Drain Fly Larva in the Cat Bowl

Finding tiny black worms in your cat’s water bowl or fountain can be disgusting and concerning at the same time. The first questions most people tend to ask are: Are they dangerous? And where did they come from?

fly larva cat bowl

You can find numerous types of worms in your cat’s water bowl. However, the most common are drain fly larvae, also known as drain worms—a midpoint between eggs and adult flies in the drain fly life cycle.

Since they don’t bite or transmit diseases, these black worms are generally harmless to humans, cats, and other pets you may have in your home. However, this doesn’t mean it’s okay to leave them there. Typically, it takes 21–27 days for drain flies to go through their life cycle, from eggs to larvae to pupae to adult flies.

Since an adult female fly can lay batches of up to 100 eggs at a go, you may find yourself dealing with a drain fly infestation that could be difficult to control.

Having drain flies in your home could also indicate problems with your plumbing system that need to be addressed. Drain flies breed in moist areas, including shallow, stagnant water that contains organic matter. So, leaks in your pipes, sewage drainage, or overflows provide a continuous breeding ground.

What are drain fly larvae?

Drain flies (also known as moth, sewer, sink, or filter flies) are tiny, dark-winged, non-biting gnats. They have broad, fuzzy wings covered with tiny hairs and are about 1.5 mm to 5 mm long.

drain fly larva

The four life stages of drain flies include eggs, larvae, pupae, and adult flies. Female flies lay batches of between 30 and 100 eggs in moist places that contain organic matter—ample food for the larvae. Such areas can include your drains, septic tanks, floor drains, sewers, compost piles, storm drains, or areas of sewage-contaminated soil.

Your cat’s water fountain or bowl  also presents an ideal breeding ground, especially if it’s not cleaned and the water changed regularly. The water provides moisture and debris, such as small pieces of food that fall into the water when your cat drinks, in turn providing the organic matter necessary for drain fly larvae to survive.

Depending on the temperature, the eggs then hatch into larvae in 32–48 hours. The drain worms, also called maggots, are legless and about 3.5 mm to 10 mm long, with a dark breathing tube extending out over their gelatinous film. They appear pale in the middle and darker in color towards the ends. You may see subtle brown and black lines if you look closely, which is why they are identified as black worms.

dirty pipes with organic matter buildup

Drain worms feed on decaying organic matter, microorganisms, algae, and sediments on moist media. This organic matter is commonly found in dirty drains, which is where the name “drain flies” comes from. The larvae live for 8–24 days before entering the pupal stage for 1–2 days and then become adults.

How to identify drain fly larvae

Not all worms that you might find in the cat’s water bowl are drain worms. Thus, before you start looking for solutions to get rid of them, it’s important to confirm that you do in fact have drain worms. 

drain fly larva and pupa

The first telltale sign that you could be dealing with drain fly larvae is the presence of adult drain flies around your home. Though much of their activity happens at night, you can spot drain flies resting on your walls during the day.

If you cannot see any adult flies, place some sticky tape over the drain in the room where the cat’s water bowl is. Leave the tape for a few hours or overnight before checking to see if flies or larvae are attached to it.

Where do drain fly larvae come from?

While they may congregate near sinks and drains, drain flies do not enter your home through your drains. They gain access through various entryways, including any small cracks and crevices they can fit through.

drain flies in a pipe

Once inside your home, the female flies will find places with an abundance of moisture and organic material in which to lay their eggs. These places might include drains and drain traps, garbage disposals, toilet tanks, sump pits, home overflow pipes, septic tanks, sewage treatment facilities, and moist compost.

Wet places with standing or stagnant water that is rich in organic material, including your cat’s water bowl or fountain, provide the ideal breeding ground.

How did they get into the cat’s water?

Failure to regularly clean the bowl and change the water provides the ideal breeding ground for drain flies. The combination of water and tiny pieces of food forms a bacterial film around the bowl. The film provides the nourishment the drain worms need to survive, attracting females to lay eggs in the bowl.

How to get rid of the drain flies

The most efficient way to eliminate drain flies and their larvae is by destroying their breeding grounds.

Thoroughly and regularly clean the cat’s water bowl or fountain to remove organic buildup, and change the water at least once or twice a day. Also, ensure you change your fountain’s filter as and when required.

To eliminate the larval breeding sites in your drain lines, you can use DIY methods or store-bought products, including chemical and enzyme cleaners.

DIY methods include pouring vinegar and baking soda or hot water down the drain to try and dislodge organic buildup. You can also use a pipe brush to scrub the interior and exterior to remove any organic matter. Kill adult flies using homemade traps made from either apple cider vinegar or a combination of sugar, water, white vinegar, and liquid soap.

If the DIY methods don’t work, you can opt to use store-bought products. These include:

  • Enzyme drain cleaners like Green Gobbler, Drano Max Build-Up Remover, or the Drano Max Gel Clog Remover.
  • Other products, like the DF-5000 Drain Fly Eliminator, Natural Armor Fruit and Drain Fly Killer, InVade Bio Drain Solution, American Bio-Systems Drain Gel, Gentrol Insect Growth Regulator (IGR) Aerosol, and Forid Drain Gel cleaner.

How to prevent drain flies

Regular cleaning and maintenance of your pipes, toilet bowls, sinks, bathtubs, and showers is the starting point to preventing drain flies. Without access to an ideal breeding ground, they will look elsewhere. Occasionally flush out any stagnant water, even if the toilet or drain is not in use. Fix leaks, and eliminate all standing, shallow water or stagnant pools.



The black worms you see in your cat’s water bowl are generally harmless. They don’t bite or transmit diseases. However, it is better to try and keep them out of your cat’s water supply if possible. Cleaning the bowl or fountain and changing the water at least once a day is the most efficient way to keep them out.

Last update on 2024-07-21 / Affiliate links / Images from Amazon Product Advertising API