Potential of varietal resistance and Piper guineense seed oil to control infestation of stored millet seeds and processed products by Tribolium castaneum (Herbst)

Three grain forms (whole grains, broken grains and wholemeal flour) of three improved varieties (ICMV-N89305, GB8735 and Sosat) developed at ICRISAT (Patancheru, India), one cultivar which originated in Zimbabwe (Zimbabwean dwarf) and two local cultivars (Ex-borno and Gwagwa), of pearl millet were compared for their susceptibility to the rust-red flour beetle (Tribolium castaneum) under tropical storage conditions. The biological effects of Piper guineense seed oil were also investigated on T. castaneum confined on similar forms of millet (Ex-borno) contained in ventilated or unventilated devices. Mortality of adult flour beetles was significantly higher on whole grains of GB8735 (99.7%), Gwagwa (92.3%) and Ex-borno (77.7%) stored in unventilated glass jars than on whole grains of Zimbabwean dwarf (52.7%) stored in a similar device. The number of adult progeny was significantly higher in wholemeal flour than in broken grains; the number in broken grains was significantly higher than that in whole grains. Development of the beetle was significantly longer in GB8735 than in Zimbabwean dwarf and the overall susceptibility of Zimbabwean dwarf was significantly higher than that of ICMV-N89305. Susceptibility of the cultivars increased with decreasing grain particle size; susceptibility indices for the whole grains, broken grains and wholemeal flour were 2.52, 12.14 and 14.16, respectively. Significantly fewer beetles developed in grains/products treated with P. guineense seed oil than in untreated grains/products. The unventilated glass jar retained the biological efficacy of the two lower dosages (20 and 40 mg/10 g) of P. guineense seed oil significantly more than the ventilated clay pot or polypropylene sack.