Introduction

As mosquitoes play the role of  vectors of many diseases, researchers have been prompted to discover newer methods to control  them. Mosquito species belonging to genera Anopheles, Culex and Aedes are vectors of pathogens causing malaria, filariasis, Japanese encephalitis, dengue, chikingunya and yellow fever¹. Controlling mosquitoes at the larval stage is easy as target specificity of the larvicide used can be ensured. But repeated use of synthetic insecticides for mosquito control has been associated with environmental pollution and some mosquito species have developed high levels of  resistance to certain insecticides².  As an alternative plant products may be used. These plant products can be used as larvicides, adulticides or as repellants depending upon the type of activity they possess. These offer a safer alternative to synthetic chemicals and can be obtained by individuals and other communities at a very low cost.

Oscillatoria is a genus of filamentatous blue green algae which is  named for the oscillation in its movement. Filaments in the colonies can slide back and forth against each other until the whole mass is reoriented to its light source³.  Oscillatoria has been used for bioremediation of heavy metal contaminated effluents by earlier workers⁴. In this study, Oscillatoria amended with BG₁₁ medium and with zinc, copper, chromium and cadmium (1mg/ L) were used along with untreated algae (control).

Materials and Methods

Mosquito larvae

Larvae of Culex quinquefasciatus used in this study were collected from ditches in Chennai. They were acclimatized to laboratory conditions and used for this study. The larvae  were maintained at 27°C ± 2°C temperature and 70-85% relative humidity and they were fed with a mixture of  dog biscuits and yeast powder in the ratio 3:1.

Preparation of algal extract

Oscillatoria was cultured in BG₁₁ medium under 12:12 hours light, dark regime at 24 ± 1°C for 30 days⁵. Medium supplemented with 1mg/ml of heavy metals such as copper sulphate, cadmium chloride, chromium sulphate and zinc sulphate  and without  metals (control) were used to grow the algae. After 30 days the algal cultures were centrifuged, the pellets were sonicated, extracted with acetone and dried and the powder was used for further experiments.

Experimental procedure

The larviciding activity against Culex quinquefasiatus was evaluated as per the standard procedure⁶.  Late III  instar or early IV instar larvae were used for the experiments. The larvae were placed in glass bottles containing 500 ml tap water. Different concentrations of algal extracts were made (50, 100, 150, 200 and 250 mg/L) and placed in the bottles containing the larvae. Untreated larvae were used as control. Four replicates of each concentration were carried out under standard laboratory conditions. The mortality of larvae were monitored after 24 hours and recorded.

Statistical Analyses

Values obtained were subjected to log probit  regression analysis to obtain LC ₅₀ and LC₉₀ values with 95% confidence limits⁷.

Results

Untreated Oscillatoria had a LC₅₀ of 279.61 mg/L (Table I).  Oscillatoria  amended with zinc was the most effective with an LC₅₀ value of 73.75 mg/L. Copper  Oscillatoria   had an LC₅₀  value of 93.49 mg/L.  And Ocillatoria  with cadmium and chromium  had  LC₅₀ values of  106.82 mg/L and 142.77mg/L respectively.  The results showed that treated Oscillatoria was more effective than untreated Oscillatoria. The 95% confidence limits  of both  treated and control Oscillatoria are given in Table II.

Table 1: LC50 and LC90 Values for treated and untreated Oscillatoria extracts.

Table 2: 95% confidence limits for treated and untreated Oscillatoria extracts.

Discussion

Several strategies have been adopted to control diseases transmitted by vectors. Larviciding is an important strategy used in vector control programmes around the world. The use of larvicides dates back to as early as 1899, when Ronald Ross  applied  kerosene on anopheline larval  breeding sites in Sierra Leone⁸. Culex mosquitoes are often  nuisance biters and are not easily controlled by insecticide treated nets or residual spraying.  Therefore controlling this vector is best achieved  at the larval  stage⁹.

Oscillatoria often inhabits depths of thermally stratified lakes and it is very tolerant to organic pollutnts. Oscillatoria amended with metals was used for this study to see if there was  any difference between the larviciding potential of both treated and untreated algae. Treated  Oscillatoria was definitely more effective than untreated algae as all had lower LC₅₀ values. Among the treated ones, Oscillatoria treated with zinc was the most effective.

Most blue green algae have biotoxins  which could be responsible for the larvicidal effect. Some species of Oscillatoria are known to produce toxins. They include toxins such as neurotoxins and hepatotoxins called microcystins¹⁰. These toxins could have caused  the larvicidal effect. Oscillatoria is also reported to produce aplysiatoxins which can cause allergic reactions in people¹¹.  Oscillatoria is readily  found in stagnant water, watering troughs, on damp earth and in other habitats. It is one of the easiest algae to maintain  in the laboratory.

Therefore algal extracts can be used as potential larvicides in vector control programmes as high pressure field application of these extracts can be done.

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