EFFECTIVENESS OF AZDIRACHTA INDICA OIL AS A POTENTIAL BIO-INSECTICIDE ON ANOPHELES GAMBIAE AND PERIPLANETA AMERICANA IN IBADAN, NIGERIA

Abstract

Conventional insecticides are characterized by a lot of health and environmental problems. Consequently, there have been calls for alternatives with minimal toxicity. There is inadequate knowledge about the bio-insecticidal effects of extracts of Azadirachtha indica on insects of public health importance in Nigeria. The study was therefore designed to test the relative bio- insecticidal effects of Azadirachta indica oil on Anopheles gambiae and Periplaneta americana which are vectors of public health importance in Nigeria. The study was experimental in design. Oil was extracted from ground seed kernel of Azadirachta Indica plants using N-hexane as solvent. The insects used included Anopheles gambiae (larvae and female adults) and periplaneta americana (nymphs and adults). They were laboratory reared and randomized into test and control groups. Larvicidal tests were carried out on 600 third and fourth instar stages of Anopheles larvae. This was done with aliquot of extracted oil emulsified with a surfactant (Tween 80) at concentrations ranging from 100 to 500 ppm. A control experiment was carried out using solution of the surfactant with similar concentrations. Mortality was observed and recorded at every 24 hours for five days. Repellency tests were carried out by exposing experimental animals that were previously treated with the oil mixed with paraffin at 10-40%v\v concentrations, to 70 adult female Anopheles gambiae in netted cages. This was observed at hourly interval for a period of four hours. Repellency was estimated in mean value from a number of insect that did not bite within 60 seconds. The contact toxicity effect was assessed on 20 nymphs and 20 adults of Periplaneta americana by administering the oil diluted with glycerol (40.0%) to the dorsal thoracic portion of the insects. The effects were observed and recorded within five days of exposure. Data were analyzed using descriptive statistics and ANOVA. The oil yield accounted for 40.0% weight of the ground seed kernel, giving 432ml\kg. The larvicidal effect was significant across the concentration of the emulsified Azadirachta oil (P<0.01) ranging from 91.56-100%, compared to control experiment ranging from 5-15% (LC50 and LC90: -1666.86 ppm and -2880.94 ppm respectively). Hundred percent larval mortality of Anopheles gambiae was first recorded within three days at 500ppm. All the concentrations of the oil solution also caused 100% inhibition of pupae formation. The repellent effect of adult Anopheles was significant (p<0.05) across the concentrations but with varying degree of protection. The highest repellent effect was observed at 40.0% (v/v), ranging from 95-100% and the lowest at 10.0% (v/v) from 92-100%. However, the variation in repellency with exposure time across the concentrations was not statistically different from each other (p>0.05). The oil (40.0% in glycerol) when tested on Periplaneta americana gave 100% feeding deterrence, reduced the locomotive rate and caused 100% mortality in nymphs after 30 minutes of exposure and 60% mortality in adults after five days. The capacity of Azadirachta indica as potential bio-insecticide against Anopheles gambiae and Periplaneta Americana was established. The specificity and effectiveness of Azadirachta indica makes it a viable alternative insecticide to be used primarily to achieve improved vector control.