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Mosquitoes and Stormwater Facilities

Image of a mosquito.

Mosquitoes can be a serious nuisance. Large populations of mosquitoes can make it unpleasant to be outside and they keep families indoors when they should be out enjoying warm weather. 

Mosquito populations can be controlled by limiting the number of places they can breed. Any small, stagnant pool of water can promote mosquito breeding, even as little as a teaspoon of water. In most cases, the source of mosquito breeding can be found in public areas or backyards.

Stormwater management facilities are often blamed for large numbers of mosquitoes, because the facilities contain rainfall runoff. But if a stormwater management facility is designed and maintained properly, it should not promote mosquito breeding.

If you know the facts about mosquitoes and how to prevent them, your family can be back outside, enjoying the warm weather, in no time.

Download a PDF flyer of the material presented on this page. (PDF, 847KB)


Image of Did You Know? graphic on mosquitoes.


Mosquitoes Facts

►  Know Your Mosquitoes: There are about 200 species of mosquito in the U.S. and they differ in their preferred habitat or the types of animals they bite.

  • The most common species of mosquito in Montgomery County is the non-native Asian Tiger Mosquito. They are aggressive, only bite during the day, breed in small containers and travel up to 300 feet from where they hatch.

  • Another common mosquito is Culex pipiens which is most active at night. They are gray-brown colored, lay eggs in both large and small bodies of water and can fly about a mile or two.

Graphic of trash on the ground with standing water.
Bottles and trash can collect standing water, making them into breeding grounds for mosquitoes.


► Breeding: Mosquitoes breed in stagnant, standing water. They lay their eggs in standing water, and depending on the species, the eggs take about a week to reach maturity. As little as a teaspoon or bottle cap of water left standing for more than a week is enough for eggs to develop into adult mosquitoes


Natural Prevention: Mosquitoes cannot complete their life cycle to become adult mosquitoes if:

  • Water is flowing, like in a stream, river or fountain;

  • Predators, such as fish frogs, salamanders, dragonflies or aquatic insects prey on them; or

  • Water drains within a week.  



Life Cycle of a Mosquito
Image of mosquito eggs

Image of mosquito larvae.

Image of a mosquito pupa. Image of an adult mosquito
Egg: Mosquito eggs are oval and about .6mm long and either laid singly or as an egg raft in water. Larva: Larva move through the water in a serpentine pattern. Some species can grow to ½ inch long. Pupa: About the size of a sesame seed. They remain at the surface of the water unless they are disturbed. Adult: Process begins again with a female adult mosquito collecting a blood meal (for protein) to lay her eggs. Adults typically live 2-3 weeks.



Do Mosquitoes Breed in Stormwater Management Facilities?

If designed and maintained properly, stormwater management facilities should not promote mosquito breeding.

Image of DEP staff sampling a pond for mosquitoes.
DEP staff surveying a stormwater
management facility for mosquitoes.

Stormwater management facilities hold and treat rainfall runoff. Without stormwater management, the runoff would go directly into our streams, causing flooding, polluting our waterways and eroding streams. There are thousands of stormwater management facilities in the County. 


Types of stormwater management facilities include:

  • Wet and dry ponds
  • Underground storage
  • Low-impact development, including rain gardens
  • Environmental site design
  • Infiltration trenches

Some of these facilities hold water permanently, while others hold stormwater runoff temporarily.


Stormwater Management Facilities that Hold Water Permanently

Stormwater management facilities that continuously have water, such as wet ponds, typically do not promote mosquito breeding because they contain predators that feed on mosquitoes.  Mosquitoes are a favorite food for aquatic predators. Their larva is eaten by fish and other aquatic wildlife. Wet ponds and stormwater wetlands serve as aquatic habitats with plenty of natural predators to control the mosquito population. 


Stormwater Management Facilities that Hold Water Temporarily

Dry detention basins, bioretention and rain gardens are designed to detain and infiltrate stormwater. Facilities fill up during storm events and then release the water within two to three days. Because these systems are designed to hold water for only short periods of time, less than the week needed for mosquito growth, they do not provide mosquitoes with enough time to reach maturity.


I Think the Nearby Stormwater Management Facility is the Source of Mosquitoes in My Area.  What Should I Do? 

If you see large numbers of mosquitoes around a stormwater management facility, please contact the Montgomery County Customer Service Center at 3-1-1 or send an email to


Preventing Mosquitoes in Your Neighborhood

It is important that we all do our part to eliminate mosquito breeding habitat. One of the best ways is to check for standing water in backyards and on porches and balconies. 


Image of a mosquito.

Checklist for Preventing Standing Water:  

  • Flush birdbaths and saucers under potted plants at least once a week.

  • Turn over children’s wading pools, buckets, wheelbarrows, canoes and garbage can lids.

  • Clean out roof gutters and down-spout screens.

  • Secure any outdoor tarps or pool covers so that there are no folds where water can collect.

  • Get rid of puddles from window air conditioners.

  • Fix dripping outdoor water faucets.

  • Dispose of trash such as plastic bags, bottle caps, open drink cans or bottles, styrofoam cups or food wrappers.

  • Throw away used tires. If you have a tire swing, drill holes in the bottom of the tire so water will run out.

  • Check recycling bins if they are left out in the rain. Dump water out of recyclables.

  • Add an aerator or fountain to birdbaths or ornamental ponds. Mosquitoes do not breed in moving water.

  • If you own a rain barrel, make sure that you use debris screens, keep the barrel tightly closed and use collected water within a week.  If the water is not used within a week, a larvicide such as Mosquito Dunks should be used to control for mosquitoes. 

Most importantly, share this information with your neighbors. Download a PDF flyer of the material presented on this page. (PDF, 847KB)


Image of rain barrel
Check your rain barrel debris screens regularly and dump out unused water within a week.


Larvicides only kill mosquito larvae and not the pupae or adults. They are preferred over the use of adulticides because larvicides are effective at killing mosquitoes, and at the same time, they minimize the impacts to the environment and non-target organisms.  They are also not harmful to animals such as squirrels, raccoons and dogs if they are accidentally ingested.


Recommended Larvicides

  • VectoLex ® (Bacillus sphaericus) is a naturally occurring bacterium found in soil and aquatic environments. It produces a stomach poison (endotoxin) which is specifically toxic to mosquito larva after it is ingested.

Because VectoLex ® only targets mosquito larvae, it is safe to use in natural water bodies or in water that flows to natural water bodies, such as stormwater facilities and storm drains.

Do not use VectoLex ® in drinking water reservoirs. It can only be purchased and applied by a licensed applicator with aquatic certification.  

  • Bacillus thuringensis var. isralensis (B.t.i.) is another bacterial larvacide that is most readily available from local stores (i.e. Mosquito Dunks ®). However, this larvicide kills other aquatic insects in addition to mosquito larvae, therefore it should not be used in water that flows into natural water bodies.

Mosquito Dunks can be used in small containers of water around residential properties, private rain gardens, rain barrels and other places that do not flow into streams, lakes or rivers.



Virus Information


Mosquito Control


Pesticides and Mosquitoes