March 2008

Best Practices for Reducing the Risk of West Nile Virus in
Stormwater Management Ponds

1.0 Introduction

West Nile Virus (WNV) is now established in Ontario1 since its introduction in 2001. The disease is transmitted to humans by mosquitoes which become infected by feeding on birds that have WNV. Birds such as crows, ravens and blue jays are very susceptible to WNV infection. The collection and testing of these types of dead birds confirms the presence of WNV in the bird population. Surveillance programs also monitor the presence of and types of mosquito larvae in stagnant water bodies. Adult mosquitoes caught in light traps are also tested to monitor levels of viral infection.

Although several species of mosquitoes act as vectors of WNV, the Culex spp., which includes both Culex pipiens and Culex restuans, is the predominant mosquito vector of WNV in Ontario due to its preference of feeding on both birds and humans and abundance in the urban environment.

It is important to reduce potential Culex mosquito larvae habitat to decrease the risk of WNV. Whether or not stormwater management (SWM) ponds are favourable Culex mosquito larvae habitat depends on several factors which are discussed in this document. Assessing the risk to human health from mosquitoes that emerge from a SWM pond is based on the biology of the mosquito species present, analysis of local surveillance data to determine if WNV is established in the bird and mosquito population in the vicinity of the SWM pond and determining the likelihood of WNV transmission to humans from infected mosquitoes based on current and forecasted weather conditions. Ontario’s surveillance data indicates that all 36 Health units have reported positive WNV activity either in birds, mosquito pools or horses and many Health units in Ontario have reported human cases.

The purpose of this document is to identify best practices for SWM ponds to reduce mosquito larvae habitat and decrease the risk of WNV.

2.0 The Function of a SWM Pond

Urban development interferes with the natural hydrologic cycle by increasing stormwater runoff into streams, rivers and lakes as a result of impervious areas such as roads, driveways, sidewalks and roofs. The ultimate goal of stormwater management is to maintain the health of streams, lakes and aquatic life as well as provide opportunities for human uses of water by mitigating the effects of urban development. Stormwater management, including the use of SWM ponds, strives to maintain the natural hydrologic cycle, prevent an increased risk of flooding, prevent undesirable stream

1. The Ministry of Health and Long-term Care web site provides further information on WNV.

erosion, and protect water quality². Residential, commercial and industrial land uses result in non-point source pollution (e.g., sediment, oil, grease, pet waste, lawn care products, heavy metals etc.) which if left unchecked may reach nearby streams and rivers. Stormwater wet ponds mitigate the flow of stormwater and collect sediment and associated pollutants.

Both wet and dry ponds hold water as a result of heavy rainfalls to reduce stream bank erosion and impacts to receiving streams and rivers. An appropriately designed SWM pond that is properly operated and well maintained3 is not likely to allow for the accumulation of standing water which, if left to become stagnant, is conducive to attracting mosquitoes such as those in the genera Culex which have been identified as key vectors of WNV. The scientific literature supports the concept that a diverse and healthy ecosystem (i.e., good water quality that will support an abundance of aquatic life stages of invertebrate and vertebrate predators and the presence of terrestrial predators such as birds, bats and flying insects) keeps mosquito populations in check. Ontario studies also support these findings. A SWM pond monitoring study4, prepared for the Ministry of Transportation, concluded that the collective data (both anecdotal and statistical) shows that mosquito larvae are less likely to be present in SWM ponds that more closely resemble natural, stable aquatic habitats.

SWM wet ponds are designed foremost as treatment facilities to capture pollutants from stormwater runoff and not to function as a natural pond/wetland. As such, SWM ponds do not necessarily have the full and diverse ecosystem of water quality and quantity, plants, fish, benthos, animals, insects and natural predators as compared to a natural wetland. The life cycle of a SWM pond will require sediment removal from the main section of the pond on an as-needed basis (e.g., after 15 years or longer). This dredging activity may severely disrupt any established ecosystem developed over time in and around the SWM pond.

2.1 Wet Pond

A wet pond consists of a forebay and main cell into which stormwater is retained. Wet ponds control both stormwater quantity and quality. Standing water that is allowed to remain for more than a week can become stagnated and act as an ideal mosquito larvae habitat. Even during periods of dry weather, the continual flow of groundwater into some wet ponds may act to reduce the stagnation of water.

