Threat: The Tomato Hornworm
These little guys can do quite a lot of damage in your tomato patch. They are hungry and will eat through the leaves of your plants & take large chunks out of the fruit themselves. Here are some photos of them on the plants, you can see some of the damage they have done (the stems have no leaves on them where they have been eaten)
& Here is a photo of one that I picked off the plant. The best way to deal with hornworms organically is to pick them off the plants as you see them (look carefully, they are very camaflouged, you can see the damage they have done, and droppings easily).
Once you pick them off the plant, you have to decide what is best to do with them. I have always killed them with the scissors that I am using to prune (may sound barbaric, but it is part of being an organic farmer).