Fumigant Toxicity of Citruspeel Oils against Adult and Immature Stages of Storage Insect Pests

The biological action of citruspeel oils was shown to depend on a strong fumigant action. Bioassays conducted in air-tight glass chambers showed that all the six citrus oils tested had vapour toxicity towards adults of Callosobruchus maculatus F., Sitophilus zeamais Motsch. and Dermestes maculatus Deg. The 24-h LC₅₀ value of limepeel oil (a typical citrus oil) vapour against C. Maculatus was 7·99 μl litre^?1 which made it 1·5 and 1·6 times less toxic against the smaller S. zeamais and the larger D. maculatus adult insects. When immature stages were fumigated, limepeel oil vapour had 24-h LC₅₀ values of 7·8 and 21·5 μl litre^?1 against eggs of C. maculatus and D. maculatus respectively, and 9·1, 17·8 and 23·1, 23·9 μl litre^?1 against early larvae, pupae of C. maculatus and late larvae, pupae of D. maculatus respectively. X-ray studies showed that fumigated C. maculatus larvae within cowpea grains died immediately without further development. The bioactivities of five other citruspeel oils were similar to that of limepeel oil. Bioassays showed that sorption of citruspeel oil fumes occurred in the presence of grains or strips of dried fish, and that this tended to reduce the amount available for fumigant action outside the materials. The problems presented by sorption may hinder the development of citrus oils into practical fumigants for large-scale treatments of stored commodities.