2. For more information refer to Understanding Stormwater Management: An Introduction to Stormwater Management Planning and Design
3. Ontario Ministry of the Environment, Stormwater Management Planning and Design Manual (revised March 2003).(See 6.1 and 6.2)
4. MTO Larvae Mosquito Surveillance. Prepared by Stantec Consulting Ltd. July 2005.

2.2 Dry Pond

Dry ponds are usually constructed for flood control whereby they capture and detain runoff during storm events and, if properly constructed, will slowly release the runoff over a period of 2-3 days. Dry ponds that are poorly designed, so that water does not drain within a week, can become a mosquito larvae habitat. Some dry SWM ponds are designed with a small permanent pool of water in the sediment forebay (which allows sediment to settle before the runoff is released) that functions to improve stormwater quality.

3.0 Factors that Provide a Suitable Mosquito Larvae Habitat

The following factors in a SWM pond provide an ideal site for mosquito larvae to thrive.

• Presence of Standing or stagnant water – The life cycle of Culex mosquitoes includes an aquatic phase that feed on detritus, algae, bacteria, and fungi which is abundant in standing or stagnant water. Female mosquitoes readily seek warm (~20oC) and highly organic water to lay their eggs and a complete lifecycle may take only a week in mid-summer.
• Dense Aquatic Vegetation – Dense emerging aquatic vegetation, such as cattails and phragmites (common reed), allow water to pool and stagnate. This also prevents aquatic predators such as minnows and aquatic insects that feed on mosquito larvae from accessing any pooled water. The presence of other aquatic plants, such as arrowhead, pickerelweed and spatterdock, are less of a problem since these allow for easier movement of predators into shallow water.
• Shallow water and gentle side slopes – SWM ponds that have been constructed with a consistent depth (>1.2 m) reduces the likelihood that aquatic emergent plants will establish which may interfere with aquatic predators’ ability to access and feed on mosquito larvae. Steep slopes (e.g., 4:1) are preferred to maintain sufficient depth of water at the edge of the pond. SWM ponds with gentle slopes allow for shallow water preferred by mosquito larvae.
• Sheltered Location – A SWM pond that is constructed in an open location such that its water surface is exposed to wind action is less conducive to mosquito larvae development. Culex larvae breathe through tubes at the water surface and prefer sheltered areas where the water is calm.

4.0 Best Practices

A Working group representing the ministries of Environment, Transportation, Natural Resources, and Health and Long-Term Care, Toronto Regional Conservation Authority and several Health Units was established to develop best practices aimed at reducing the potential of SWM ponds of being a favourable habitat for developing mosquito larvae that transmit WNV.

One of the first activities undertaken was a jurisdictional and literature search to determine if SWM pond design manuals produced by other jurisdictions included information on mosquito control. The review also identified relevant scientific papers on mosquito control in SWM ponds both pre and post construction. A Report⁵ of the relevant papers and technical documents has been prepared and will be continually updated to include new sources of information.

In addition, monitoring of SWM ponds in Ontario has been conducted over several years. Data has been collected to determine what key factors are contributing to mosquito larvae habitat in SWM ponds and what measures can be taken to change these conditions.

The following Best Practices are recommended to reduce mosquito larvae habitat in SWM ponds.

4.1 Design and Construction Considerations

Good design and construction6 of a SWM pond, which prevents the accumulation of stagnant water, can reduce maintenance but not eliminate the need for a regular maintenance program. Some of the following design best practices may not be practical in all situations or may conflict with necessary functioning or safety requirements. For example, human safety considerations of preventing children from falling into a SWM pond must be considered in the design of a SWM pond (e.g., slope of the sides of SWM ponds).

4.1.1. Dry Pond:

• Consider the water flow volume in designing a pond and allow for complete drainage of water within a week. Some dry SWM ponds are designed with a small permanent pool of water in the sediment forebay, which functions to improve stormwater quality.
• Ground water depth must be considered so that ponding water will not be present.
• Utilize construction features6 that will reduce or prevent drain blockage.
• Incorporate the natural hydraulic grade of the area to allow gravitational flow for drainage. The use of mechanical devices such as filters and pumps require increased maintenance over time and are susceptible to breakdown which may contribute to problems of stagnant water accumulation.
• Drainage piping must drain fully and the slope of the pond should allow for sediment accumulation over time.

