Insecticidal activity of Monoon longifolium, Helianthus annuus, and Croton hirtus against Callosobruchus maculatus

Abstract

Acetone (ACT) and hexane (HXN) extracts of Monoon longifolium (Sonn.) B. Xue & R.M.K. Saunders (Fabaceae) leaf, aerial parts of Helianthus annuus L. (Asteraceae), and Croton hirtus L’Her (Euphorbiaceae) were tested at concentrations of 16.66 – 200 mg/ml for mortality against cowpea weevil, Callosobruchus maculatus, in stored cowpea. Seeds were dressed with each extract, air-dried, and incubated with weevils in closed Petri dishes for 336 h at ambient laboratory conditions. Mean mortality data were generated for different time intervals per concentration. Mortality increased with incubation time for ACT and HXN extracts; all plants reached a peak of 100% at the maximum time, 336 h. High insecticidal activities were recorded for ACT extract (LT₅₀, 17 h. at 200 mg/ml) and HXN extract (LT₅₀, 140 h at 16.66 mg/ml) of H. annuus, as well as C. hirtus ACT extract (LT₅₀, 133 h at 200 mg/ml). With reference to LC₅₀, the order of insecticidal activity was: H. annuus ACT (11 mg/ml) > C. hirtus HXN (25 mg/ml), which suggested H. annuus as the most potent biopesticide against Callosobruchus maculatus. C. hirtus is recommended for long-term preservation of stored cowpeas using either ACT extract at 33.33 mg/ml, or HXN extract at 200 mg/ml for 240 h and 168 h., respectively. Furthermore, H. annuus ACT extract (200 mg/ml) and HXN extract (66.66 mg/ml) of M. longifolium also provided longterm preservation after 168 h and 240 h., respectively.