Pest Control/Drain Flies
We have a jetted tub and started seeing swarms of drain flies on the wall of the tile wall around the tub and shower. Most of the web articles suggested that the drain flies come up from the drain but this didn't seem to be the case. I removed the side panel of the tub and found hundreds of the drain flies in here. They could be breeding in the crawl space below the tub or perhaps moister has gotten into the sheet rock around the tub. My bet would be that there's moisture in the crawl space but I don't really know. Any suggestion for next steps to deal with this? Temporarily I put duck tape around the tub side panel and that's improved the fly situation for now although it looks pretty bush league!
Sorry for such a long delay in getting back to you (computer bugs are a lot harder for me to control than the bugs that exist in reality).
Drain flies are difficult to control - the key is finding their breeding media and disposing or destroying it. Killing the adults will not totally do it, as we've got to break the breeding cycle. There are about a half dozen or so different species of flies that are referred to as drain flies. Some, such as fruit flies like to breed in fermenting materials, while others need decomposing material in which to lay their eggs. My guess is that you are dealing with the latter rather than the former.
Typically the critters that are found around drains themselves (as opposed to fruit flies which can be found all over the place) breed in the gelatinous material that lines the inside of the drain pipe. There used to be a "old wives tale" that putting bleach down the drain was a good thing. Not so much - it kills too many of the wrong type of single-celled critters - but not the poly-celled flies.
Michael, try using a bottle brush, or something similar, to clean as far down the inside of the drain pipe as possible. Your goal is to remove as much (if not all) of the breeding material as you can. You can also use a pyrethroid insecticide (flushing agent) to destroy the adults you find after removing the side panel.
If moisture has gotten into the sheetrock - it's possibly adding to the problem, but might not be THE problem. Maybe you could use a de-moisturizing product in conjunction with cleaning out the drain.
When I retired from the industry, there were a lot of new products coming to market (especially for the commercial pest control firms) to fight these critters - so you may be able to find something that will aid in your fight, but the primary method of control - is to eliminate the breeding media.
I sincerely hope this helps.