The volatility and mosquito repellency of the essential oil of Hemizygia welwitschii Rolfe (Laminaceae) and its prototype pharmaceutical formulations were examined and compared with those of a commercial mosquito repellent. At ambient temperature (28°C), neat Hemizygia oil had a volatility of 4.47±1.02 mg/cm2/ h for 15 hours, after which the rate declined. The oil evaporated at 200 μg/ cm2/ h from 10% w/w liquid paraffin solutions, and at 60 μg/ cm2/ h from soft paraffin during the first 20 hours, after which the decline in evaporation rate was generally bi-phasic. The oil demonstrated concentration-dependent repellency against the mosquito Aedes aegypti. Its liquid paraffin solutions (5% to 15% concentration) prevented up to 50% of starved mosquitoes from feeding on a blood meal for 6 hours. Lower concentrations were less effective and inhibited feeding for only 1 hour. A 15% w/w oil-in-water emulsion formulation of Hemizygia oil produced 55% feeding deterrence in the mosquitoes for 2 hours. Neither the solution nor the emulsion formulations of the oil provoked any adverse reaction on human skin.

A laboratory study was carried out to assess the effects of seed treatment and fumigation of artificially infested cowpea with the volatile oil of air-dried leaves of Ageratum conyzoides L. (Asteraceae). The oil exerted acute toxicity on adults of the cowpea weevil, Callosobruchus maculatus F. upon fumigation. Maximal mortality of 95–97% was inflicted by 60 μl oil. Application of oil dressing on cowpea seed exhibited remarkable insecticidal activity-against the weevil. Significant oviposition deterrency and complete inhibition of emergence of adult insects (F1 offspring) from oil-treated beans were evident at doses of 2.5 μl to 10 μ1/9.5 g beans. No adverse physical effects on the beans were noticed at these doses. The major isolate of the essential oil, precocene I, was four times as active as the oil.

We tested the effectiveness of volatile oils of Lippia adoensis Hoschst, Cymbopogon citratus (DC) Stapf., Eugenia uniflora L., Lantana camara L., and Chromolaena odorata (L.) King and Robinson at a dose range of 5 to 50 p.1 in protecting 9.9 g (50 seeds) of stored cow pea from infestation by the cowpea weevil, Callosobruchus maculatus F. The effectiveness of the oils as fumigants ranked as follows: L. adoensis > C. citratus > E. uniflora. Oils of L. camara and C. odorata were unsuitable for fumigation purposes. At doses of 5 to 20 μl, oil of L. adoensis protected cowpea from weevil attack. At 5 μl, volatile oil of L. adoensis was as effective as 20 μ1 each of E, uniflora and C. odorata, and only slightly inferior to C. citratus. Insecticidal activity of L. adoensis against C. maculatus was unequalled by any other oils tested. The effective doses of these oils did not impair the viability of treated cowpea.

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.