Mosquito larvicidal properties of Mirabilis jalapa (Nyctaginaceae) against Anopheles stephensi, Aedes aegypti & Culex quinquefasciatus (Diptera: Culicidae)

Sir,

Mosquitoes are the most important single group of insects known for their public importance, since they act as vector for many tropical and subtropical diseases such as dengue fever, yellow fever, chikungunya, malaria, filariasis and encephalitis of different types including, Japanese encephalitis1. Larviciding is a successful way of reducing mosquito densities in their breeding places before they emerge into adults. Larviciding largely depends on the use of synthetic chemical insecticides – organophosphates (e.g. temephos and fenthion), insect growth regulators (e.g. diflubenzuron and methoprene), etc. Although effective, their repeated use has disrupted natural biological control systems and sometimes resulting in the widespread development of resistance. These problems have warranted the need for developing alternative strategies using eco-friendly products2. We undertook investigations of certain plant species traditionally used as insecticidal agents, as well as other endangered plant species, with the aim of identifying lead compounds for the development of new plant based insecticidal agents3.

Mirabilis jalapa Linn (Nyctaginaceae) is a perennial herb and is known as “Gulambasa” in Ayurveda. The presence of oxymethyl anthraquinone, trigonelline, arabinose, galactose, beta-sitosterol in leaves has been reported. It is used in the traditional system of medicine in the treatment of piles, abscess, boils and ulcers45. There is no information available on the larvicidal activity of M. jalapa against Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus. Therefore, the present study was carried out to determine the larvicidal efficacy of M.jalapa leaves extract against malaria, dengue and filariasis vector mosquitoes.

The leaves of M.jalapa were collected from in and around Gingee, Tamil Nadu, India, and were authenticated by a plant taxonomist from the Department of Botany, Annamalai University. A voucher specimen was deposited at the herbarium of Plant Phytochemistry Division, Department of Zoology, Annamalai University, Annamalai Nagar, Tamil Nadu, India.

Cx. quinquefasciatus, Ae. aegypti and An. stephensi were reared in the Vector Control Laboratory, Department of Zoology, Annamalai University. The larvae were fed on dog biscuits and yeast powder in the 3:1 ratio. Adults were provided with 10 per cent sucrose solution and membrane feeding on goat blood. Mosquitoes were held at 28 ± 2°C, 70-85 per cent relative humidity (RH), with a photoperiod of 12 h light : 12 h dark. The dried leaves (1 kg) were extracted with four different solvents, namely, benzene, chloroform, ethyl acetate and methanol (500ml), individually and the extract was evaporated in a rotary vacuum evaporator. Standard stock solutions were prepared at 1 per cent by dissolving the residues in ethanol, which was used for the larvicidal bioassay.

The larvicidal activity of the plant crude extracts was evaluated as per the method recommended by the World Health Organization6. Batches of 25 third instar larvae were transferred to a small disposable paper cups, each containing 200 ml of water. The appropriate volume of dilution was added to 200 ml water in the cups to obtain the desired target dosage, starting with the lowest concentration (25-250 mg/l). Four replicates were set up for each concentration, and an equal number of controls were set up simultaneously using tap water. To this, 1 ml of ethanol was added. The LC₅₀ (lethal concentration that kills 50 per cent of the exposed larvae) and LC₉₀ (lethal concentration that kills 90 per cent of the exposed larvae) values were calculated after 24 h by probit analysis7

The results of the larvicidal activity of crude benzene, chloroform, ethyl acetate, and methanol solvent leaf extracts of M. jalapa against the larvae of three important vector mosquitoes, viz. An. stephensi, Ae. aegypti and Cx. quinquefasciatus are presented in the Table. Among extracts tested, the highest larvicidal activity was observed in leaf methanol extract of M. jalapa against An. stephensi, Ae. aegypti and Cx. quinquefasciatus with the LC₅₀ and LC₉₀ values as 57.55, 64.58, 84.53 ppm and 104.20, 120.28, 159.25 ppm, respectively (Table). Regression analysis showed that the mortality rate (Y) was positively correlated with concentration of exposure (X). The result of log probit analysis (95% confidence level) showed that LC₅₀ values gradually decreased (benzene < chloroform < ethyl acetate < methanol).

Table Larvicidal activity of different solvent leaf extract of M. jalapa against An. stephensi, Ae. aegypti and Cx. quinquefasciatus

It is a well recognized fact that plant extracts and phytochemicals could be developed into products suitable for mosquito control, because many of these are selective, often biodegradable to non-toxic products, and may be applied to mosquito breeding sites in the same way as conventional insecticides. Our result showed that the crude benzene, chloroform, ethyl acetate and methanol solvent extracts of leaf of M. jalapa had significant larvicidal properties against three vector mosquitos viz. An. stephensi, Ae. aegypti and Cx. quinquefasciatus. This result was also comparable to our earlier reports of the LC₅₀ values of benzene, hexane, ethyl acetate, methanol, and chloroform extract of Eclipta alba against early third-instar larvae of Ae. aegypti which were 151.38, 165.10, 154.88, 127.64, and 146.28 ppm, respectively8, and for the larvicidal efficacy of benzene, hexane, ethyl acetate, methanol, and chloroform leaf extract of Cardiospermum halicacabum against Cx. quinquefasciatus and Ae. aegypti, the LC₅₀ values were 174.24, 193.31, 183.36, 150.44, and 154.95 and 182.51, 200.02, 192.31, 156.80, and 164.54 ppm, respectively9. The leaf oil extracts of Eucalyptus tereticornis showed 100 per cent mortality at 160 ppm against the larvae of An. stephensi10. The acetone extracts of Nerium indicum and Thuja orientelis have been studied with LC₅₀ values of 200.87 and 127.53 ppm against III instar larvae of An. stephensi11. The isolated compound neemarin from Azadirachta indica exhibited LC₅₀ and LC₉₀ values of 0.35 and 1.81 mg/l for An. stephensi12. Compared with earlier reports, our results revealed that the experimental plant extracts were effective against An. stephensi, Ae. aegypti and Cx. quinquefasciatus and the plant M. jalapa exhibited larvicidal activity against three important vector mosquitoes. These results could encourage the search for new active natural compounds offering an alternative to synthetic insecticides from other medicinal plants.