Question
Almost a month ago, I have had a mouse die in the wall of my home. It smelled really bad and eventually the smell went away.  After that, I noticed large larvae in my kitchen and some large flies as well.  However, over the past few weeks, I haven't noticed that many flies anymore.  Last week, I went on vacation and when I returned, I noticed maybe 20-30 large dead flies as well as the same if not more house flies.  Over the past few days, I continue to see the regular house flies on a consistent basis but not the larger blow flies.  I am concerned about this.  Is this just stages of flies from that dead mouse or does this signify something different.  What is the best way to kill the flies and their source? I'm hoping that since the mouse has stopped smelling that means that it has stopped decaying that that eventually there won't be attraction for the flies to lay eggs.  How long should I expect to see flies after this incident?

Answer
Crystal,

I think there may be a couple of different things going on here. The large black flies are probably blow flies that developed on the dead animal in the wall. These will stop once the animal decomposes which it apparently has already done.

The "house flies" may not be house flies at all. House flies generally don't enter houses except the occasional one that flies in through an open window. You almost never see numbers of house flies indoors. The fly that enters houses is a cluster fly and it looks very similar to a house fly except for some golden hairs behind the head.

Cluster flies are not associated with the dead animal. They entered the house last fall and have spent the winter in a type of dormancy. They are now becoming active and are trying to get outside. They cause no damage except for the nuisance.

Take a look at this page http://www.livingwithbugs.com/cluster.html for more information about cluster flies.


Jack DeAngelis, PhD
Extension Entomologist (ret.)
author Living with Bugs: http://www.livingwithbus.com/bug_book.html

update:

I forgot to mention control of cluster flies in my original answer. In late spring (now) the flies are leaving the house so no control is needed. In a few weeks or less they will be gone. Next fall new flies may try to once again enter the house to spend the winter (remember they cause no damage). The best 'control" is to prevent them from entering the house in the first place. See the page cited above for some suggestions and where exactly they are coming from.