What is azadirachtin and which neem product strength should I buy?

Azadirachtin is the main active compound in neem that interferes with insect feeding, moulting and egg laying. Neem products are sold by their azadirachtin content in ppm, commonly 300, 1500, 5000 or 10000 ppm and higher. A higher ppm means you use a smaller quantity per litre, so do not just compare bottle price, compare the ppm and the dose. For field spraying many farmers use a 1500 ppm formulation at the label dose, while a stronger 10000 ppm or above is used at a lower rate. Raw cold pressed neem oil from a local ghani has a variable, often lower azadirachtin level, so it is cheaper but less standardised. Neem cake is a different product, used in soil to feed plants and suppress some nematodes and grubs, not for spraying. Pick a formulation that carries a CIB registration label, and follow the per litre rate on that label. Your KVK can tell you which ppm and crop combinations are recommended locally.