Comparative toxicity of four local botanical powders to Sitophilus zeamais and influence of drying regime and particle size on insecticidal efficacy

The toxicity of Azadirachta indica seed powder and leaf powders from Plectranthus glandulosus, Steganotaenia araliacea and Annona senegalensis to adult Sitophilus zeamais was determined in the laboratory. The influence of drying regime and particle size on the insecticidal efficacy of P. glandulosus and S. araliacea powders was also assessed. The seeds and leaves were either dried under shade or sunlight, pulverized until the particles passed through a 1 mm or 0.2 mm mesh-sieve. Maize grains were admixed with the powders at the rates 0, 5, 10, 20 and 40 g/kg for the assessment of mortality over a 14-d or 30-d period, as well as for population increase and damage. Within 14-d exposure, P. glandulosus powder caused the highest weevil mortality followed by A. indica. A. indica powder was by far more effective in reducing grain damage and population increase compared with all the other powders during three-month storage period. Powders from the leaves dried under shade compared with those dried under sunlight and the smaller compared with the larger particle size, caused higher weevil mortality. Neem seed powder and powdered leaves of P. glandulosus have a great potential in the protection of maize against S. zeamais infestation in subsistence storage systems.