Bioinsecticidal efficacy of Eucalyptus camaldulensis (Dehn) and Hyptis suaveolens (L.) Poit. leaf extracts on Callosobruchus maculatus (Cowpea weevil)

The modern method of controlling insect pests, using synthetic insecticides has caused serious problems to human health and the environment. Hence, the need for alternative, cheap, readily available and acceptable insect pest control methods, among which are the use of plant leaf extracts. The aim of this study was to determine the bioinsecticidal efficacy of Eucalyptus camaldulensis and Hyptis suaveolens against cowpea weevils (Callosobruchus maculatus). The bioassay of the powdered methanol extracts of leaves of E. camaldulensis and H. suaveolens was carried varying quantities (3 g, 6 g, 9 g and 12 g) of powdered extracts of the respective plant samples were mixed separately with 50 g of cowpea. Synthetic insecticide, Al2(PO4)3 was used as positive control while the negative control contained no treatment. Proximate compositions of the treated and untreated cowpeas were done using standard procedures while the extracts were characterized using Gas Chromatography-Mass Spectroscopic. Although mortality rates of the C. maculatus in presence of the extracts did not follow any dose-dependant trend, the respective dosages used for treatments inhibited the growth and activities of Callosobruchus maculatus. A high rate of seed damage was observed in the untreated cowpea seeds. Gas Chromatography-Mass Spectroscopy of the extracts identified the presence of seven potential insecticidal compounds (Bargamotene, Globulol, Humulene, Linolein, Prasterone, Terpinen-4-ol and Traumatic acid) in Hyptis suaveolen. In the Eucalyptus camaldulensis extract, 13 potential insecticidal compounds (Barosma camphor, Carvacrol, Carbazon, Irone, Isoborneol, 5-methyl, 1,4-Naphthoquinone, Oleic Acid, Phytol, Piperitone, Pyrazole-3-carboxylic acid, Terpinen-4-ol, Viridiflorol) were identified. The bioinsecticidal activities of these plant extracts could be attributed to the presence of phytochemicals and isolated insecticidal compounds.