Jeffrey Hahn, Asst. Extension Entomologist
Many people are familiar with boxelder bugs, multicolored Asian lady beetles, cluster flies, and other nuisance insects that can be found in homes. But occasionally less familiar insects are found inside. On one such occasion, a homeowner reported finding a lot of wasps nesting in her home. In fact, she had identified them from the internet as Cerceris fumipennis.
This wasp species is a solitary wasp in the family Crabronidae. It has gained fame recently as a method for detecting emerald ash borers (EAB). This native ground nesting wasp hunts buprestid beetles, including EAB, which it paralyzes and carries back to its nest to feed its larvae. Location of a nest with captured EAB indicates the presence of EAB in the area. More information on Cerceris wasps can be found here.
This wasp's status is unclear in Minnesota and nests have not been discovered so far. Ultimately, you would not find this wasp indoors in the winter as they do not overwinter as adults.
A sample of insects was requested from the homeowner with the expectation being that paper wasp queens, Polistes spp., which overwinter gregariously in homes would be found. It was therefore quite a surprise to find that not only were the insects not paper wasps, but were instead flies. A run through the diagnostic keys identified them as soldier flies (family Stratiomyidae). These flies are typically black and yellow insects that can appear to be wasp- or bee-like.
A run up to the museum, and with the help of John Luhman, the soldier flies were identified specifically as Ptecticus trivittatus. This is a species that is particularly associated with composts. The conclusion was that these soldier flies most likely originated from a compost nearby in the neighborhood and they found their way to this home in which to overwinter. There had a mild stretch of weather when the homeowner first noticed the flies which would have been sufficient to cause overwintering insects to become active. There are other flies, such as cluster flies and face flies, that overwinter in structures so this made sense.
The homeowner kept insisting that these insects were nesting in her home. She was continuing to see consistent numbers of them; at one point she was seeing as many as 20 - 30 at a time. She eventually asked whether these flies could be associated with composts. They had brought a worm compost box indoors so the worms would not freeze. It was in a plastic tub with a cover but there were small air holes. She wondered whether the soldier flies could be in the compost. That of course was the source of the problem and why they were seeing such persistent numbers. The adult flies would be short-lived but to put an end to the problem it was important to erect some kind of screening so the flies could not escape.
A good example of how the identity and biology of an insect has a direct impact on its management.