Expert: Jack DeAngelis - 7/2/2009

Question
We have drain flies, and the problem has become huge.  We previously had some type of small flies for about a year, and within the past month they have been taken over by a different type of fly.  The small ones are gone, it's a strange concept.  Anyway, the large ones we have figured out are drain flies.  We had a plumber come out after many attempts of doing different things ourselves.  He has found the problem in the crawl space and it going to fix it, but the flies in the crawl space need to be somewhat gone as it is un-bearable for him to work under the house until some have ceased.  So, what do you suggest we use to get rid of them?  Fogger?  Other bug bombs? (Also on a side note, any idea what the other flies could have been?  They were quite similar to fruit flies.  Is it possible they were fruit flies?)

Answer
Amy,

This may be one of the few times I'd suggest using a fogger (total release aerosol insecticide). I don't generally like foggers because they do a fairly poor job of application plus they tend to be expensive. However, in this case a fogger might work to reduce the fly population in this fairly confined space. Use only one at a time and be sure the crawl space is aired-out before anyone goes in following the application. Try to find a product that contains pyrethrum (or pyrethrins) as the insecticide (check the label).

Here's a link to some information about drain flies http://www.livingwithbugs.com/drain_fly.html that may be useful.

I'm not sure what the smaller fly might have been. Drain flies are themselves very small and there are not many flies that are smaller.

Jack DeAngelis, PhD
Extension Entomologist (ret.)
My website about home and garden pests: http://www.livingwithbugs.com