A natural component in Neem Oil called Azadirachtin will impair the systems of chewing insects when ingested. They will die off in a few days. The bonus is that once it is sprayed and dried, it will not harm other insects that come in contact with it. It has to be ingested. This product has been used in India for centuries. I highly recommend you search Neem Oil and India as a phrase. Historically, the story goes like this, locusts came and devastated large parts of India. The trees left standing with little damage to their leaves were the Neem Trees. They were skipped for a very good reason.
The nut from the Neem Tree is pressed and produces 100% cold pressed Neem Oil with all its natural components. Why is this important? All Neem Oil is not the same. We are misled and I hope to educate you so you can by the product you want for your garden.
Any oil based spray will smother and kill soft bodied insects but (generally) not caterpillars or hard bodied insects… any oil. The oil of choice must be sprayed onto the soft bodied insects to kill them. Neem Oil as an oil has this property too. If you spray it on soft bodied insects it will smoother them. However, that process is not what kills the chewing insects. It is the component called Azadirachtin that only comes in Neem Oil that does the killing. That is why you want 100% cold pressed Neem Oil. Your salad oil in your pantry can be used as a smothering oil.
Many products show the words NEEM OIL in bold on their packaging. Notice 100% Cold Pressed is not on them. Nor is the word Azadiracthin. The words you will find in the ingredients is often Hydrophobic Extract of Neem Oil. That means the company removed the good stuff, Azadiractin, and gave you Neem Oil that is useless but for smothering soft bodied insects and as a fungicide (all oils have fungicide properties). It may also state it is an insecticide because smothering oils can be called that.
You DO NOT have the azadirachtin that sits on the leaves for 7-10 days and kills chewing insects if you purchase a Hydrophobic Extract. This video explains the difference between Hydrophobic Extract of Neem Oil and Cold Pressed Neem Oil.