Fumigant toxicity of five essential oil constituents against major stored-product insect pests of food grains

Insect pests attacking stored food grains cause substantial losses that undermine food security in sub-Sahara Africa. Subsistence farmers predominantly use traditional plant-based products to protect their grains in storage. In an attempt to seek alternatives to synthetic fumigants, laboratory screening of essential oil constituents were conducted to evaluate their fumigant toxicity against five stored-product insects, Sitophilus oryzae L., Rhyzopertha dominica F., Tribolium castaneum (Herbst), Oryzaephilus surinamensis L. and Callosobruchus chinensis F. Five essential oil constituents, a- humulene, caryophyllene oxide, myrcene, R- (+)-a- pinene and R- (+)-âpinene were each evaluated at four (0, 1, 5 and 10 μl/L air) rates in space fumigation chambers with four replicates per concentration. Results revealed a strong dose-, insect speciesand time-dependent fumigant toxicity in which caryophyllene oxide, myrcene, a- humulene, R- (+)-a- pinene and R- (+)-âpinene caused 18-100%, 49-100, 55.4-100, 47.3-100 and 32.5- 100% adult mortalities of all test insects, except the most tolerant species, T. castaneum, at 10 μl/L air 168 h after treatment. Against all the test insects, except T. castaneum, Probit regression analysis produced end-point LC50 values of 0.03- 8.5, 0.03-7.0, 0.01-4.82, 0.01-8.20 and 0.03-6.5 μl/L air for the five essential constituents, respectively. The observed fumigant potency levels could possibly be explained by variations in compound structure-insecticidal activity relationships that influenced their degree of penetration into the insect cuticle and neurotoxicity. These findings provide the scientific basis for using essential oils as fumigants against stored-product insect pests and hence, potential alternatives to synthetic fumigants in both subsistence and commercial agriculture. Further studies are recommended to evaluate the grain fumigation potency, biosafety and broad spectrum bioactivity of these essential constituents against insect pests of stored food commodities.