Braconid wasps

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Definition as written by critterologist:
Here's a picture of a tomato hornworm that has been parasitized by braconid wasps:
(Click for full-size) ...

Braconid wasps are tiny, ranging in length from 1/16 to 5/16 inch, with stout bodies. Host pests include various species of aphids, garden webworm, tomato hornworm, armyworms, strawberry leaf roller, and tent caterpillar, among others.

Parasitic wasps are also beneficial and include braconid wasps which attack aphids, caterpillars and the larvae of many beetles. Ichneumonid wasps can help to control whiteflies, aphids and some caterpillars.

A healthy plant can withstand a lot of bugs, Paul says, and if the gardener holds off for about a week, natural predators — such as ground beetles, damsel bugs and braconid wasps — may move in and help control the problem.

It's a bit off-putting to see this creature on your plants, but you're better off letting him be and letting the wasps do their job. Once they hatch, they'll be enough braconid wasps to keep your garden hornworm free.

Tomato hornworm: Handpick and squash the caterpillars, but spare those that carry the white cocoons of braconid wasps on their backs. The wasps are their natural predators; to attract them, plant dill, and let your cilantro flower.

See also: Plant, Insect, Spring, Predator, Stage