Laboratory evaluation of three underutilized Nigerian plants as cowpea seeds protectants against cowpea beetle, Callosobruchus maculatus (Fab.) [Coleoptera: Chrysomelidae]

Powders and methanolic extracts of Clerodendrum capitatum, Phyllanthus fraternus and Tithonia diversifolia were evaluated for their insecticidal activities against Callosobruchus maculatus (Fab.). Plant materials were tested for contact toxicity, oviposition and adult emergence at rates of 0.5, 1.0, 2.0, 4.0 and 5.0 g (w/w) for plant powders and 0.1, 0.2, 0.5, 1.0 and 2.0 ml/20 g (v/w) of cowpea seeds for plant methanolic extracts. The results of the plants powders showed that T. diversifolia was the most effective as it caused 55.0, 60.0, 67.5, 100.0% and 100.0% mortality at the concentrations of 0.5, 1.0, 2.0, 4.0 and 5.0 g/20 g of cowpea seeds within 24 h of application. The least effective plant powder was C. capitatum which evoked the insect mortality of 77.50% at rate 5.0 g within 24 h of exposure. Similar trend of results were obtained on the plant methanolic extracts. Extracts caused more mortality, prevented oviposition and emergence of adult cowpea beetle, more than the plant powders. The calculated lethal dose (LD50 and 90) and concentrations (LC50 and 90) of the plant powders and methanolic extracts showed that T. diversifolia had the lowest values while C. capitatum had the highest across all period of exposure. Tithonia diversifolia methanolic extract completely inhibited oviposition and adult emergence at concentrations 0.2–2.0 ml. Tithonia diversifolia powder and methanolic extract were found to be the most effective in protecting cowpea seeds against C. maculatus. This can be a better alternative to synthetic insecticides since it is abundant in our environment.