Question
We recently bought a new construction house.  Last night we noticed about 20 house flies "piled" into one corner of our dining room ceiling.  Why are they staying in this confined area?

Answer
Rachael,

These are probably cluster flies rather than house flies. Cluster flies look a lot like house flies but enter houses in large numbers and tend to "cluster" together. They are actually parasites of earthworms so tend to be most common in homes that are surrounded by well managed turf like golf courses, cemeteries, athletic fields, and horse pastures that support healthy earthworm populations. The flies are harmless but annoying once they get indoors. Here's an article that explains a bit more life history and control http://www.livingwithbugs.com/cluster.html that includes a picture.

Jack DeAngelis
http://www.livingwithbugs.com