I've been having a problem with hornworms eating my tomato plants. They're so big, ugly, and nasty that I hate picking them off the plants. The other day, I found several hornworms with little white things all over them. I got out my Rodale's Encyclopedia of Organic Gardening (Thanks L's!), and read about it. It said that the little white things are parasitic wasp eggs that will kill the hornworm. So I left the wasp-infested hornworms on the plants so the good wasps will hatch and lay more hornworm-killing eggs.