TOXICITY OF LEAF, BARK AND STEM EXTRACTS OF EUCALYPTUS (EUCALYPTUS CAMALDULENSIS L.) AGAINST SITOPHILUS ORYZAE L. AND RHYZOPERTHA DOMINICA F.

Abstract

The experiment was conducted to study the effectiveness of leaf, bark and stem extracts of eucalyptus as botanical insecticides for the control of rice weevil, Sitophilus oryzae L. and lesser grain borer, Rhyzopertha dominica F. in the laboratory conditions during March to October 2017. Methanol was used to prepare the extracts and four doses viz. 0.5, 1.0, 1.5 and 2.0% along with a control of the plant extracts were applied for insect bioassay, residual and repellent effects. Results revealed that the stem extracts of eucalyptus plant showed the highest toxic effect both for the rice weevil (mortality, 34.53%) and the lesser grain borer (mortality, 40.27%) whereas leaf extract showed the lowest toxic effect for the rice weevil (mortality, 11.20%) and the lesser grain borer (mortality, 17.43%). Mortality percentages were increased proportionally with the increased time intervals. In case of residual effect for rice weevil, the highest number of adult emergence was recorded in control (78.33) treatment. Stem extract showed the lowest adult emergence (36.53) and the highest infestation inhibition on rice weevil (67.42%). On the other hand, the highest number of adult emergence in control (100.0) for lesser grain borer whereas the lowest (45.59) was in stem. Similarly, the highest infestation inhibition was found 69.97% in stem also. Among the eucalyptus plant part tested, stem extract showed the highest mean repellent effect for both rice weevil (73.33%) and lesser grain borer (74.67%) whereas leaf extract showed the lowest mean repellent effect for rice weevil (56.33%) and lesser grain borer (59.33%). On the basis of repellency rate, it was found that stem and bark extracts were in the same repellency class i.e. Class IV and leaf extract was in Class III. All the doses had direct toxicity, residual and repellent effects but the highest response found at 2.0% concentration and the toxicity was increased proportionally to the level of concentration. On the basis of LD50 values, the stem extract showed the highest toxic effects against rice weevil and lesser grain borer. Considering all the concentrations, the order of toxicity of different parts of eucalyptus plant extracts against rice weevil and lesser grain borer were found as stem> bark> leaf.