Every summer I put the house plants outside on the covered patio. They are out there usually May-September. This year not too long after I brought them in, I noticed small houseflies in the house. These are not fruit flys or gnats. They are houseflies. They are small, but definitely bigger than the ones that you get from bananas. Could they be coming from the plants? I can't find anything dead or rotten in the house and don't know where else they could be coming from. They are disgusting and driving me nuts. What to do?!