Comparative chemical compositions and insecticidal activities of the volatile oils of <i>Rosmarinus Officinalis</i> and <i>Callistemon viridis</i> against <i>Callosobruchus maculatus f</i>

Abstract

Background and aim: Use of synthetic chemicals in controlling destructive activities of cowpea weevil, Callosobruchus maculatus is marred by shortcomings which necessitate the search for safer, cheaper alternatives like volatile oils. This work examined the constituents and effects of Rosmarinus officinalisand Callistemon viridis volatile oils on Callosobruchus maculatus.

Method: Hydro-distilled volatile oils obtained from fresh R. officinalis and C. viridis leaves were evaluated for insecticidal activity using seed-dressing method at 2.5, 5 and 20 µL per 50 cowpea seeds measured into Petri-dishes. Acetone (300 µL) was added and shaken to ensure uniform seed coating. Controls were treated with acetone only. Treated seeds were transferred into 250 mL plastic vials with tight covers and inoculated with five pairs of male and female adult Callosobruchus maculatus. On day 14, insects were removed and eggs laid on each seed counted. Adult insects’ emergence was determined between days 23 and 40. Chemical constituents of the oils were determined by GC-MS.

Results: Both volatile oils significantly (p<0.05) reduced oviposition, adult emergence and other parameters with increase in concentrations. Anti-oviposition effects of 2.5, 5.0 and 20 µL of R. officinalis (represented by 81.70, 90.79 and 91.26 % reductions in fecundity compared to control), surpassed that of C. viridis (69.88 and 91.40 % reductions). While 1,8-cineole (31.72%) and camphor (30.11%) formed major constituents of R. officinalis, 1,8-cineole (72.98%) and α-pinene (23.17%) predominated in C. viridis.

Conclusion: Although C. viridis seems more potent than R. officinalis, the volatile oils of the plants exhibited oviposition and adult emergence inhibitory activities on the weevils.