Some insecticidal plant extracts for controlling maize weevil, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae)

Toxicity of crude extracts from three selected plants, namely; Waan-nam (Acorus calamus L.), Noi-naa (Annona squamosa L.) and Lian (Melia azedarach L.) was done under laboratory conditions at temperature of 27+-3 deg C and 80+-20 percent RH. By the use of dry film method, for the toxicity test, the result revealed that crude extract from waan-nam was more toxic to maize weevil than crude extract from noi-naa seed. Due to low toxicity of lian to maize weevil, thus, lian was dropped out of the next investigation. The efficacy and long-term residual effect of crude extracts from waan-nam root and noi-naa seed were also studied. Crude extracts of both plants were treated to maize seeds and the number of dead and alive maize weevils was counted. The results indicated that crude extract from waan-nam root at the concentration of 10 percent (w/v) controlled effectively the number of maize weevil and its F1 for 120 days after treated (DAT). Additionally, the same results on the control of maize weevil was obtained from crude extract of noi-naa seed at the concentrations of 5 and 10 percent (w/v) whereas at the concentration of 2.5 percent (w/v) it controlled the number of maize weevil for 90 DAT. Moreover, all concentrations of crude extract from noi-naa seed controlled effectively the number of F1 maize weevil for 120 DAT. Treated crop seeds with crude extracts from plant may affect seed germination as time progresses. Therefore, the effects of crude extracts from waan-nam root and noi-naa seed applied to maize seeds, on seed germination percentage were evaluated. The between-paper method was employed for germination test. It was found that maize seeds treated with crude extract from waan-nam root almost completely lost their germination percentage while seed germination was high up to 90 DAT when treated with crude extract from noi-naa seed at 2.5 percent (w/v) concentration.