Exploring synergistic insecticidal effects of binary mixtures of major compounds from six essential oils against Callosobruchus maculatus.

The cowpea seed beetle, Callosobruchus maculatus, causes significant damage to stored grains of food legumes. Given the environmental and health hazards associated with synthetic fumigants and residual insecticides, there is an urgent need for safer alternatives. This study investigates the insecticidal potential of six different essential oils and their major components based binary mixtures activities against C. maculatus. The EOs were extracted by hydrodistillation and characterized by gas chromatography-mass spectrometry. The insecticidal activity of each EO was evaluated by fumigation in vitro. The major components of the EOs showing significant insecticidal activity (1,8-cineole, carvacrol, pulegone, and eugenol) were evaluated individually and in sublethal binary mixtures to determine synergistic effects at a 1:1 ratio. Notably, the EO of R. officinalis exhibited the highest fumigant toxicity, with an LC of 29.06 µl/l air in males and 44.27 µl/l air in females at 48 h. The major components were more potent than the complete EOs, particularly 1,8-cineole (males: LC = 17.83 µl/l air and females: LC = 28.08 µl/l air at 48 h) and pulegone (males: LC = 23.04 µl/l air and females: LC = 38.25 µl/l air at 48 h). Binary combinations of these compounds were even more effective than single compounds, particularly the eugenol and carvacrol mixture, which resulted in 76.67% mortality in males and 63.33% in females at 48 h, and the 1,8-cineole and carvacrol combination, which produced 70.00% mortality in males and 60.00% in females. Interestingly, males C. maculatus were more susceptible to the treatments than females. These results highlight the potential of natural fumigants as environmentally friendly solutions for seed preservation and sustainable pest management.