Question: I have found several big green caterpillars covered with something that looks like white eggs on two of my tomato plants. What are they? How can I get rid of them?
What you are seeing is a good example of Nature helping you to control some of the pests in your garden. The green caterpillars are tobacco hornworms, which can chew up big portions of a healthy tomato plant before you even know they’re there. The white things are the cocoons of braconid wasps, parasitic wasps whose larvae feed on the hornworm, ultimately killing the caterpillar. The wasps, which are very small and not a stinging type, are considered beneficial insects in large-scale agricultural production and in home gardens.
Here’s how it works: A female braconid wasp lays eggs under the skin of a hornworm. The eggs hatch, and the larva begin eating the caterpillar’s insides and chew their way out through the host’s skin when they mature. On the outside, they spin the little oval cocoons along the caterpillar’s back and sides. By the time the adult wasps emerge, the hornworm is weakened and will soon die.
If you see a green hornworm on tomato plants – and they’re hard to spot, being exactly the color of tomato leaves! – pluck it off and dispose of it. But if you find one with white cocoons all over its back, leave it in place. It’s hosting another generation of parasitic wasps that will seek out and destroy hornworms on your tomatoes.