Into the Hole

It was a beautiful mid-October day; we were walking along the edge of an open woodland when H spotted something on the ground. That something was a bright green caterpillar on the brown and tan of the bare ground and the fallen leaves. The caterpillar was moving, but not on its own, it was paralyzed and being dragged by an inch-long wasp.

We watched as the wasp dragged the caterpillar quite rapidly across the ground, over fallen leaves and through scattered tufts of grass –

I recognized it as one of a group of ground-nesting parasitic wasps and we followed along as it traveled 40-50 feet. 

After a few minutes the wasp arrived at her destination and put down the caterpillar –

Then began opening a previously constructed burrow by removing pebbles and soil that were blocking the entrance –

When the burrow was open, she turned and pulled the paralyzed caterpillar into the burrow –

The wasp emerged and proceeded to pick up small pebbles and insert them into the burrow to block the entrance –

Finally, the wasp turned to scrape loose soil over the entrance and fly off –

Back at home, and after looking at the photographs, I identified the wasp as a common thread-wasted wasp, which is found from Canada to Central America. The species stings the caterpillars of moths and butterflies to feed their own larvae. The sting paralyses the caterpillar, thus ensuring a fresh meal for its own larva.

The common thread-wasted wasp lives in areas with bare soil where the female excavates a burrow a few inches deep with a brood chamber at the bottom. After completing the burrow the female closes and conceals the entrance, then flies around the area to memorize landmarks so she can find the burrow again.

At that point the hunt begins; when the female wasp finds a suitable caterpillar she stings it and takes it back to the burrow where she lays a single egg on the immobilized caterpillar. After laying the egg, the female wasp again closes the burrow with pebbles and soil. Occasionally the wasp will use a pebble as a hammer to pack soil tightly into the burrow entrance – one of the few instances of a tool-using insect.

The egg hatches after two days and the wasp larva feeds on the live but not moving caterpillar for about a week. It pupates and then emerges the next year to begin the cycle anew.