how do I prevent aphids
 

I picked a beautiful huge head of cauliflower yesterday, took it in the house, washed it off then left it on the counter for about 30 minutes, when I looked at it again I saw millions of aphids all over it. I did not see a single one when I was washing it.

how do I prevent these bugs from getting in my cauliflower because they are a pain to get out, I got most out but I don't think I got them all.