Photo credit: Karen Macon Jackson

Q: I found this 4-inch green caterpillar on my tomato plant.  It had diagonal stripes and a horn at the rear end. It seemed to have eggs like rice on its back.  My tomato plant seems okay though. 

A: Tomato hornworm caterpillars eat tomato leaves, branches, and green fruit. They overwinter in cocoons in the soil, then emerge as moths in late spring-early summer to lay greenish yellow eggs on the undersides of host plants leaves. The larval caterpillars hatch out and feed for about a month before the cycle begins again.  They can decimate a tomato plant but usually just do incidental damage.

The white elliptical rice-like things on the hornworm’s back are cocoons—but not hornworm cocoons. Your hornworm was parasitized by a beneficial insect, either a braconid or trichogramma wasp.  A while agon, these tiny wasps laid eggs on the hornworm, which hatched and entered the hornworm, consuming it from the inside while they grew, just like aliens in a sci-fi movie.

They have now emerged and made their cocoons. They saved your tomato plant! The parasitized tomato hornworm feels sick and cannot cause further damage.  The cocoons contain developing tiny wasps that will emerge and parasitize other hornworms.  So, don’t destroy the hornworm now—it’s a source of more beneficial wasps.