Efficacy of plant extracts against the cowpea beetle, Callosobruchus maculatus

Traditionally used African plant powders, with a known effect against the cowpea beetle Callosobruchus maculatus in stored cowpea, were extracted with water. The extracts, 13 volatile oils, 2 non-volatile oils and 8 slurries, were evaluated for their toxic and repellent effects against the beetle. Application of volatile oils led in most cases to a reduced number of eggs on treated beans. The volatile oils of Cymbopogon nardus and C. schoenanthus caused the majority of the eggs not to develop into adult beetles. Repellent effects were found for Clausena anisata, Cymbopogon citratus, Cymbopogon nardus, a mixture of Cymbopogon citratus and Cymbopogon flexuosus, Hyptis spicigera, Tagetes minuta and for two samples of Ocimum basilicum. Non-volatile oils were not repellent and had no effect on the number of eggs laid, but the development of these eggs was hampered, most so by Azadirachta indica oil. None of the slurries had a toxic effect on the beetles, but the slurries obtained from Carica papaya, Dracaena arborea and Tephrosia vogelii were repellent, whereas the slurry from Azadirachta indica leaves was attractive. Oils, both volatile and nonvolatile, were easily extracted from plant material and showed promising results as a protective agent for stored cowpea.