Integrated Strategies for the Control and Prevention of Dengue and Malarial vectors.

Abstract

Mosquitoes are vectors of many life-threatening human diseases. In the absence of vaccine and treatment for most of these diseases, the only solution is to control mosquitoes by using insecticides. Any significant reduction in mosquito populations effectively lowers disease transmission risk in an area. At present, broad-spectrum synthetic insecticides are widely used in mosquito control especially when disease outbreaks threaten human health. Continuous use of these chemicals sooner or later results in development of resistance in vector species, biological magnification of toxic substances through the food chain, and adverse effects on environmental quality and non-target organisms. In the present study, the emphasis is given on larval mosquito control using plant products (botanicals) mixed with reduced rates of some synthetic insecticides (temephos, deltamethrin, mixture of allethrin and resmethrin, and methoprene) or a microbial control agent (Bacillus thuringiensis israelensis) for obtaining a sustainable and more effective method of mosquito control. Combined use of these insecticidal products may exploit physiological tradeoffs to stop or slow down resistance evolution. Low rates of synthetic chemicals are usually softer to the environment and non-target species. Extracts of many plant species have shown biological activity against mosquitoes and other insects. These compounds are biorational products which are relatively non-toxic to people and exhibit fewer or no sideeffects to the environment. Unfortunately, when used alone plant chemicals are not as efficacious as the other insecticides in mosquito management. In the present study, extracts of fourteen indigenous (medicinal) plant species belonging to eleven families (Amaranthaceae, Apocynaceae, Asteraceae, Cannbaceae, Cucurbitacea, Convolvulaceae, Euphorbiaceae, Gentianaceae, Meliaceae, Solanaceae, Zingiberaceae) were tested against laboratory reared Anopheles and Aedes larvae. Ether, methanol and water were used as solvent for phytochemical extractions. Essential oils were distilled with Soxhlet apparatus. Mixtures of plant extracts with B. thuringiensis israelensis (Bti) and synthetic insecticides were evaluated for joint action mainly in terms of toxicological effect. For all bioassays, the mortality data were collected 2, 4, 8, 16, 32, 64 and 128 hrs. Posttreatment against 2nd and 3rd instar larvae and analyzed using standard statistical procedures including ANOVA and Tukey HSD test. Probit analysis was also used to calculate LC50 and LT50. In the initial trials with plant extracts alone, the highest mortality (70-90%) was obtained with ether extracts and the minimal toxicity (30-65%) was observed with aqueous extracts against both Anopheles and Aedes larvae. Ether extracts of Citrullus colocynthis (L.) produced the highest mortality (88.2%) followed by Datura stramonium L. (86.61%) and Swertia chirayaita (Roxb. ex Fleming) H. Karst. (83.78%). Among the aqueous extracts, D. stramonium L. showed the highest mortality (52.33%). In these trials, all mortality responses were found to be time and concentration dependent. In the mixing trials of plant extracts with chemical insecticides or Bti, most combinations lead to additive and synergistic interactions. Extracts from Azadirachta indica A. Juss, C. colocynthis, D. stramonium, D. wrightii Regel and S. chirayaita mixed with low to moderate rates of synthetic insecticides or Bti produced up to 100% mortality of both Anopheles and Aedes larvae. Among different combinations, the most potent treatments were ether extracts from C. colocynthis, D. stramonium, A. indica (leaves) and S. chirayaita with Bti (LC50: 68 ppm 32 hrs. post-treatment and LT50: 1.04 hrs. at 10% concentration mixture of plant extracts with Bti) and with synthetic insecticides (LC50: 77.7 ppm 32 hrs. post-treatment and LT50: 1.55 hrs. at 10% concentration of mixture of plant extracts with insecticides) against Aedes larvae. In some combinations, mixture contents antagonized effect of each other. Moreover, Anopheles larvae were found to be more susceptible than Aedes larvae. Different outcomes of the mixing trials are probably attributed to individual or combined effects of the ingredients on the insect endocrine system (through enhanced metabolic detoxication). Nevertheless, future studies should explore underlying mechanisms for variance in the joint mortality. In the phytochemical analyses, flavonoids, alkaloids, steroids, tannins, terpenes and anthraquinones were found in different plants used in this study. Among them, flavonoids, steroids and tannins were more common in the promising plant species and hence might have played a role in their toxicity to mosquito larvae. To summarize, the results of this laboratory study indicate that the extracts of indigenous plants C. colocynthis and D. stramonium combined with reduced rate of chemical insecticides or Bti can provide an effective control of mosquito vectors of human diseases such as dengue fever and malaria. Integrated use of these compounds may provide an eco-friendly solution to mosquito management with no or minimal side effects on human health and non-target species. Nevertheless, the use of these mixtures should be further analyzed under the field condition to determine their efficacy and residual activity against mosquito populations and non-target effect in the natural larval habitats.