Just Flush the Toilet, Vol. 5, No. 24

Just flush the toilet?  It doesn’t happen very often but sometimes the solution to a pest problem is just this easy.
 
The situation runs something like this. A homeowner calls to ask about insecticides to control a fly problem that has suddenly appeared in the house.  Often they have been seeing a few flies for some time but the problem has gotten progressively worse.  Sometimes these are large flies, flies the size of house flies, but more often they are smaller flies and are described as “gnats.”  Sometimes the complaint involves more than one kind of fly.  
 
These can be difficult problems to solve because there are quite a few species of flies that can be involved.  The solution requires finding out where the flies are breeding or entering the building and either eliminating the breeding source or excluding the flies from the building. 
 
Different species of flies breed in different situations.  Sewage gnats and phorid flies are often, but not always, associated with sewage or drains; fruit flies are most often associated with fruits and vegetables, garbage or “empty” drink containers, and fungus gnats occur around potted plants.  House flies are often associated with garbage, but they can also breed in pet feces or moist pet food, as can bottle flies and flesh flies.  Flesh flies look a lot like house flies and often breed in carcasses of rats, birds or other small animals that have died in the attic or other areas of the house.  See the challenge?
 
One clue that can help focus the search for the source is a conversation that runs something like this.  Where are you seeing these flies?  “They are mostly back in the spare bed room” (or some other infrequently used area of the house).  Is there a bathroom there?  “Yes, but we never use it unless we have company.” Is there water in the toilet bowl?  “Let me lift the lid and see.  No there’s not, and I see some of those little gnats in there!”  Ah, try flushing the toilet a few times and see if that stops them over the next few days.
 
What has happened is that the water in the bowl and P-trap of this seldom used toilet has dried up, allowing flies that were breeding deeper in the sewage system to enter the living area.  Flushing the toilet replaces the water and re-establishes this liquid exclusion barrier, which doesn’t just keep out flies but helps exclude odors as well.  Of course, this can also happen when the P-Trap of a seldom-used sink, bathtub or shower dries out.  Better check those too.
 
Another common cause of this type of problem occurs when there is breach in the wax seal on a toilet, allowing small flies, such as phorid flies and/or sewage gnats to enter the building.  Flushing the toilet won’t help in this case but replacing the wax seal will.
 
See pages 37-43 of Publication 2443, Control Household Insect Pests, for more information on flies that occur in the home. 

Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service. The information given here is for educational purposes only. Always read and follow current label directions. Specific commercial products are mentioned as examples only and reference to specific products or trade names is made with the understanding that no discrimination is intended to other products that may also be suitable and appropriately labeled. 

Mississippi State University is an equal opportunity institution.

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