Ground black pepper and its 95% ethanol crude extract were highly toxic to the rice weevil, Sitophilus oryzae (L.), when they were used to surface treat wheat subsequently infested with the insect. This toxicity was attributable to the presence of piperine in black pepper. However, the results of topical application of the crude extract or a purified extract to S. oryzae and the cowpea weevil, Callosobruchus maculatus (F.), and TLC analysis indicated that the activity may be ascribed to its other chemical components in conjunction or synergism to piperine. The LD50’s for topical application of the crude and purified extracts obtained from the 24-h dosage-mortality data were 3.4 and 4.8 µg/insect for adult S. oryzae and 4.5 and 7.2 µg/insect for adult C. maculatus, respectively.