What are the white TicTac-like structures hanging off the back of this tobacco hornworm (larva of Carolina sphinx moth)?
These are the cocoons of a Braconid wasp (Cotesia congregata)! The female adult wasps use their long ovipositors to lay eggs inside a live hornworm caterpillar, along with viruses that take down the internal defense responses of the hornworm. The larvae hatch inside the caterpillar and feed on their host. After 12-16 days, the wasp larvae chew through the skin of the dying caterpillar to spin their silken cocoons. About 4-8 days later the 2-3mm adult wasps emerge and fly off, following chemical scents released by the damaged plants and feeding caterpillars to find more hornworms.
To discourage tobacco hornworms in your tomatoes, allow the wasps to complete their metamorphosis – they will cause the host caterpillar to feed slowly and die before pupating and will also release more parasitoid wasps.