The antifeedant activities of Piper guineense Schum et Thonn (Piperaceae), Aframomum melegueta (Rosk) K. Schum (Zingiberaceae), Aframomum citratum (Pareira) K. Schum (Zingiberaceae) and Afrostyrax kamerunensis Perkins and Gilg (Huaceae) seed extracts were investigated in laboratory dual- and no-choice bioassays using third-instar Spodoptera littoralis (Boisduval) larvae. In the dual-choice test, the hexane and methanol extracts of A. melegueta showed potent dose-dependent antifeedant activity at concentrations of ≥300 ppm and the water extract at ≥500 ppm, as illustrated by significantly lower leaf consumptions. Aframomum citratum methanol and water extracts exhibited antifeedant activity at ≥300 and ≥1000 ppm, respectively, but the hexane and ethanol extracts did not affect feeding at any concentration. Piper guineense ethanol and water extracts showed dose-dependent antifeedant effects at ≥300 and ≥500 ppm, respectively, and the methanol extract was active only at 1000 ppm. None of the extracts of the highly aromatic A. kamerunensis exhibited antifeedant activity at any of the tested concentrations. In the no-choice bioassays, extracts with antifeedant activity in the dual-choice tests also showed dose-dependent feeding inhibition. The hexane and methanol extracts of A. melegueta were effective in the no-choice tests at ≥100 and ≥500 ppm, respectively, and the water extract at ≥300 ppm. Similarly, the A. citratum water and methanol extracts were active at ≥500 ppm and the P. guineense water and ethanol extracts at ≥100 ppm. GC/MS chromatography of A. melegueta hexane and methanol extracts revealed volatile constituents with known anti-insect activity. The hexane and methanol extracts of A. melegueta, the methanol extract of A. citratum and the water and ethanol extracts of P. guineense may have potential for use by subsistence farmers.

Aqueous extracts (5%) of the seeds Parkia clappertoniana (Jacq), Azadirachta indica A. Juss, and Piper guineense Schum and Thonn caused 90% mortalityof the worker caste with P. guineense, and 86% mortality with A. indica and P. clappertoniana respectively within 3 hr of topical application. At 24 hr, mortality of Macrotermes nigeriensis (Sjostedti) workers was 100% with the same concentration of each extract. Oral toxicity at 3 hr to the worker caste using cuttings of cassava, Manihotesculenta Crantz impregnated with each extract gave 65% mortality with Piper, and 45 and 29% with Azadirachta and Parkia, respectively. Mortalityby oral toxicity reached 100% in 12 hr with Piper, 95 and 69%, respectively with Azadirachta and Parkia. Increasing the concentrations of each extract to 15% did not significantly (P > 0.05) increase the toxicities of the extracts to the worker caste even at prolonged exposure periods.

Six amide alkaloids of Piper guineense were tested for antifeedant activity against fifth instar larvae of Chilo partellus in choice assays. These amides include piperine, Δα, β-dihydropiperine, wisanine, Δα, β-dihydrowisanine, trichostachine and N-isobutyl-trans-2-trans-4-eicosadienamide. Piperine and its dihydrosaturated derivative exhibited the most potentantifeedant activity. Δα, β-dihydrowisanine and trichostachine were intermediate in activity followed by wisanine. The N-isobutyl derivative was the least active. These results suggest that the presence of a methylenedioxybenzene and an alicyclic amide group in the compound may be crucial for high antifeedant activity.

The efficacy of the powder and volatile oil of Piper guineense Schum & Thonn at protecting cowpea against seed beetle, Callosobruchus maculatus (F.) was tested in the laboratory. At low concentrations the powder significantly reduced oviposition and adult emergence while oviposition was completely suppressed at a higher concentration of 42%. The same concentration provided adequate protection for the same amount of cowpea placed in containers of different volumes. The powder, however, did not show any fumigant effect. Volatile oil at 0.002 and 0.005% significantly reduced oviposition and adult emergence, while oviposition was completely suppressed at 0.02% and above. Weight loss of seeds at 90 days after treatment with volatile oil decreased as concentration of the oil increased. The practical significance of the results in small scale storage of cowpea is discussed.

Steam distillates of seven Nigerian plants, Lippia adoensis Hoschst, Azadirachta indica A. Juss, Monodora tenuifolia Benth, Piper guineense Schum & Thonn, Petiveria alliacea L., Citrus sinensis L. (variety Nigerian green orange), and Cymbopogon citratus L. were tested for acute toxicity on four major insect pests. The pests were larvae of Acrae eponina Cramer, adult cotton stainer Dysdercus superstitiosus (F.), the cowpea flea beetle, Ootheca mutabilis Sahlberg and the pod sucking bug Riptortus dentipes F. Essential oils of L. adoensis, M. tenuifolia and P. guineense were toxic to the insects. O. mutabilis was the most susceptible with LD50 of 0.15, 0.32 and 0.52 respectively for M. tenuifolia, L. adoensis and P. guineense, while R. dentipes was the least susceptible. There was cessation of feeding in A. eponina treated with essential oil of C. sinensis and P. alliacea.

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