Larvicidal activity of medicinal plant extracts against Anopheles subpictus & Culex tritaeniorhynchus

Mosquitoes are the major vector for the transmission of malaria, dengue fever, yellow fever, filariasis, schistosomiasis and Japanese encephalitis (JE)1. In India, malaria is one of the most important causes of direct or indirect infant, child and adult mortality with approximately two to three million new cases arising every year. Anopheles subpictus Grassi is distributed throughout India, Afghanistan, Borneo, China, Malaysia, Philippines, Sri Lanka, Java and Indonesia. It is a dominant species in Haryana and Uttaranchal states2. Though it is a non-vector species, same infected specimens with malarial parasite have been reported from India, Indonesia and Java3. An. culicifacies is the main vector of malaria, and An. subpictus is a significant secondary vector in Sri Lanka4. An. subpictus is recognized as the secondary vector of malaria in South East Asia, with a large number of cases being reported from India. 2,400 million (about 40%) of the world's population56. India contributes 77 per cent of the total malaria in Southeast Asia7. Cx. tritaeniorhynchus is a primary vector of Japanese encephalitis (JE) virus, with a distribution throughout Southeast Asia and South Asia. Keiser et al8 have reported that global annual incidence and mortality estimates for JE are 30,000 to 50,000 and 10,000 respectively.

Annona squamosa, commonly known as custard apple is a native of West Indies and is cultivated throughout India, mainly for its edible fruit. Das et al9 reported that the ethanol leaf extract of A. squamosa was found to have the most promising larvicidal activity against Cx. quinquefasciatus larvae. The larvicidal and mosquitocidal activities of ethanolic water mixture extract of A. squamosa and Centella asiaticai were effective against larvae of An. stephensi10. The leaves methanolic extract and the seeds petroleum ether extract of A. squamosa showed larvicidal activity when tested against An. stephensi, Cx. quinquefasciatus and Aedes aegypti11. Chrysanthemum indicum is a traditional herb used to treat various disorders, hypertension symptoms and several infectious diseases in Korean and Chinese medicine12. Beninger et al13 reported that the leaves of C. morifolium extracted sequentially with hexane, ethyl acetate, and methanol were found to reduce the growth of Trichoplusia ni larvae. Tridax procumbens is commonly used in Indian traditional medicine as anticoagulant, antifungal and insect repellent, in bronchial catarrh, diarrhoea and dysentery14. The entire plant is used for the treatment of malaria, leishmaniasis, vaginitis, dysentery and gastrointestinal disorders15.

Though larvicides play a vital role in controlling mosquitoes in their breeding sites, these also show a negative impact in areas of beneficial and non-target organisms. In view of an increasing interest in developing plant origin insecticides as an alternative to chemical insecticide, this study was undertaken to assess the larvicidal potential of the extracts from the medicinal plants against two medically important species of malaria vector, An. subpictus and JE vector, Cx. tritaeniorhynchus.

Material & Methods

Collection of plant materials: The bark of Annona squamosa L. (Annonaceae), leaf of Chrysanthemum indicum Linné (Compositae), Tridax procumbens Linn. (Asteraceae) were collected from Chitheri Hills, Dharmapuri District, Tamil Nadu, India in March 2009 and the taxonomic identification was made by Dr. C. Hema, Department of Botany, Arignar Anna Govt. Arts College for Women, Walajapet, Vellore, India. The voucher specimen was numbered and kept in the research laboratory of Unit of Bioactive Natural Products, Post Graduate & Research Department of Zoology, C. Abdul Hakeem College, Melvisharam, Vellore District, Tamil Nadu, India for further reference.

Preparation of plant extracts: The leaf and bark were dried for 7-10 days in the shade at the environmental temperatures (27-37°C day time). The dried bark (600 g) and leaf (700 g) were powdered mechanically using commercial electrical stainless steel blender and extracted with hexane (2,200 ml, Fine Chemicals, Mumbai), chloroform (1,000 ml, Fine Chemicals, Mumbai), ethyl acetate (2,500 ml, Qualigens, Fine Chemicals, Mumbai, India), acetone (1,000 ml, Qualigens), and methanol (2,800 ml, Qualigens), in a soxhlet apparatus (boiling point range 60–80°C) for 6 h. The extracts were filtered through a Buchner funnel with Whatman number 1 filter paper. The extract was concentrated under reduced pressure 22 - 26 mm Hg at 45°C and the residue obtained was stored at 4°C. The residues were then made in to a 1 per cent stock solution with acetone (stock solution). From the stock solution, 1000-4.69 mg/l, dilutions were prepared with dechlorinated tap water. Polysorbate 80 (Qualigens) was used as an emulsifier at the concentration of 0.05 per cent in the final test solution1617.

Mosquito culture: An. subpictus and Cx. tritaeniorhynchus larvae were collected from rice field and stagnant water areas of Melvisharam and identified in Zonal Entomological Research Centre, Vellore, Tamil Nadu, to start the colony and larvae were kept in plastic and enamel trays containing tap water. They were maintained and all the experiments were carried out at 27 ± 2°C and 75–85 per cent relative humidity under 14:10 h light and dark cycles. Larvae were fed a diet of Brewer's yeast, dog biscuits and algae collected from ponds in a ratio of 3:1:1, respectively as per the method of Kamaraj et al17.

