Insect Quiz February, 2005

February Quiz 2005

One day you notice a dozen small insects in your bathroom.   They fly a little but mostly seem to walk or run along the counter.   As you look more closely, they are dark-colored and look like flies.   You cannot see any obvious places where they are coming from.

1.   What kind of insect are they?

This insect is a phorid fly also known as a humpbacked fly.   It is about 1/8 inch long with a hump-shaped thorax that is tannish to dark brown on top and light colored along its sides.   If you examine a wing under magnification, you'll find two strong veins at the top with four parallel veins radiating out toward the margin.   Although a phorid fly looks similar to a fruit fly, a phorid fly lacks the red eyes that a fruit fly has.   A phorid fly also moves in a rapid, jerky motion, often preferring to walk or run than fly.  

2.   Where are they coming from?

You can find phorid flies in many types of moist decaying organic material. They are commonly found in drains, especially in bathroom sinks and showers, as well as food residues in trash containers, rotting food, infrequently used garbage disposals, dirty mops and old dish rags, potting soil, organic material on the bottom of pet cages, and sewage from broken sewer lines.   They have even been known to infest mausoleums and feed on human corpses!  

3.   What kind of damage do they cause?

These flies are essentially just a nuisance, especially when they occur in large numbers.   Phorid flies can potentially vector disease organisms by physically contacting them when they visit rotting foods and generally unclean areas and then transmitting disease organisms to other sites where people can contact them.   However, they are generally not considered to be a medical problem.

4.   What is the best control?

The best method to control phorid flies is to locate and remove the moist organic material that the larvae develop in.   If they are in a drain, use a hard-bristled brush to remove the film of organic gunk the larvae are infesting.   It may be tempting to spray the adult flies with an insecticide to control the problem but as long as a food source is available, the phorid flies will continue to occur.   Finding the food source can be challenging and often requires detective work to locate the infestation.   Remember that they can be found in a variety of different places.   Be flexible in your thinking as the problem could occur in unexpected sites. The keys are they are in moist, organic material.