Five experiments were conducted during 1989 to determine the effect of palm and groundnut oils on oviposition, emergence and longevity of adultCallosobruchus maculatusF. infesting cowpea, and on germination of the cowpea seed. Effect of olive oil on longevity of the weevil was also tested. The results indicated significant reductions in oviposition, progeny emergence and longevity by a single application of groundnut oil or palm oil at 2.5–30 ml/kg seed. Olive oil also reduced adult longevity. The longest adultC. maculatussurvivors died at about five days on oil‐treated cowpea and at 7–8 days on untreated cowpea seed. Mean adult longevity ranged from 2.2 to 2.7 days for insects in treated seeds compared with 3.1–3.8 days for those in untreated cowpea seed. The adult emergence pattern suggested the need for repeated applications for prolonged protection of cowpea grain. Cost‐benefit ratios of 0.013, 0.031 and 0.279 for palm, groundnut and olive oils, respectively, indicated that use of the oils against the weevil was economic.