Early last winter I was speaking with my dentist (also a close personal friend) who told me that he had had a Boxelder (Acer negundo) shade tree cut down in front of his clinic. I asked why and he answered, "bugs!"
Apparently, "millions" of bugs came from the tree into his building and he was concerned that his patients might not find this particularly professional or hygienic. I had never heard of such a thing and so contacted my County Forester who once again helped me out.
Emerging in the spring from eggs deposited in crevices of the bark, "Boxelder bugs", Leptocoris trivittatus (Say), suck the sap from developing leaves and deform them in the process. They also feed on the tree, flowers, and seeds. There is usually one generation yearly although in warmer climes there may be two.
In the fall the bugs crowd together on the south face of the tree, rocks, buildings; anything exposed to the sun. Once a large mass has formed, they then fly to a nearby building to hibernate for the winter as adults. They become a nuisance and are considered a household pest. In the spring, they return to the trees where they remain during the growing season.