Seeing as you are looking for a pesticide, I'm guessing it's a bit of an emergency otherwise there are other (slower) ways of getting rid of pests -- it also depends on *what* you are trying to kill. In the past I've always used the cayenne and garlic recipe YBG mentioned.... that is, when my ladybugs and wasps didn't get to them first ;-)

What particular pest are you trying to get rid of, and where on the plant are they located?