Knowledgebase

Gnat infestation #211972

Asked September 11, 2014, 3:41 PM EDT

My basement is suddenly infested with gnats!  Thousands of them.  I have no potted plants in my basement, so I'm not sure why they are there.  I apply a flying insect spray and now my floor is covered with their dead bodies.  I vacuum them up and within days, the floor is covered again.  We did have a leaky shower which required us to tear down an inside wall.  We fixed the leak, but haven't repaired the wall yet or the floor drain which we did seal.  How can we get rid of these gnats??

County Colorado

Expert Response

Given there were plumbing problems, it is most likely either a moth fly: http://bspm.agsci.colostate.edu/files/2013/03/Moth-Flies.pdf

or a phorid fly: http://bspm.agsci.colostate.edu/files/2013/08/Phorid-Flies.pdf

Elimination of the food source is the means to manage these.  As they feed on microbes associated with wet environments, drying out the food material is the primary approach.  

I am guessing you have some damp site created by the leak that is providing the conditions for them to breed and they will continue to develop as long as the site remains favorable for breeding.  
An Ask Extension Expert Replied September 12, 2014, 11:35 AM EDT
Not sure if my bugs are the kind you mentioned, but i don't think I have any moisture that wasn't there before they showed up. I've arranged to have the repairs made to the walls, ceiling & floor next week so maybe that will take care of it. 
The Question Asker Replied September 12, 2014, 1:07 PM EDT

Can't tell what they are from the pictures - either phorids or moth flies.

If they are phorids then they are feeding on bacteria associated with some food material lodged at some point in the plumbing.  A thorough clean-out, which usually involves a formulation of cleaning agent that foams and can penetrated to pockets in the plumbing, is needed to eliminate the food source.

If pipes have leaked because of a break then the flies can breed in the soil next to the break, and adults travelling in the pipe then may emerge in the home.

If you want the insects identified I think you will need to collect some and mail them to CSU.  If you want to do this send them to:  Whitney Cranshaw, Dept BSPM-1177, Colorado State University, Fort Collins, CO 80523.

However, the two sheets I sent notes about do have picture that can allow identification of these two, at least from each other.  So check the sheets referenced in the first note before you consider trying to send anything.

An Ask Extension Expert Replied September 12, 2014, 1:31 PM EDT
Here are a couple more pics of my bugs. 
The Question Asker Replied September 13, 2014, 12:59 PM EDT
I still can't tell from the pictures.  Possibly moth flies.  But something doesn't look right and they may be something else - perhaps hackberry psyllids.  If you want these identified they will have to be collected and shipped for identification. 
An Ask Extension Expert Replied September 22, 2014, 1:45 PM EDT

Loading ...