5. A Jurisdictional and Literature Review of Stormwater Management Ponds and West Nile Virus, May, 2007. Available from Standards Development Branch, Pesticides Section MOE.
6. Ontario Ministry of the Environment, Stormwater Management Planning and Design Manual (revised March 2003).

4.1.2 Wet Pond:

• Where possible, seal equipment, screen vent holes and grates etc., in areas that hold water for any period of more than a week to prevent female mosquitoes from laying eggs in standing water.
• Use screened covers or grates that are easily accessible for inspection and maintenance staff.
• Consider designing inlet and outlet pipes to be submerged at all times to prevent access by adult egg-laying mosquitoes provided flow measurements are not inhibited.
• Consider the use of various valves, one way flaps or collapsible tubes to discourage mosquito entry into areas where standing water will be present.
• Design ponds to have a consistent depth (>1.2 m) to limit the establishment of aquatic emergent plants that easily establish in shallow water.
• Design ponds edges to have a steep slope (e.g. 4:1) since mosquito larvae prefer shallow stagnant areas; however, human safety factors also need to be considered in design features.
• Consider the use of heavy duty plastic liners in shallow areas to discourage unwanted aquatic vegetation that is not desirable.
• The shape of the SWM pond should be designed to prevent dead zones or standing water (e.g., long and narrow with no bays).

4.2 Ownership and Stewardship

A SWM pond that is functioning properly is unlikely to be conducive to mosquito larvae habitat. However, this can only be determined by visiting the actual site and assessing the SWM pond. As a first step, it is important to determine the ownership of the SWM pond to establish who has stewardship responsibilities for regular inspections, maintenance and record management. Local Health Units have authority to inspect and conduct larval mosquito surveillance in a SWM pond to determine if there are problems that may contribute to the spread of WNV.

Initially, a SWM pond may be owned by the property developer and later transferred to the local municipality or property owner. In any case, the owner should take responsibility for the inspection, monitoring, maintenance and record management of the SWM pond including any contracts for mosquito control, if necessary.

4.3 Monitoring

All SWM ponds should have an initial inspection (by knowledgeable personnel) to determine if they are operating properly and to assess local conditions in the vicinity of the SWM pond. Additional monitoring can then be considered, as resources allow and in consultation with public health unit (i.e., the health unit can assist in providing information as to whether or not the SWM pond is located in an area where WNV activity is present), should an initial assessment indicate that the SWM pond is poorly designed and/or poorly maintained and conducive to a mosquito larvae habitat.

All persons visiting a SWM pond should be made aware of health and safety considerations including the need for personal protection measures to avoid being bitten by mosquitoes (e.g., wearing appropriate clothing and using insect repellents according to label directions7).

Consider the following best practices for the monitoring of SWM ponds:

• Determine current ownership/stewardship of the SWM pond and identify an appropriate contact person. Update the information as needed.
• Use a monitoring and inspection check list form for each SWM pond8 (refer to the later section on record management for more details)
• Delegate a person(s) to visit each SWM pond and rate the pond according to its location to nearby urban areas and its risk of being a mosquito larvae habitat. For example, use a 1-10 rating where 1 is a low risk (i.e., the SWM pond is far from an urban centre and poor mosquito larvae habitat) and 10 is a high risk (i.e., the SWM pond is near an urban area and is a favourable site for mosquito larvae to quickly develop to adults and have a potential to transmit WNV).
• Use a mosquito larvae surveillance form (see Appendix 1 for an example of a standardized form) to record data on the characteristics and conditions of SWM ponds conducive to mosquito larvae habitat. Also, identifying the species of mosquito larvae is encouraged to determine whether or not these are WNV vectors.
• Contact the local Public Health Unit9 to determine if assistance is provided for larvae speciation. Some Public Health Units will only assist with municipal owned SWM ponds. Contact the Ministry of Health and Long-term Care7 for additional resources such as private companies that may offer this service.
• Develop an inspection schedule based upon available resources and other considerations such as major rain events and the priority assigned to each SWM pond (e.g. ponds with a high risk rating should be inspected more frequently). Also keep in mind the short life cycle of mosquitoes. Some Culex mosquito species can mature to adults from an egg stage in less than a week under ideal conditions (See Figure 1). This may require prompt follow up to address any maintenance problems that were identified through inspection activities.

