To get rid of/ward off hornworms in your garden, you can sprinkle some cornmeal on the soil. They cannot digest it, and any grubs under the soil will end up eating it as it seeps into the ground. You'll have to pick these big guys off by hand, though. The horns are harmless to humans, and can usually be found on the underside of a leaf. Also, their poop looks like little blackberries.