Field occurrence of bruchid pests of cowpea and associated parasitoids in a subhumid zone of Burkina Faso: importance on the infestation of two cowpea varieties at harvest.
A study was carried out in the subhumid zone of Bobo-Dioulasso in Burkina Faso during the rainy seasons of 1999 and 2000 to determine the relationship between the dynamics of bruchid and parasitoid populations in the fields and cowpea infestation at harvest. The variations in insect numbers were weekly estimated by net captures during the growth of early (KN-1)- and late-maturing (Local Moussa) cowpea cultivars. Adults of Callosobruchus maculatus and Bruchidius atrolineatus appeared early in the fields before cowpea flowering, and their population regularly increased over time. Infestation occurred earlier in the early-maturing cultivar. Dinarmus basalis was the only larval parasitoid recorded. Its adults appeared later than the bruchids and their parasitism activity increased over time. At harvesting, 35% of the early cowpea pods and 74.5% of the late cowpea pods were infested by bruchid eggs. Of the bruchid eggs laid on the early and the late cultivars, 58.5 and 72.0%, respectively, were parasitized by Uscana species. The level of cowpea seed infestation by bruchids at the beginning of storage was estimated to be 1.0 and 2.7% for the early and late cowpea cultivars, respectively. The parasitism rate of bruchid larvae by D. basalis at this period was estimated to be 7.8% for the early cowpea cultivar and 18.2% for the late cultivar. These results are discussed in view of developing an integrated control method based on the enhancement of the pest natural enemies in fields and/or in storage systems in combination with beneficial farming practices.