The insecticidal activity of essential oils extracted by steam distillation from six aromatic plants from Togo, Cymbopogon schoenanthus (L.) Spreng., C. citratus (D.C.) Stapf., C. nardus (L.) Rendle, Lippia multiflora Moldemke, Eucalyptus citriodora Hook., Diplolofium africanum Turcz., was tested against the cowpea weevil, Callosobruchus maculatus F. Concentrations of 6.7; 10.0; 16.7 and 33.3 μ1/1 were evaluated. The chemical composition of the essential oils used was established by GC/MS analysis. Only C. schoenanthus oil showed significant insecticidal activity at 6.7 μl/1 oil (LC50 2.3 μ1/1). At concentrations higher than 33.3 μ1/1, all test plants showed similar insecticidal activity; more than 90% of the test insects died within 24 hours of exposure. Oviposition was also reduced significantly by the essential oil treatments.

Piper guineense Schum and Thonn, mixed with cowpea seeds, Vigna unguiculata (L.) Walp, was significantly superior (P < 0.05) to the large, red and mild Capsicum annum var connoides (Miller) and the small, red and pungent Capsicum frutescens L. in causing up to 96% mortality to adults of Callosobruchus maculatus F. within 48 hr. When applied as ground pepper, P. guineense was more active insecticidally than when applied as whole fruits.

Adult bruchids in contact with P. guineense showed significant reduction (P < 0.05) in the numbers of eggs laid, and in the numbers of F1, offspring that emerged compared with adult bruchids exposed to the same doses of C. annum or C. frutescens respectively.

Neither of the peppers reduced the viability of the cowpea seeds or the absorption of water needed to soften the seed testa for decortication before cooking or processing.

At a cost price of U.S.A. $0.003 per 1 g of black pepper, and an effective application rate of 1 g black pepper to 20 g cowpea, there is a saving of over $750 by disinfesting a ton of cowpea with black pepper for a storage period of 2–3 months in a metallic drum which costs less than $50.00