directly related. The time variation explains 73.2% of mortality at the lowest concentration, 0.0156%; 81% at the concentration of 0.0312%; 74% at the concentration of 0.0625%; and 60% at the concentration of 0.125%, compared to the positive control which showed an R² of 73.7%.