Abstract

The effect of grain moisture content on toxicity of silica dust against adult populations ofProstephanus truncatus and Sitophilus zeamais was investigated at 8.6, 13.7, 14.1, 15.4 and 17% moisture content levels of maize grains. Concentration levels of 0.25 to 50 mg/50 g maize grains were tested against P. truncatus and 0.25 to 7 mg/50 g maize grains for S. zeamais. The LC50 of the silica dust increased with increase in moisture content. P. truncatus was more tolerant to silica dust at all levels tested ranging from 0.25 to 50 mg/50 g compared to S. zeamais with lower levels of 0.25 to 7 mg/50 g. Both insect species were able to survive at low moisture content level of 11% for 7 days with a survival rate of 89% for P. truncatus and 27% forS. zeamais without food indicating that mortality recoded was not influenced by relative humidity but silica dust activity. For P. truncatus, the highest mortality of 95% was at 8 mg/50 g and 13.6% grain moisture content whereas for S. zeamais the same was 96% at 2.25 mg/50 g and 8.6%. The results are discussed in relation to the possible use of silica dust for the control of the two insect species as an alternative to insecticides.

Key words: Stored product insect pests, maize, relative humidity, silica dust, control.