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These are the cocoons of a braconid wasp, one of your best garden friends! The adult lays eggs on the back of the caterpillar, your tomato hornworm or others. When the eggs hatch, they have a pre-made dinner of caterpillar! Then they pupate to form these cocoons on your caterpillars. You can actually see some holes in the end of the white tubes. That is where the young wasp has emerged as an adult. At that point, it's looking around for another caterpillar to lay its own eggs.
The adult wasps are very small and they may seem like gnats if you see them. Most likely, you won't, because they are so small.
This is an excellent reason not to overuse insecticides in your garden. We can't control every pest insect, but we can rely on our beneficials to do a lot of the work for us. If we kill them or their food supply, then we're constantly in a pest cycle that is out of balance.