On Wednesday, March 20, 2019 at 10:43:28 AM UTC-6, wrote:
On Wed, 20 Mar 2019 07:58:03 -0700 (PDT), honda.lioness
wrote:

On Tuesday, March 19, 2019 at 12:19:24 PM UTC-6, wrote:
Septic or city sewer?

City sewer.

I suppose you could put a little insecticide in each trap to knock
them down. Once you break the cycle they will usually stay gone a
while but they are living in the sewer pipe. Be sure your traps are
staying full of water (no bad vents)

Thank you, Andy and for sharing your experiences. I use all drains regularly, so I believe the three traps (in my small condominium) are staying full of water. The daily count of flies-that-go-to-heaven is down to about one-third of its recent peak. The temperatures have also dropped where I am, so my study may be biased.

Eggs laid, 30-100 per female fly. Hatch in two days.
Larva stage 8 to 24 days. Larvae can survive in low oxygen conditions.
Pupa stage one to two days.
Adults breed only once, usually right after emerging from the pupa.
Adults die after 3 to 4 days with no food. If food is available, they will live around 7 to 21 days with liquid carbohydrates available.

https://agrilife.org/extensionento/p...s/drain-flies/

I have a couple of plastic containers near the drains with a mixture of apple cider vinegar, sugar and a drop of dishwashing soap (for fragrance) that usually catch a fly or two each day at this point.

I have repeated once so far the multi-hour soaks of the piping upstream of the traps, using the Home Depot bio-enzymatic drain cleaner. I have been using the bulb of a turkey baster and a hose clamp to stopper off the drain pipes and do these soaks, just above where they join the u-traps. This approach is a little easier than taking apart all the drain piping beneath the sinks and scouring with a brush. (I have dis-assembled and scoured in the recent past.)