INTRODUCTION
The geo-climatic condition of Bangladesh supports fast proliferation and growth of natural fauna and flora. Many of the herbs and shrubs are found to have promising medicinal properties, mosquito larvicidal and mosquito repellent properties. Owing to the fact that application of synthetic larvicide has envenomed the surroundings as well as nontarget organisms, natural products of plant origin with insecticidal properties have been tried as an indigenous method for the control of a variety of insect pests and vectors in the recent past1-5. Phytotoxicological activity ofTagetes erectawas demonstrated by Sharma and Saxena6against second and third instar larvae ofAnopheles stephansi. Azadiractin, the active ingredient ofAzadiracta indicahas long been recognised for its mosquito larvicidal capability. Long before the advent of synthetic insecticides, plants and their derivatives were being used to kill the pests of agriculture, veterinary and public health importance. Insecticidal activities of plant-derived compounds have been evaluated and few of these exploited commercially7. Laboratory and field trials of plant extracts and purified chemicals showed mosquito larvicidal activity. Larvicidal activity of certain extracts of plant parts such asAcorus calamus, Adhatoda vesica, Croton tiglium, Mentha arvensis,Ocimum basilicum,andVitex negundoagainstCulexlarvae was reported by Deshmukh8. Also, the alcoholic extract ofArtemisia annuawas reported to be more toxic than the hexane extract and acetone extract ofArtemisia vulgarisagainstCulexlarvae. Similarly, the extract ofCuscuta reflexawas found more effective than the extract ofCannabis sativa,againstCulexlarvae9. According to Sharma and Shrivastava10, the extract ofCuscuta reflexawas the most toxic toCulexlarvae, followed by extracts ofArtemisia annua, Carica papaya,andLantana indica.Latha11reportedPiper longumandZingiber wightianumextracts at 80 mg/l causing complete mortality onCulex quinquefasciatusand 60 mg/l for Culex sitiens. Fruit and seed extracts ofAbrus precatoriouswere found toxic to adult mosquitoes12and crude extract ofWithospermum arvensewere found toxic toAedes aegyptilarvae13. Several such reports related to the toxic properties of plant products against mosquito larvae like peel oils of three citrus fruits againstCulex pipiens14and larvicidal properties of leaf extracts ofSolanum nigrum15are available.
Insecticides of plant origin have been extensively used on agricultural pests, and to a vary limited extent, against insects vectors of public health importance, which deserve careful and thorough screening. The use of plant extracts for insect control has several appealing features, as these are generally more biodegradable, less hazardous, and rich storehouse of chemicals of diverse biological activity. Moreover, herbal sources give a lead for discovering new insecticides. Therefore, biologically active plant materials have attracted considerable interest in mosquito control programmes in the recent times. The present study deals with the screening of locally available herbs and shrubs for mosquito larvicidal properties.
MATERIALS AND METHODS
Plant materials collected from the foothill forests of Chittagong Hilltracs, Rajshahi and some other parts of the country were segregated as leaf, twig, flower, tuber, root, etc and were air dried in a shady place to retain their active ingredients intact. Dried materials were powdered in a table model grinder for extraction and each of the powdered plant material (500 g) was soaked in methanol in a airtight wide mouth bottle and kept for 7 days. After that, the cold extracts from the bottles along with methanol were filtered and kept in petridishes for drying at room temperature. The average yield of crude extract (500 g) of powder was 3.5 g. Dried extracts were used for larvicidal bioassay as per WHO standards. Stock solutions were prepared by dissolving plant extract (1 g) in water (1l) to make its strength 1000 ppm. Teepol (1 ppm) was also added as emulsifying agent. Different concentrations were prepared by adding required doses of stock solution in beakers containing water (250 ml).
Laboratory-rearedAedes albopictusandCulex quinquefasciatuswere used for larvicidal bioassay under laboratory conditions (27o ± 2 °C and 75 ± 5 % RH). Twenty five mosquito larvae of third or early fourth instar were released in each beaker for 24 h with a concurrent control for every set of experiment. Three replicates were kept for each concentration. No food was added in the beaker as per WHO norms. Mortality was recorded after 24 h of treatment and mortality corrected by Abbot‘s formula. Data were analysed by probit analysis16.
RESULTS AND DISCUSSION
Out of 19 plants, pericarp extract ofZenthoxylum limonellawas found to possess the most effective larvicidal activity againstAedes(s) albopictusandCulex quinquefasciatuswith LD90 values at 0.47 ppm and 0.73 ppm, respectively, followed by seed extract ofPiper nigrum, Piperlongum,and leaf extract ofSpilanthes acmella(Table 1). Among the others, peel extract ofCitrusaurantifolia,leaf extract ofMentha arvensis,and flower extract ofTagetas patulawere also found to have larvicidal properties. Leaf extracts, ofLippia nodiflora,Chrysanthemum cinerarifolium,Leucus linifolia,Lantana camara,fruit extract ofCapsicum annuum, root extract ofMoringaoleifera,and rhizome extract ofZingiber officinalisalso showed larvicidal (LD90) activity against both the species with a higher dose.
As compared to the other herbal extracts, concentration ofZenthoxulum limonellaextract for LD90 value was very low for both the species. Results ofPiper nigrumwere also encouraging considering other plant extracts tested againstAedes albopictusandCulex quinquefasciatus.LD50 values ofZenthoxulum limonellashowed at 0.01 ppm and 0.02 ppm and forPiper nigrum,at 0.56 ppm and 0.65 ppm accordingly. It may be the plant alkaloid that have toxic effect on mosquito larvae. According to Saxena5, et al. plant alkaloids resulted in a significant loss in fecundity Percentage of test mortality and fertility in the adult species of mosquitoes. Sharma and Saxena6also found that the petroleum ether extract ofTagetes erecteshad toxic effect on larvae ofAnopheles stephensiand on its significant growth index. In the present study, post-treatment data analysis on growth and fertility could not be carried out. Mwangi and Mukiama17observed that one fraction ofMelia volkensifruit karnel extract had growth inhibition activity at low concentration, whereas two other fractions had acute toxic effects on the mosquito larvae. As per Pushpalatha and Muthukrishnan18, leaf extracts ofVitex negundo, Nerium oleander, and seed of extract ofSyzygium jambolanumat very low concentration had larvicidal activity againstCulexquinquefasciatusandAnopheles stephensi,and also extended the duration of larval instars pupation. Mudrigal,13et. al.reported that methanol extract ofLithospernum arvensewere toxic toAedes aegyptilarvae. Fractionation ofZenthoxylum limonellaextract may give more active compound for larval control of mosquitoes. It is evident from the present study that herbal extracts might have promising larvicidal efficacy. Plants are rich source of bioactive organic chemicals and offer an advantage over synthetic pesticides as these are less toxic, less prone to development of resistance, and easily biodegradable. Screening and identification of effective compounds available in Bangladesh will certainly bring more success towards the control of mosquitoes.
ACKNOWLEDGEMENTS
The authors are grateful to Professor Dr. Md. Nurul Islam and Shah Hosain Ahmad Mahdi for the help providing the laboratory facilities and to Professor M. Khalequzzaman in the statistical analysis of the data.
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