Insecticidal Effects of Crude Extracts of Six Officinal Plants on the Cowpea Weevil, Callosobruchus maculates F. (Col.: Bruchidae)

The cowpea weevil, Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae), is one of the most important stored product pests of several pulse crops including chickpea (Cicer arietinum L.), cowpea (Vigna unguiculata (L.) Walp.), lentil (Lens culinaris Medik.), common bean (Phaseolus vulgaris L.) and soya (Glycine max Mer.). This pest cause considerable economic damage especially under conditions of tropical and subtropical subsistence agriculture. Infestation quickly becomes obvious as a result of the presence of eggs stuck to the outside of seed together. In the recent years, plant extracts have received much attention as pest control chemical agents. The application active compounds, which are less persistent, are harmless for the environment.
In this study, the contact and fumigant toxicity of extracts of Ziziphora clinopodioides Lam., Lavandula officinalis Chaix., Laurus nobilis L., Rosmarinus officinalis L., Satureja hortensis L. and Salvia officinalis were investigated against adult repellency, egg hatching, and oviposition rate of the cowpea weevil, Callosobrochus maculatus F. The extract of the examined plants was provided by the polar (methanol) and non-polar (N- hexan) solvents. The adults of the cowpea weevil were originally collected form Esfahan’s grain warehouses and colonized at the favorite experimental conditions. The experiments were conducted using a completely randomized design. All experiments were carried out at 27±2˚C, 55±5% RH and 12L: 12D photoperiods. Insecticidal effects of the extracts were determined as mortality percentage 24, 48 and 72 hours after treatment. The data of mortality percentages were corrected by Abbott equation. Repellency effects of the extracts were determined one hour after treatment and were continued until five hours.
The contact toxicity at the highest concentrations of Z. clinopodioides, L. vera, L. nobilis, R. officinalis, S. hortensis and S. officinalis caused 82.21, 94.07, 73.74, 77.76, 88.23 and 92.65% mortality of adults, respectively. The calculated LC50 in contact toxicity was 14.87, 9.72, 32.30, 19.20, 21.48 and 23.59% for extract of Z. clinopodioides, L. officinalis, L. nobilis, R. officinalis, S. hortensis and S. officinalis, respectively. in fumigant toxicity, the highest concentration of extracts caused 63.48, 97.72, 55.96, 81.17, 86.47 and 90.85% mortality of adults and 83.15, 88.88, 71.73, 41.84, 53.60 and 76.74% mortality of eggs. The calculated LC50 in fumigant toxicity of various plant extracts on the adults of cowpea weevil was 47.07, 4.91, 61.77, 11.07, 6.01 and 20.42% for Z. clinopodioides, L. officinalis, L. nobilis, R. officinalis, S. hortensis and S. officinalis, respectively. At the highest concentration, (1µl/ml) oviposition deterrence rate was recorded 92.81, 99.20, 97.22, 100.00, 91.60 and 95.80% by extracts of Z. clinopodioides, L. vera, L. nobilis, R. officinalis, S. hortensis and S. officinalis, respectively. The crude extracts of L. vera and R. officinalis repelled 100 and 64.00% of insects at the highest concentration (30µl), respectively. The extracts of Z. clinopodioides and S. officinalis showed the high repellency potential on the adults of cowpea weevil and both of them repelled 80% of studied insects after the 5-h exposure time to the highest concentration (30 μl/ ml). The highest and lowest egg mortality was obtained by the extracts of L. officinalis and R. officinalis as 89 and 42%, respectively. The present study revealed the high efficiency of these plant extracts in non-chemical control of C. maculates. Based on our findings, at the low concentration of all examined plant extracts, more than 50% mortality was observed on the population of cowpea weevil in the laboratory conditions.
Among the examined plants, L. officinalis and Z. clinopodioides are introduced as the most effective plant species in control of the adults of cowpea weevil based on the low values of LC50 which are obtained in the experiment of contact toxicity.