Has a pest control professional confirmed that wasps are present in any areas other than wall voids? They would not be interested in getting access to pipes, either empty or (especially not) when carrying water. Social wasps like Yellowjackets (the type most often nesting in wall voids) build paper-like nests, which would not hold together in a pipe with any water pressure. If piping is experiencing a drop in water pressure, we suspect another cause unrelated to the wasps is behind it. Uncut grass and unpruned trees will not impact where Yellowjackets nest when they are the type nesting in wall voids; some species nest in the ground, but if anything, they would be nesting in that yard, not one with regular mowing. Where they forage for food outside of the nest will have little to do with yard care of a neighbor. Trees do not need regular pruning unless they have health issues or limbs at risk of falling due to decay.

Does she have Spotted Lanternflies in the yard? These invasive insects have spread into just about all Maryland counties by now (including Frederick Co.) and, as they feed on plant sap (not causing serious damage), they secrete a sugar-water-type waste liquid called honeydew. Honeydew can drip onto cars parked under or near trees, or any other structure that might be hosting lanternfly populations. Wasps seek sources of sugar, especially when other sources like nectar are declining at the end of the season, and so can often be found congregating around lanternfly populations in order to lick up the honeydew sugar.

That might explain why wasps are near a car, since they otherwise would be highly unlikely to enter a car or have anything to do with one. Once in a very rare while, a small Paper Wasp nest might be built on a car that isn't moved for long periods, but even then, Paper Wasps make a much smaller nest than Yellowjackets, and the wasps themselves look different in color and shape. If you wanted to share photos of any wasps that you can safely take pictures of, we can try to confirm their ID.

Social wasps do not maintain their nests over the winter; all colony members die as soon as we experience a few frosts/freezes. Then, the nest will be abandoned, and only a handful of "queens" will overwinter alone, in more sheltered areas away from the nest they were born in. The nest is not re-used the following year. The social wasp web page linked above goes into more detail about their life cycle and habits. Although the temperature protection of nesting in a wall void might delay the abandonment of a nest in that location, nest activity will still eventually stop sooner rather than later, because the wasps will not have any food to keep raising young and sustaining the adults. (They do not make or store honey the way honey bees do, so they cannot stockpile food to get through the winter.)

We suggest that you have a pest control professional with experience in dealing with wasp nests in buildings assess the situation and remove the nest, if they are able. Even though this is not to prevent a colony from expanding in this case, since they're at the end of the season and lifespan for the workers, it's to help prevent future issues such as rodents seeking the dead wasps as a source of food, and thus potentially causing their own damage.

Miri