Single applications of insecticide againstChilo partellusat different times in the pre-flowering stages of maize and from early whorl to soft dough stage in grain sorghum were compared with multiple applications. Based on yield and the incidence of damaged plants, late treatments were more beneficial than early treatments. Two serial applications would have sufficed to eliminate the greater part of the observed yield loss. The cost- benefit ratio seemed to favour a reduction in the number of applications currently recommended for maize, although more than one application seemed to be needed in grain sorghum.