An Australian company is promoting the planting of "Neem Trees" (4 per acre) as a means of getting rid of sandflies and mosquitoes. My company is interested in this idea and wants to see if the claims are based on any scientific evidence. Help on this would be greatly appreciated.
The book, "Neem: A Tree for Solving Global Problems" (available from Richters) has a very good review of the uses of neem. There are many references to the scientific literature and an extensive list of research contacts. There is a wealth of data indicating that neem extracts have strong effects on insects. Neem compounds mimic insect hormones and act to confuse the normal hormonal balance in insects. When exposed to neem compounds insects cannot reproduce and their populations plummet. The precise effects depend on the insect type and their life cycle stage.
According to the above-mentioned book, neem kills the larvae and disrupts moulting in the yellow-fever mosquito (Aedes aegypti) and is toxic to larvae of the house mosquito. "They stop feeding and die within 24 hours after treatment." It goes on to say that "in one test, crushed neem seeds thrown in pools proved nearly as effective at preventing mosquito breeding as methoprene..." There is no mention of any effect on sandflies, but over 200 species of insects are sensitive to neem.
These are the effects of applying neem compounds on insect larvae or on the leaves on which they feed. I could not find any mention of a repellent effect by the mere presence of neem trees. There does not seem to be any indication that neem trees can lower mosquito populations, other than by the effect of seeds dispersing over breeding pools. This is not to say that neem cannot have this effect; only that this effect was not noticed by the ad hoc panel of the National Research Council that wrote the book.
I would note that neem trees are common in Ghana when I visited in 1994. I contracted malaria despite the presence of neem trees within 30 metres of where I was staying. I am suspicious of claims of repellent action. However, the degree to which neem varies in chemical properties from country to country is still unknown and may be important in explaining differences in efficacy of neem products produced in different regions.