Laboratory experiments were conducted to study the effects of an ethanolic neem seed extract (NSE) from Azadirachta indica A. Juss on the development and ovipositional behaviour of the autogenous mosquito Anopheles hilli Woodhill and Lee.
The immature stages of An. hilli (eggs, 4 larval instars and pupae) were continually exposed to NSE/brackish water solutions containing various concentrations of azadirachtin.
There was no evidence to suggest that NSE affected the hatchability of eggs. Larval mortalities were high, frequently 100%, for all concentrations of NSE tested, even as low as 2 ppm of azadirachtin. For many larval instars, a significant positive dose-mortality response was recorded. Larvae exposed to high doses (e.g. 200 ppm of azadirachtin) died quickly, often within one day. Larvae exposed to lower concentrations frequently underwent some development, but died eventually.
Histological studies revealed gross changes in the internal structure of 4th instar larvae exposed to NSE/brackish water solutions containing 30 ppm of azadirachtin. These changes indicated possible interference with feeding.
Contrary to other studies, NSE produced high pupal mortalities at concentrations of 2 ppm of azadirachtin and above. Concentrations below 0.7 ppm of azadirachtin failed to cause significant pupal mortalities.
In choice experiments adult females showed very strong oviposition non-preference for NSE/brackish water solutions containing 200 and 300 ppm of azadirachtin. The non-preference for a solution containing 100 ppm of azadirachtin was variable.