Toxic properties of certain botanical extracts against three major stored product pests

Acetonic extracts of melon pumpkin, Cucurbita maxima (Duchesne), Sweet Orange, Citrus sinensis (L.) and Sour Orange, Citrus aurantium (L.), were evaluated for insecticidal activity against adults of three stored product pests viz., Sitophilus oryzae (L.), Rhyzopertha dominica (Fabricius) and Tribolium castaneum (Herbst), by filter paper diffusion method (direct contact application) and fumigation methods. The insect responses towards the treatment depended on the type of plant, the time of exposure and also the mode of application. In a test with the filter paper diffusion method (contact application), C. maxima leaf extract showed 100% mortality to S. oryzae and R. dominica within three days after treatment (DAT) at the rate of 8.5 mg/cm, whereas, only 65% mortality was observed against T. castaneum at this dosage. The application of the crude plant extracts, C. sinensis and C. aurantium in the same concentrations, caused 89 % and 76% mortality to S. oryzae and R. dominica respectively 72 hrs post treatment. Both the extracts produced comparatively less toxicity to T. castaneum and C. sinensis within the same duration of time. Among the 3 insects tested T. castaneum was the most tolerant having least mortality against all the phytochemicals. The plant compounds lack volatile toxic chemicals as indicated by the absence of significant mortality in fumigation bioassays.