hmm well that could be anything really, so step one is IDENTIFY THE ENEMY. Step two BUY INSECTICIDE. the stage of growing you are in should also determine what kind of insecticide you should use on your plants. I would personally use anything by Safer's, as it's organic and can be used even up until harvest. It's gentle on the plants, and effective. Neem oil works as a preventative, insecticidal soap is a milder insecticide but works really well on most pests, and then safer's also has a product called End All, which will wipe out any stragglers/stubburn pests that won't die.