Another difficulty is caused by the fact that many of the neem trees scattered around the world are (for all intents and purposes) genetically distinct. This means that conclusions drawn from one may not be exactly applicable to the others. Extracts from neighboring trees, for instance, may differ in their mixtures of ingredients.
There is no current evidence that this has caused any practical problems. Eventually, however, certain elite types will undoubtedly be selected and propagated.
In an era when many people are desperately seeking alternatives to synthetic pesticides, it is ironic that neem's very uniqueness is slowing its acceptance by regulatory authorities. Neem components incapacitate pests by repelling them, stopping them from feeding, or upsetting their growth—only indirectly by killing them. Its varying modes of action, its complex and synergistic mixture of ingredients, and its lack of standardization all raise barriers that trouble pesticide regulators.
Writing regulations to cover neem has been made even more difficult because no standard of potency has yet been developed. For consistency of composition, a mixed product from nature cannot compete with a single molecule from a laboratory. For instance, the mix of active ingredients may vary with the sample's age, the locality where it was grown, the genes of the tree it came from, or the method by which the sample was handled or shipped. Moreover, the analytical techniques are tricky and, for the moment at least, the various reports of neem's level of efficacy cannot all be trusted.
Although the tree is easy to grow, the specific horticultural and climatic conditions that maximize its potency are still unknown. Extracts from trees grown in different parts of the world currently show differing levels of activity, and the relative differences vary with the types of insects being tested. Sorting out just how genetics and environment—not to mention handling methods and insect species—influence neem's various ingredients is a knotty problem. Experience may eventually prove, for example, that the best-looking seeds from the fastest-growing trees on the most advantageous sites produce the poorest pesticides.