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Fungus Gnat infestation in home with no plants

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From: Maureen Sullivan
Springfield, VA
Hoping you can offer some advice as we are desperate. Situation: last Christmas we somehow unknowingly brought fungus gnats into our home by either a mini Trader Joes Christmas potted tree or live plant received as a gift. Shortly thereafter we notices what seemed like fruit flies mainly downstairs; we have no other soil potted plants in the house. We noticed them flying around the house, but downstairs where we have sticky pads we noticed them under the door. The downstairs is finished with a bedroom/recroom and then the basement section that has tile floor washer dryer and storage. We have a sump pump in the basement with french drains that were installed over 10 years ago due to water coming in the house built in 1966. We called our bug guy who had them tested and confirmed they were fungus gnats. He tried spraying around the house which didn't help. He then said that he thought since we had no plants in the house they were likely either multiplying the sump pump so we sealed those off with masking tape but saw no indications that was where they were coming from. He then put some powder in the french drains and we put three humidifies running 24/7 in the basement and bingo that seemed to do 97 % of the trick. The powder he used he said would last for months. But they were not 100 % gone. Then all of a sudden about a month ago they came back with a vengeance. We don't know what to do anymore and are very frustrated. What do we do now and how do we resolve this problem. The bug guy suggested we have the french drains flushed but I am not sure what that even means or how to go about it or who to call. Regarding the french drains. The were installed on a concrete wall painted in the basement and actually I believe go around the entire perimeter of the downstairs- just you can only see them along that wall. There is some paint that has fallen off above the drains due to moisture in certain spots so there is some moisture there. It has not rained here in quite some time. We are in Northern Virginia. I was wondering if you might have some suggestions on who we should call about this- a plumber, a different bug guy? Any suggestions on what we could do ourselves if anything would be much appreciated. thank you.

Extension Message
From: Richard Hentschel
Extension Educator, Horticulture
DuPage/Kane/Kendall Unit
Hello, Without completely understanding of how your french drains are constructed in relation to your walls, I can share information on Fungus Gnats. Their food source is organic matter and most often found where there are houseplants. Other locations include floor drains that are relatively unused and in the duct work of certain kinds of AC units in a home. they prefer high humidity and cooler temperatures such as would be provided by houseplants sitting on a cool windowsill, basement bathrooms, and cooling ducts in the AC unit. In your references to your french drains, this may also be possible. Since they feed on the tiniest of bits of organic matter, cleaning and scrubbing the french drains especially if they are exposed might be an option. Soapy water and something like a toilet bowl brush could be used. You want to avoid chemicals that could harm the lawn or shrubs a the point of sump pump discharge. Your cleaning should also include the sump pump pit too. If you can use plenty of rinse water to flush what you have broken loose into the sump and out of the home

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