Black Worms in the Toilet With a Septic Tank

Many species of insect pests that would prefer to spend their lives outdoors can easily become indoor pests when something draws them inside and compels them to stay there. A prime example of this behavior is the drain fly. Although less common than other indoor flies such as the common housefly and fruit fly, many homeowners report the presence of drain flies when the flies have found a favorable breeding site inside the home.

Drain Flies

Drain flies, also known as moth flies or filter flies, are tiny, nonbiting flies that are easily recognizable by their dark-colored wings. Drain flies earn their name due to their proclivity to enter homes through sink drains in kitchens, bathroom sinks, bathtubs, showers and the like. A drain fly infestation is more likely to develop in standing water, particularly standing water that has been at rest long enough to become somewhat contaminated. Drain flies persist on the microorganisms and other materials that contaminate water, so once they have found a source of water with enough contaminants on which they can subsist, they are unlikely to leave. A septic system, particularly if it has not been pumped on a regular basis, may contain even more contaminants than a toilet connected to a traditional city water supply.


Homeowners often are not aware of a drain fly problem until they notice the larvae, which resemble small, black worms floating in the water. Drain flies prefer to breed in the same contaminated water in which they feed and will continue to do so until they are compelled to leave or until the water is removed. Again, because of the presence of fecal form bacterias and other contaminants in a toilet connected to a septic system, the chances are very good that the black worms you observe are drain fly larvae. Adult drain flies are capable of laying anywhere from 10 to 200 eggs at one time, and within nine to 15 days, the drain fly larvae will pupate into adult form.

Sanitation & Exclusion

Preventative steps to exclude the drain flies from ever entering your home is the best way to avoid a drain fly problem. Never leave standing water in sinks, drains, bathtubs or showers. Check for excess moisture or organic debris around damaged or faulty septic lines, septic tank openings, drain fields and other parts of the septic system. If there is algae or mold growing in these moist areas, the water is likely contaminated enough to attract drain flies, so fix the leak and clean up the excess moisture to compel the flies to leave. If you can't find a source around your home, the drain flies could be traveling to your home from a sewage treatment plant or a neighbor's damaged septic system.

Other Control Methods

When good sanitation and exclusion measures are exercised, chemical pesticides are rarely necessary for total drain fly control. Pyrethrin insecticides will kill adult drain flies, but remember that whatever source of contamination in the septic system drew the flies to your toilet in the first place will likely continue to attract drain flies until the contamination is taken care of.