Efficacy of vegetable oils as grain protectant against Sitophilus oryzae (L.) and Rhyzopertha dominica (F.) in stored maize

The investigation was carried out to evaluate the toxicity of vegetable oils viz., castor, corn and karunj oils against Sitophilus oryzae and Rhyzopertha dominica in stored maize. Karanj oil was most effective against S. oryzae with the lowest LD50 and LD95 of 0.0433 and 0.0674 μl/cm2, respectively followed by corn (0.0494 & 0.0978) and castor oil (0.4037 & 0.9080) in film method. The same trend was also observed with R. dominica but slightly higher values of LD50 and LD95 than S. oryzae except castor. R. dominica was more susceptible to castor oil in comparison to S. oryzae. Grain treatment produced more than 90% mortality of adults of S. oryzae with all the oils at 2.5 ml/kg of grains in both the exposure periods (10 & 20 days) with the maximum of 100% in karanj oil followed by castor and corn oils. Karanj oil was significantly superior over other oils having less number of progeny production and % damage. In case of R. dominica, maximum mortality of 66,6% was recorded in karanj oil at 2.5 ml/kg with the exposure time of 20 days, S. oryzae was more susceptible to the vegetable oils than R. dominica. Although increased time of exposure of insects to the oils gave higher mortality, it also gave more time to the insects, which were alive, to oviposit and feed. So there was corresponding increase in progeny production and damage.