Whopper of a Wasp

A group of campers at the nature center spotted a group of these wasps on a downed tree in the woods southeast of the Museum. What caught their attention was both the extremely long ovipositor (over 5” long!) and the pale green sac that would sporadically inflate as they watched the wasp.

Among the questions raised by the group, (What is this? Is it dangerous? Can it sting me with that long thing?) the most asked was, “What is it doing?”

This is the time of year when the giant ichneumon wasps have mated and the females are laying eggs. This is a parasitic wasp. It is particular about the host it is looking for – a pigeon horntail larva. Ironically, pigeon horntails are also a kind of wasp. The females lay their eggs on the surface of the tree and once hatched, the larvae burrow into the wood where they will live for up to two years.

Along comes a female ichneumon wasp. Scientists aren’t sure exactly how she finds the larvae inside the tree but they suspect she is relying on smells or sounds produced by the larvae. Once she has zeroed in on a larva, she inserts her ovipositor into the tree. A sac at the base of her abdomen fills with a liquid that is released and starts to decompose the wood so she can reach the larva. The sac also helps to stabilize and support the female wasp during this time. Rotating her body to create the correct orientation, she is now able to deposit an egg on the larva. The entire process of laying one egg can take up to one hour. Once she’s done, the wasp withdraws her ovipositor and moves on to find the next victim. She’ll only live 27 days, so she doesn’t have time to waste.

The ichneumon larva develops and hatches from the egg to start feeding on the horntail larva. It stays in this stage of its life through the winter, waiting to pupate inside the tree in the spring. In June, the new adult emerges, mates, and starts the cycle again.