Larvicidal bioassay: During preliminary screening with the laboratory trial, the larvae of An. subpictus and Cx. tritaeniorhynchus were collected from the insect rearing cage and identified in Zonal Entomological Research Centre, Vellore. From the stock solution, 1000 mg/l extract was prepared with dechlorinated tap water. The larvicidal activity was assessed by the procedure of WHO18 with some modification and as per the method of Rahuman et al19. For the bioassay test, larvae were taken in five batches of 20 in 249 ml of water and 1.0 ml of the desired plant extract concentration. The control was set up with acetone and polysorbate 80. The numbers of dead larvae were counted after 24 h of exposure and the percentage of mortality was reported from the average of five replicates. The experimental media in which 100 per cent mortality of larvae occurs alone were selected for dose response bioassay. Hundred per cent mortality was observed in bark methanol extract of A. squamosa at concentration of (500, 250, 125, 62.5, 31.25, 15.63 and 7 mg/l), leaf methanol (300, 150, 75, 37.5, 18.75, 9.38 and 4.69 mg/l) and ethyl acetate (500, 250, 125, 62.5, 31.25, 15.63 and 7 mg/l) extract of C. indicum, leaf acetone (800, 400, 200, 100, 50, 25 and 12.5 mg/l) and ethyl acetate (600, 300, 150, 75, 37.5, 18.8 and 9.4 mg/l) extract of T. procumbens tested against An. subpictus and Cx. tritaeniorhynchus (Table I).

Table I Larvicidal activity of different concentrations against An. subpictus and Cx. tritaeniorhynchus (mg/l)

Dose-response bioassay: From the stock solution, different concentrations ranging from 4.69-1000 mg/l were prepared. Based on the preliminary screening results 100 per cent mortality of larvae extracts alone were tested at different concentrations. Crude different solvents of leaf, and bark extracts prepared from A. squamosa, C. indicum and T. procumbens was subjected to dose response bioassay against An. subpictus, and Cx. tritaeniorhynchus respectively. The numbers of dead parasite and mosquito larvae were counted after 24 h of exposure, and the percentage mortality was reported from the average of five replicates. However, at the end of 24 h, the selected test samples turned out to be equal in their toxic potential.

Statistical analysis: The average larval mortality data were subjected to probit analysis for calculating LC₅₀, LC₉₀, and other statistics at 95 per cent fiducial limits of upper confidence limit and lower confidence limit, and Chi-square values were calculated by using the software developed by Reddy et al20. P<0.05 was considered to be statistically significant.

Results & Discussion

The activity of crude plant extracts is often attributed to the complex mixture of active compounds. In the preliminary screening, potential larvicidal activity of plants, different solvents crude extracts of three plants was noted (Table II, Fig. A, B and C). Five different solvents were tested against An. subpictus and Cx. tritaniorhynchus and 100 per cent larval mortality was observed in the bark methanol extract of A. squamosa, leaf ethyl acetate and methanol extract of C. indicum and leaf acetone and ethyl acetate extract of T. procumbens (Table II).

Table II Preliminary screenings of different solvent extracts against fourth instar larvae of An. subpictus and Cx. tritaeniorhynchus at 1000 mg/l

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Fig.

Graph showing the larvicidal activity of (A) Annona squamosa bark (B) Chrysanthemum indicum leaf and (C) Tridax procumbens leaf crude extracts against the fourth instar larvae of An. subpictus and Cx. tritaeniorhynchus at 1000 mg/l. Values are mean ± SD of 5 replicates.

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All plant extracts showed moderate toxic effect on An. subpictus and Cx. tritaeniorhynchus after 24 h of exposure at 1000 mg/l; however, the highest mortality was found in bark methanol extract of A. squamosa, leaf ethyl acetate extract of C. indicum and leaf acetone extract of T. procumbens against the larvae of An. subpictus (LC₅₀=93.80, 39.98, and 51.57 mg/l; LC₉₀=524.90, 145.70, and 226.56 mg/l); and bark methanol extract of A. squamosa, leaf methanol extract of C. indicum and leaf ethyl acetate of T. procumbens against the larvae of Cx. tritaeniorhynchus (LC₅₀=104.94, 42.29 and 69.16 mg/l; LC₉₀=443.79, 172.34, and 287.21 mg/l) respectively. The data obtained were analyzed using Chi-squared test, comparing experimental and control groups, with a significance level established at P<0.05. (Table III).

Table III LC₅₀, LC₉₀, and other statistical analysis of different solvent plant extracts against, fourth instar larvae of An. subpictus and Cx. tritaeniorhynchus

Crude extracts of leaves or bark of these plants have been tested earlier by several investigators21–29 and larvicidal and antifeedent activities have been shown against An. subpictus and Cx. tritaeniorhynchus, and also Leishmania maxicone30. C. fuscatum and C. myconis flowers powders have been tested for the antifeeding and growth regulatory activity against Spodoptera littoralis28. The leaves essential oils of T. procumbens are more effective repellents activity at 6 per cent concentration against An. stephens29 and leaves methanol extracts and purified compound (35)-16,17-didehydrofalcerinol (1) showed effective antileishmanial activity against promastigotes of Leishmania mexicana30.

In conclusion, our findings showed that leaf and bark extract of A. squamosa, C. indicum and T. procumbens can be developed as ecofriendly larvicides. Also our results open the possibility for further investigations of the efficacy of larvicidal properties of natural product extracts.