Synergistic Effects of Wood Ash and Essential Oil on Fecundity, Pupal Eclosion and Adult Mortality of Callosobruchus maculatus (Coleoptera: Bruchidae) Cowpea Seed Weevil

Aims: The use of residual synthetic chemicals, although effective against stored product pests, has led to environmental degradation. However, the risk of target pests developing resistance to insecticides in addition to the high costs of synthetic insecticides has pushed researchers to find alternative control methods. Plant-based insecticides represent a suitable alternative control method because they are less toxic to non-target organisms and biodegradable. The present study investigates the potency of essential oil and wood ashes in the control of the stored product pest Callosobruchus maculatus (Coleoptera: Bruchidae).

Study Design: Bioassays were performed in glass jars. Insect rearing, oil extraction and wood ash preparation were done according to an established protocol. For each treatment, a randomized complete block design with four replications was used.

Place and Duration of Study: Laboratory of biology of the Faculty of Science, University of Ngaoundere, Cameroon from February 2013 to February 2014.

Methodology: Wood ash from two different plants and essential oil were used. We evaluated the single effects of our products before combining each wood ash at 0.4, 2, 10, 20 and 40 g/kg with the essential oil of Lippia adoensis at the rate of 0.05 mL/kg on different fitness parameters of C. maculatus. Data on insect mortality rate and adult emergence were assessed.

Results: In single applications, the susceptibility of C. maculatus was significantly (P = .05) higher to essential oil than wood ash with LC50 values of 2.64 mL/kg and 139.64 g/kg, respectively after the first day exposure. The essential oil was also more effective in reducing egg laying and adult emergence. However, the combination of wood ash and essential oil caused 98.69±1.32% mortality within six days of exposure, compared to 73.53±3.80% for wood ash and 77.44±2.91% for essential oil. The co-toxicity coefficients of the combination of the two substances for mortality (275.9), fecundity (562.61) and fertility (438.92) were higher than 120, suggesting a significant essential oil-ash synergistic interaction.

Conclusion: Essential oil was the most toxic in single applications for all treatments. But when applied in combination, a lowest dose of wood ash (0.8 mL/kg) instead of 40 g/kg was able to kill 98.69±1.32% of adults. Therefore, combining essential oil with wood ash could significantly (P = .05) improve on the efficacy of controlling C. maculatus in storage facilities. Knowledge gained from this study could be exploited by low income farmers as it could provide a cost effective strategy for crop storage without the use of enormous quantities of wood ash.