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Storm Water Management Ponds
West Nile virus Surveillance Study 2003 - Results


What are Storm Water Management Ponds (SWaMP)?

Storm water management ponds are ponds that are designed to handle water overflow. They are also called retention or detention ponds. There are two types of ponds: wet ponds and dry ponds. Wet ponds always have some water present. Dry ponds fill and empty throughout the season. These ponds can support a wide range of animal life including birds, fish, reptiles and insects.

What was the purpose of the study?

Mosquitoes that can spread West Nile virus lay eggs in areas where water is still (stagnant) such as in storm sewers, old tires and containers. SWaMP have been considered low-risk for mosquito breeding in the past due to movement on the water’s surface. It is possible, however, that environmental conditions may result in pockets of stagnant water. In 2003, the Durham Region Health Department conducted a study to determine if some conditions lead to increased chances of mosquito breeding at wet SWaMP.

What types of conditions at SWaMP were studied?

The study examined plants, floating debris, shoreline conditions and pockets of stagnant water. Additionally, the depth and water temperature of the SWaMP were examined. This information was used, along with sampling data, to assess the likelihood of mosquito breeding at each particular site.

How did the Health Department measure mosquito breeding at the SWaMP?

Mosquitoes deposit eggs onto the surface of the water. After one to several days, the eggs hatch and become larvae. It is these larvae that were used to measure mosquito breeding. Health Department staff visited each selected SWaMP in Durham Region once every two weeks. At each visit, staff sampled the water from selected sites along the shoreline. Each sample was examined for mosquito larvae. The number and type of mosquito larva were recorded. This information was combined with the SWaMP condition data to assess the risk of mosquito breeding.

When was the study conducted?

At the end of April, Health Department personnel began visiting each SWaMP to gather information on the location, general characteristics and suitability for the study. Not all SWaMP in the Region were included in the study (a map of included ponds is available by clicking here). Sampling began in May and continued through August, the typical mosquito-breeding season.

What did the study find?

Testing of the SWaMP for mosquito larvae yielded the following conclusions:

  1. wet SWaMP are most likely to support the growth of Culex species of mosquito over any other species of mosquito,
  2. the most important time of the year for finding larvae at these ponds is between July and September,
  3. wet SWaMP can serve as effective breeding grounds for the deposit, maturation and proliferation of West Nile Virus vector mosquitoes,
  4. presence of cattails, large amounts of vegetation and standing water are the best predictors for finding Culex larvae,
  5. adequate maintenance of SWaMP, including reducing vegetation, cattails and pockets of standing water along the shoreline would be an effective measure in reducing the likelihood of finding mosquito larvae at these sites.

Is the Health Department planning on treating the SWaMP with pesticides?

This study has provided guidelines on the need for and type of mosquito control measures for SWaMP. Measures include improved maintenance, removal of debris and control of vegetation. Decisions on future preventive measures will be made on a case-by-case basis using the findings from this study and the established risk associated with a particular pond.

Where can I obtain more information?

A full copy of the study report, including the methods of data collection, results and conclusions can be found on the Durham Region Health Department website at durham.ca - West Nile Virus.

Additionally, more information on West Nile virus can be obtained by calling Durham Region Health Department, Environmental Help Line 905-723-3818 or 1-888-777-9613.

April 22, 2013