The self-sustaining balance we seek in garden communities is only achieved through complexity.
~ Douglas Tallamy, Bringing Nature Home
If you're out watering and puttering around your tomato plants then suddenly run into one of these boogers, I say it's a good occasion to shriek and do a little creeped out dance.
Aren't they bizarre? Especially with those egg-y things hanging on to them!
Hornworms are the larval stage of a sphinx moth. During this stage the worms feed on plants in the Nightshade family, which includes tomato and tobacco.
It turns out that the eggs are actually cocoons and they are a good sign; if you have hornworms, you want them to have cocoons attached.
Why? Because the fuzzy, white cocoons contain the pupa of a braconid wasp. The braconid wasp pupae suck the organs out of the hornworm and weaken it sufficiently that it can't completely devastate your tomato plant before it keels over.
If that isn't amazing enough, there's more: Another type of wasp (pteromalid) lands on the cocoons and deposits its eggs inside them. Then those pteromalid pupae eat the pupae that eat the hornworm!
The braconids keep the hornworms from being such a devastation and the pteromalids keep the braconids from completely wiping out the hornworms which are hosts for these insect parasites.
If you remove cocoon-infested hornworms from your tomatoes, you also remove scores of braconids and pteromalids, both of which eventually become pollinators.
I read about all this in Bringing Nature Home. It's nice to feel kind of proud (instead of embarrassed!) for having parasitized hornworms in my garden. According to Mr. Tallamy, having the sorts of insects which feed on these worms is one of the many rewards for maintaining a diverse landscape with lots of native plants.