An Ally Fly in Caterpillar Combat

Tachnid fly takes aim for a tomato hornworm, Pratt, Kansas

If you're country enough to grow your own tomatoes, you probably know about tomato hornworms [1]. These finger-sized caterpillars can chew their way through LOTS of foliage, affecting growth and crop quality when present in sufficient numbers. Often, gardeners spray to prevent damage.

Sometimes, though, they catch you by surprise "“ not hard, considering they are perfectly camouflaged and tend to hide during daylight hours. I didn't know my plants were under attack until I noticed a vine tip dancing unnaturally during the heat of the day. Investigating, I discovered a grisly and fascinating case of biological control in my own garden.

A large hornworm caterpillar was under attack by a Tachinid fly. [2] This particular species, which looks like a hairy housefly, is a parasite of tomato hornworms. The female fly, sporting a stinger-like ovipositor and eager to lay eggs, flew repeatedly at the large host. The caterpillar, virtually defenseless, writhed violently to discourage its attacker from landing.

It was a slow, futile effort. Again and again, the larva tired and the fly landed, piercing through the skin and leaving a mature egg inside the caterpillar's body. I watched for ten minutes until the hornworm finally grew still and allowed the fly to finish its work.

When I returned later, the fly was gone, and the larva was wet and bleeding from its stab wounds. But it wouldn't die yet. Instead, it would continue to feed as the fly eggs hatched within and voracious maggots began to ingest it from inside out. End of problem.

That's a tough way to go, but hey, it's nature. The good-bug, bad-bug routine is constantly at work, and in this case, it saved me a chemical application. Along the way, it heightened my appreciation of the amazing world we live in.

Tachnid fly stings a tomato hornworm and prepares to lay eggs