What causes an insect phobia?

This is such a good question, i was wondering it myself a while ago. I think that maybe since bugs are ugly, as you said, people don't want to have them around. And one of the first questions is, do they bite? when people see one that they don't reckognize. since bugs are gross (because they are all crawly and ugly) and they have a change of being dangerous that is why at least I avoid them.