Poisoning larvae and adults;
Blocking the ability to "swallow" (that is, reducing the motility of the gut);
Sending metamorphosis awry at various stages; and
Inhibiting the formation of chitin.1
As noted earlier, neem extracts have proved as potent as many commercially available synthetic pesticides. They are effective against dozens of species of insects at concentrations in the parts-per-million range. At present, it can be said that repellency is probably the weakest effect, except in some locust and grasshopper species. Antifeedant activity (although interesting and potentially extremely valuable) is probably of limited significance; its effects are short-lived, and highly variable. Blocking the larvae from molting is likely to be neem's most important quality. Eventually, this larvicidal activity will be used to kill off many pest species.
By 1990, researchers had shown that neem extracts could influence almost 200 insect species. These included many that are resistant to, or inherently difficult to control with, conventional pesticides: sweet potato whitefly, green peach aphid, western floral thrips, diamondback moth, and several leafminers, for instance.
In general, it can be said that neem products are medium- to broad-spectrum pesticides of plant-eating (phytophagous) insects. They affect members of most, if not all, orders of insects, including those discussed below.
In Orthoptera (such as grasshoppers, crickets, locusts), the antifeedant effect seems especially important. A number of species refuse to feed on neem-treated plants for several days, sometimes several weeks. Recently, a new effect, which converts the desert locust from the gregarious swarming form into its nonmarauding solitary form, has been discovered.