Mosquitoes are well known as vectors for several disease causing pathogens, which affect millions of people all over the world. Aedes aegypti is known to transmit dengue, yellow fever and chikungunea; malaria is carried by Anopheles stephensi and filarial disease by Culex quinquefasciatus. To prevent mosquito-borne disease and improve public health, it is necessary to control them. But in recent years, mosquito control programmes have been suffering from failures because of the ever-increasing insecticidal resistance (WHO 1992). Besides insecticidal resistance in arthropod vectors of tropical disease, the increase in the cost of insecticides and increased public concern over environmental pollution have necessitated continued search for alternative vector-control methods, which would be environmentally safer and specific in their actions.
Synthetic insecticides have created a number of ecological problems such as the development of resistant insect strains, ecological imbalance and harm to human beings. Natural products are generally preferred because of their less harmful nature to non-target organisms and due to their innate biodegradability. A number of workers have used plant products in the control of various mosquito species. Therefore, the present investigation is to evaluate the mosquito larvicidal activity of some plant species from India.
Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus were maintained in our insectary and larvae were fed with a diet of, finely ground yeast and dog biscuits in the ratio of 3:1. Material from over 20 plants like Vilvam, Vallari, Ummatta, Atthi and Nelli were collected from all Tamilnadu.
The dried leaf and/or the bark were powdered mechanically using commercial electric blender and extracted with ethyl acetate, butanol, petroleum ether in a soxhlet apparatus separately until exhaustion under reduced pressure 22-26 mg at 45°C and the residue obtained was stored at 4°C. One gram of crude extract was first dissolved in 100 ml of petroleum ether (stock solution). From the stock solution, a different concentration ranging from 200 to 1000ppm was prepared with dechlorinated tap water. Polysorbate 80 was used as emulsifier at the concentration of 0.05%. Experiments were conducted for 24 hours at room temperature. The larvicidal activity was assessed by the procedure of WHO (1996) with some modification. For bioassay test, larvae were taken in 20 in 249ml of water and 1.0ml of the desired plant extract concentration. The positive and negative control was also maintained. The average larval mortality data were subjected to probit analysis for calculating LC50 an LC90.
In the present study, the petroleum ether crude extracts of Verittumatti and Perumtutti and acetone crude extracts of Ficus racemosa have caused 100% mortality on fourth instar larvae of all three mosquito vectors Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus at 1000ppm in 24hrs. These plants were selected for isolation and purification experiments.
However, future investigation on such aspects are vital before one can recommend the practical application of these isolated compounds of plant extracts in fresh water ecosystem for the control of vector mosquitoes.
P. Venkatesan Reader in Zoology, Loyola College, Chennai
