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 investigated under laboratory conditions 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, it was dropped out from further 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 were counted. The results indicated that crude extract from Waan-nam root at the concentration of 10 percent (w/v) effectively control the number of maize weevil and its F1 for 120 days after treated (DAT). Additionally, the same results on the control of maize weevil were obtained from crude extract of Noi-naa seed at the concentrations of 5 and 10 percent (w/v) whereas the concentration of 2.5 percent (w/v) could control the number of maize weevil for 90 DAT. Moreover, all concentrations of crude extract from Noi-naa seed could effectively control the number of F1 maize weevil for 120 DAT. The effects of crude extracts from Waan-nam root and Noi-naa seed, on maized seed germination were also 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.