I agree that Neem Oil is very effective to control spider mites, and some gnats.....but don not feed the plant with it. It will wind up on the leaves as pointed out above BUT it will also wind up in your buds and will not flush out like nutrients will and will have an adverse effect on the products finished taste.

I've used successfully a mixture of Neem Oil and "FOOD GRADE" Hydrogenperozide mixed with water to spray plants and grow medium.

Best tho is to maintain a sterile growing enviroment, and that includes sterilizing the grow medium, especially if using dirt (ugh) or any medium that does not come pre-sterilized or has been open and sitting around. Washing your hands and armrs, and not going into your grow area with the same shoes, clothes, etc you wear everywhere else, plus washing the floor, walls and any surfaces with bleach and water.

Remember cleanliness is next to godliness, and next to impossible for a dirt farmer.