Pre-harvest Spray of Neem (Azadirachta indica A. Juss) Seed Products and Pirimiphos-methyl as a Method of Reducing Field Infestation of Cowpeas by Storage Bruchids in the Nigerian

The efficacy of neem seed oil (NSO) applied in water at the rate of 20 ml/l and aqueous neem seed extract (ANSE) applied at the rate of 50 g/l, in reducing field infestation of cowpeas by storage bruchids was compared with that of pirimiphos-methyl (PMM) (Actellic 25 EC®) applied at the recommended rate of 2 ml/l to plants in 16 m2 experimental plots in Maiduguri, northeastern Nigeria in the cropping seasons of 2001 and 2002. Insecticides were applied twice, thrice or four times at weekly intervals to mature cowpea (var. IT89KD-374-57) pods. The results show that in the 2002 cropping season, two, three or four sprays of NSO or ANSE significantly reduced the number of eggs laid by bruchids on cowpea pods. In both cropping seasons, two sprays of any of the insecticides significantly reduced the percentage of pods bearing bruchid eggs and/or the number of exit holes of F1 adult bruchids on pods in the 2001 season. In plots that received two sprays of the insecticides in 2002, PMM reduced the number of eggs laid on threshed cowpea seeds significantly more than ANSE. In the same year, significantly fewer adult bruchids emerged from seeds obtained from PMM-treated pods that received two sprays than those from ANSE-treated pods that received a similar number of sprays of the 977 adult bruchids that emerged from shelled cowpea seeds in the 2001 cropping season, 28.6, 24.0, 29.6 and 17.7 % emerged from seeds obtained from unsprayed pods and pods sprayed with NSO, ANSE and PMM, respectively; 28.6, 20.2, 21.2 and 30.1 % emerged from cowpea seeds obtained from unsprayed pods and pods sprayed twice, thrice and four times, respectively. Of this number of adult bruchids, 28.2, 40.9, 28.0 and 2.8 % wer