Biopesticides from botanicals are nowadays actively encouraged in the mosquito control program because of their low mammalian toxicity, biodegradability, and target specificity. However, leaf methanolic extracts and essential oils of Callistemon rigidus and Eucalyptus camaldulensis were evaluated individually and in combination on third- and fourth-instar larvae of Anopheles gambiae. The extracts were tested individually and in combination at doses of 125, 250, 500, and 1000 ppm while essential oils were applied at 25, 50, 100, and 200 according to the standard protocol of WHO. The commercial insecticide Bi-one tested at the recommended dose of 1000 ppm was used as positive control while the solution of tap water containing 0.5 ml of methanol was used as negative control. The mortality of the larvae was recorded after 24 h postexposure. In the results, 100% mortality of the larvae was recorded with the extracts of C. rigidus and the combinations E50% : C50% and E25% : C75% of the plants as well as positive control (1000 ppm). Similarly, essential oils of the two plants and their combinations caused 100% mortality of the larvae. Among the various combinations of the extracts and essential oils, only the combination E75% : C25% of essential oils presented a synergistic effect. Therefore, the essential oil combination (E75% : C25%) of the plants E. camaldulensis and C. rigidus is recommended in order to promote the its use in the form of natural biocide in the implementation of effective insect controls against the mosquito larvae, vector of malaria.

Aims: The present investigation aimed to assess the toxic effect of hexane, acetone and methanol extracts of Cymbopogon citratus and Cymbopogon giganteus against 3rd and 4th instar of Anopheles gambiae larvae under laboratory conditions. Place and Duration of Study: Plant products were extracted from November 2017 to February 2018 in the Chemistry laboratory, while the larvicidal tests were conducted from April to June 2018 in the laboratory of Applied Zoology of the Faculty of Science, University of Ngaoundere, Cameroon. Methodology: Twenty five (25) 3rd and 4th instars of An. gambiae were subjected to methanol, acetone and methanol plant extracts of the two plants at doses of 1000, 500, 250 and 125 mg/L each while their essential oils were tested at concentrations of 200, 100, 50 and 25 mg/L. Dichlovos tested at the recommended dose of 1000 mg/L was performed as positive control while 1mL of tween-80 in 99 mL of natural breeding site water was used as negative control. Larval mortality was recorded after 24 h post treatment for plant extracts and after 1, 6, 12, 18 and 24 h post-exposure for plant essential oils. Results: At the highest concentration of 1000 mg/L, C. citratus extract caused 100% mortality of mosquito larvae while 84, 81 and 88% mortality of larvae were recorded with hexane, acetone and methanol leaf extract of C. giganteus, respectively. C. citratus (LC50=58.32 mg/L) and C. giganteus (LC50=372.36 mg/L) hexane extracts were more potent than acetone and methanol extracts of the plants. C. citratus extracts were the most effective against larvae compared to C. giganteus extracts. Essential oil of C. citratus (LC50=27.98 mg/L after 1h) was also the most toxic on mosquito larvae compared to C. giganteus (LC50=180.07 mg/L after 1h) essential oil. Conclusion: C. citratus plant and specially its hexane extract and essential oil could be taken into consideration as a new source of botanical insecticide and may be used in the mosquito control programs for An. gambiae larvae elimination in their breeding sites.