Why Are There Drain Fly Worms in the Toilet?

Many different species of fly can become household pests when they take up residence inside your home. An important way to ensure the effectiveness of measures to rid your home of the pest is to understand the behaviour of the flies and what drew them indoors in the first place. Taking a moment to consider these factors will not only help rid your home of the pest but will prevent future infestations of the same pest as well.

Drain Fly Larvae

It is not uncommon for homeowners to report black or dark-coloured worms in their sink drains or toilet bowls. These are actually not worms at all but the larval maggot form of the drain fly. Drain flies are known for their love of moist environments. Adults are drawn indoors because they detect a water source; contaminants in that water source such as bacteria, fungi, decaying organic matter, sediment and the like are one of the principal food sources of the drain fly, so contaminated, standing water is a favourite breeding ground for drain flies.


The easiest way to remove drain fly larvae from the toilet bowl is to simply flush the toilet. But keep in mind that something in the toilet water drew the drain flies in the first place, and a reinfestation is likely if you don't take steps to remove the water contaminants the attracted the adult flies to breed in the toilet water. After flushing the larvae down the toilet, monitor the toilet water for a few days to see if adult flies breed in the water again. If they do not, the infestation was likely a one-time occurrence. If drain fly larvae appear in your toilet water again, however, further action is required.

Preventing Future Infestations

First, deep clean the toilet bowl and surrounding surfaces and see if the drain flies return to breed in the toilet again. If they do, there may be decaying organic matter or another water contaminant deep in the toilet drain. Use a long-handled toilet brush or plumber's "snake" to clear out any contaminants that are deeper in the toilet drain. You may also need to use a bacteria complex cleaning agent such as DF 5000 gel to breakdown organic matter that may be attracting the drain flies.

Other Considerations

Drain fly infestations are more common during extended periods of inactivity that leave puddles of water standing for some time. For instance, homeowners often report drain flies in the home after having been on vacation, during which puddles of water have collected in kitchen and bathroom sink drains and have bee left standing long enough to be contaminated. Therefore another important component of preventive drain fly control is to make sure water is not left standing in the home for any period of time.

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