Circadian Activity of Smicronyx guineanus Voss, A Potential Biocontrol Agent of striga hermonthica (Del.) Benth

The circadian activity of adults of Smicronyx guineanus Voss (Coleoptera: Curculionidae) was studied in semi-field conditions. Pairs of weevils were observed on a 24-h cycle starting from 0800 to the following day at 0600 hrs for 21 days in cages containing Striga hermonthica (Del.) Benth (Scrophulariaceae) plants parasitising sorghum. Each female and male was observed every second hour. Occurrences of the vatious behaviours exhibited by the weevil were recorded. In decreasing frequency, the weevils were observed to be standing still, feeding, mating, walking, moving and foraging. In addition, the difference in time spent on the different parts of the host plant—classified into four groups: A- stem, leaves and bud; B- corolla; C- calyx; and D- sorghum, petiole, soil, capsule and cage—was highly significant (P = 0.0001,10 couples/days, n = 21 days). The weevil adults spent 46.8% of their time on the Striga inflorescence (bud, corolla and calyx). Striga plants were also stratified with respect to distance from the base and categories were designated as upper, central and lower parts. Smicronyx adults spent 85.2% of their time in the upper stratum, 9.8% in the central stratum and 5% of their occurrences were in the lower third of the Striga shoot. They were more active during daytime, suggesting that the best period of day to sample the weevils in the field is from 0700 to 1100 or from 1600 to 1800 hrs.