Insecticidal activity of some seed powders against Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) on stored cowpea

Seed powders of Azadirachta indica A. Juss, jatropha curcas (L.), Thevetia peruviana (L.), Monodora myristica (Gaertn.) Dunal, Piper guineense (Schum and Thonn.), and Carica papaya L. were evaluated in the laboratory for their insecticidal activity against the cowpea bruchid, Callosobruchus maculatus (F.). These seed powders were applied at the rates of 0.5, 1.0 and 2.0 g per 20 g cowpea seeds, corresponding to 2.5%, 5% and 10% W/W, respectively. Pirimiphos-methyl, a synthetic insecticide was incorporated into the study at dosage rates of 0.5, 1.0 and 2.0 g per 20 g cowpea seeds for comparison. Results showed that all the seed powders had low insecticidal activity against the bruchid. The toxicities of these seed powders increased with increase in dosage as well as increase in the period of exposure to the plant materials. Higher dosage rates (1 and 2 g/20 g cowpea seeds) significantly killed the adults of C. maculatus (P <= 0.05) comparable to the synthetic insecticide. Oviposition and adult emergence were reduced by these seed powders. The highest damage of 7.87 g weight loss was sustained by the unprotected control while the least damage of 0.82 g weight loss was sustained by the. grains treated with P. guineense and followed closely by the grains treated with pirimiphos-methyl with 1.65 g weight loss. The efficacy ranking when comparing identical dosages was in the order of Pirimiphosmethyl > Piper guineense > Thevetia peruviana > jatropha curcas > Azadirachta indica > Carica papaya > Monodora myristica > control.