Housefly infestation

I live just north of Houston and have spent my entire life in this area. My current home is the first that I’ve ever encountered a fly infestation, though. It’s to the point where I think I’ll have to sell the home.

For context, I do exterminate regularly and keep a clean home. I don’t leave food sitting out, do dishes several times a day, wipe down counters, mop, etc. I can kill all of the flies inside the house (minimum of a dozen per day) but they’re getting in through the doors.

Whether it be the front door, back door, or the garage, the flies congregate in swarms. I try to slide inside as quickly as possible – I don’t even open the door all the way – but there are so many around the doors that a few end up getting in each time anyone enters or exits.

I have no idea what is attracting them to my house. I don’t have any trash piled up, no pets or dog poop, I keep a trimmed yard, there’s no rotting vegetation or anything dead in the yard. This is my third summer in this home and so far this has happened every year as soon as it begins to heat up. They’ll go away once it begins to get cold but come spring time they arrive with a vengeance.

I’ve tried fly traps. They work – they catch flies by the hundreds – but they’re like zombies. I kill 10 and 20 take their place. I’ve tried other tricks as well like pinesol and Irish spring but nothing seems to work.

Any tips or tricks? Anything I could be overlooking that is attracting these flies?