UW-Madison Insect Lab director recalls unusual insect ID’d in Rhinelander

The Case of the Hitchhiking Bog Wasps

While most of the cases at the Insect Diagnostic Lab involve fairly common insects, I do also see my fair share of unusual cases each year. One of my favorites from 2015 involved a miniature “bog wasp” from the family Eucharitidae: Pseudochalcura gibbosa. Due to their small size, these tiny (~2 mm long) wasps would simply go unnoticed in most cases––that and the fact that you’d most likely have to be wandering around in a bog to find them. So how exactly did these tiny, easily-overlooked “bog wasps” end up being submitted to the Insect Diagnostic Lab?  Simple: a homeowner found several in a second story bedroom of their house. This simply didn’t make much sense, so I knew there must have been a deeper story at play. Whenever I get an unusual case like this in the diagnostic lab, I often have to track down additional pieces of the puzzle before things make sense.

Bog Wasp-Pseudochalcura gibbosa
The tiny, hump-backed wasp: Pseudochalcura gibbosa. Photo Credit: PJ Liesch, UW-Entomology.

In this case, this particular home was located near Rhinelander, Wisconsin, where there’s certainly an abundance of bogs. As part of their life cycle, the females of Pseudochalcura gibbosa lay eggs on Bog Labrador Tea (Ledum groenlandicum), a common shrubby plant in northern bogs. The eggs spend the winter on the plants and hatch the following spring. However, these wasps aren’t plant feeders, and their presence on Labrador Tea is temporary. What they’re really after are immature carpenter ants (Camponotus sp.) to feed on. After the eggs of Pseudochalcura gibbosa hatch, it’s thought that the wasp larvae hitch a ride on foraging carpenter ant workers back to their nest. Once they’ve dropped off their six-legged taxis in the ant nest, the tiny larvae of Pseudochalcura gibbosa behave much like a wood tick on a dog: they hang off of and feed on carpenter ant larvae and pupae. In some cases, dozens of small wasp larvae may be present on a single carpenter ant larva. Eventually the tiny wasps complete their development and leave the carpenter ant nest to head back to the bog.

Having identified the wasps as Pseudochalcura gibbosa, I was suspicious that a carpenter ant nest was also present in the home and simply hadn’t been found yet. After some detective work, the homeowner eventually confirmed the presence of carpenter ants in the house. With that final piece of the puzzle I had my explanation for how the wasps had hitchhiked from a nearby bog to an upper story bedroom: it’s was all the ants!

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