One particularly valuable class of additives are those that inhibit ultraviolet degradation. These include sesame oil, lecithin, and para-aminobenzoic acid (PABA).
Mixing neem extracts with other materials can boost their power 10to 20-fold. Among these so-called ''promoters" are sesame oil, pyrethrins (a type of insecticide mostly extracted from chrysanthemum flowers, see sidebar page 91), and piperonyl butoxide. They are used to produce a quicker kill.
Combinations with synthetic pesticides also can work well—they add rapid "knockdown" to neem's ability to suppress the subsequent rebound in the pest population. The effectiveness of neem extracts can even be boosted with the insect-killing bacterium Bacillus thuringensis (Bt) to provide a multifaceted pesticide.
Neem extracts can be applied in many ways, including some of the most sophisticated. For example, they may be employed as sprays, powders, drenches, or diluents in irrigation water—even through trickleor subsurface-irrigation systems. In addition, they can be applied to plants through injection or topical application, either as dusts or sprays. Moreover, they can be added to baits that attract insects (a process used, for instance, with cockroaches). They are even burned. For example, neem leaves and seeds and dry neem cake are ingredients in some mosquito coils.
The fact that the extracts can be taken up by plants (and thereby confer protection from within) is one of neem's most interesting and potentially useful features. As has been noted, however, the level of this systemic activity differs from plant to plant and formulation to formulation. Extracts without oil, with a little oil, and with much oil exhibit different levels of systemic action.
The systemic activity differs with the insect as well. It is not effective on some aphids, for instance. They feed in phloem tissues, where (for reasons yet unknown) the concentration of azadirachtin is very low. Phloem is the plant's outermost layer of conductive tissues and insects such as these, whose mouth parts cannot penetrate past it, are little affected by neem treatments. On the other hand, leafhoppers and planthoppers, that feed at least half the time on the deeper layer of conductive tissues (called the xylem), get knocked down.