Drain flies to say the least can be quite a nuisance. The pests feed off
of stagnant water and can be found in moist areas such as your sinks,
drains and garbage cans. They thrive off the sludge buildup inside them.
Drain flies seem to be most active in the evenings.
According to websites on the internet such as domyownpestcontrol.com you
can try and get rid of the flies. Many say to find the source of the
stagnant water, check you floor drains and sink drains. Clean them
thoroughly with hot water and then with bleach. There are also many over
the counter products, such as drain gel. According to the websites a one
time cleaning just won't work. We also talked with a pest control
company here in town and they suggested you try the drain gel and say it
seems to be the most powerful.
By the way, there is no drain fly season. They can be found year round.