Bioactivity of Ocimum gratissimum L. oil and two of its constituents against five insect pests attacking stored food products

The fumigant and repellent effects of Ocimum gratissimum L. oil and its constituents, β-(Z)-ocimene and eugenol, were evaluated against adults of Sitophilus oryzae (L.), Tribolium castaneum (Herbst), Oryzaephilus surinamensis (L.), Rhyzopertha dominica (F.) and Callosobruchus chinensis (L.). The fumigant toxicities of the oil and two of its constituents were assessed at four rates (0, 1, 5 and10 μL/L air) in space fumigation, whereas repellence of the oil and eugenol in acetone was evaluated in choice bioassays at five rates (0, 1.0, 2.0, 3.0 and 4.0 μL oil/2 g grain). Results showed that fumigant toxicity and repellence of the oil and its constituents were significantly (P<0.0001) influenced by concentration and time after treatment. At 1 μL/L air, the oil caused 98%, 99% and 100% mortality of R. dominica, O. surinamensis and C. chinensis, respectively, 24 h after treatment, whereas eugenol achieved 79%, 61% and 100% kill of the same insects. β-(Z)-ocimene produced a weaker toxicity with 8%, 11% and 59% mortality of R. dominica, O. surinamensis and C. chinensis, respectively. Except for T. castaneum which was more tolerant, LC₅₀ values for tested insects ranged from 0.20 to 14, 0.01 to 17 and 0.80 to 23 μL/L air 24 h after treatment for O. gratissimum oil, eugenol and β-(Z)-ocimene, respectively. All test insects had percentage repellence (PR) values which ranged from 37.5% to 100% and 45% to 100% for the oil and eugenol, respectively. However, C. chinensis showed a dose-dependent decrease in PR values in the eugenol bioassay with a corresponding dose-dependent mortality on treated grains. Ocimum gratissimum oil and its constituents are potential alternatives to synthetic fumigants in the treatment of durable agricultural products. Successful adoption of plant oils in the protection of food commodities promises an eco-friendly option compatible with international biosafety regulations.