7. See the Ministry of Health and Long-term Care web site for more information.
8. The Ministry of the Environment's Stormwater Management and Planning and Design Manual Chapter 6.0 Operation, Maintenance and Monitoring provides a maintenance schedule and inspection routine.
9. Ontario's 36 PHU's.

4.4 Maintenance

A maintenance program involves a long-term, stewardship commitment. The following best practices should be considered while taking into account available resources and consultation with the local public health unit to determine the potential risk of WNV activity in the vicinity of the SWM pond.

• Inspect pipes, filters etc., to check for possible clogging and the condition of gaskets to prevent the accumulation of standing water.
• Remove dense aquatic vegetation as needed.
• Reduce vegetation such as shrubs and small trees from around the perimeter of the SWM pond. This will reduce shade, harbourage and shelter from predators for adult mosquitoes and limit the nutrient source for developing larvae which may be provided by falling, decaying leaves. Maintenance personnel will also have improved access to perform cleanup, larvae monitoring and larviciding activities. The amount of vegetation that is removed must be considered along with other parameters (i.e., reduction of thermal impacts of SWM ponds provided by perimeter vegetation).
• Avoid creating ruts from service vehicles used for cutting grass or removing dense vegetation around the perimeter of a SWM pond. These ruts will allow for standing water to accumulate. Limit the amount of grass cuttings or leaf litter around SWM ponds since this is a nutrient source for mosquito larvae.
• Remove debris in and around the SWM pond during the mosquito season especially discarded containers and tires, plastic bags etc., that can hold stagnant water and act as incubators for developing larvae.
• Investigate complaints from the public regarding a SWM pond (e.g., mosquitoes, stagnant water, garbage, vandalism, odours etc.)

4.5 Records management

Proper records management should include the following:

• A contact person responsible for each SWM pond. Local residents should be able to contact a representative of the owner of the SWM pond in order to report concerns of mosquito activity.
• Cataloguing of maps, GIS location and type (e.g. wet, dry) for every SWM pond in the jurisdiction of the owner.
• Priority ranking of each SWM pond for scheduling of maintenance and possible larviciding activities based upon their potential risk of WNV. For example, a SWM pond would be ranked as a high priority and require more frequent inspections and maintenance if it is a high risk (i.e., poorly designed, conducive to mosquito larvae habitat and located close to an urban area where WNV activity has been confirmed, based on local Health Unit dead bird, mosquito pools and human reports).
• Regular maintenance schedules (e.g., annual, monthly and/or weekly)

5.0 Additional Mosquito Control Methods

5.1 Use of Larvicides:

When risk to human health from WNV is identified in an area by the local Medical Officer of Health, the use of a larvicide for mosquito control in the SWM pond should be considered by the SWM pond owner in consultation with the local Health Unit and Medical Officer of Health. The SWM pond owner is responsible for the contracting of a licensed pest management company. Larvicides must be used in compliance with provincial pesticides regulations10.

5.2. Non-Chemical methods:

The following non-chemical methods are often suggested as possible ways of controlling mosquitoes but may not be as effective as larvicides in reducing mosquito populations in a SWM pond:

• Introduction of stocks of native predatory fish (e.g., Fathead minnows – Pimephales promelas) in consultation with the Ministry of Natural Resources pond stocking requirements. Ontario pilot studies are currently being developed.
• The use of wind mills, agitators or fountains that create surface water disturbance may have some benefits in deterring mosquito larvae. However, some jurisdictions have noted that such methods have not been effective.
• Acoustic units that emit sound energy into water that resonates at a frequency that ruptures the mosquito larvae’s air bladders may have limited impact in large water bodies. Technology is constantly improving and larger or multiple units designed for industrial installation may be effective.
• The construction of bat houses or Purple martin bird houses to entice bats and birds around a SWM pond may have some aesthetic and ecological value; however, insectivorous bats and birds are opportunistic feeders and mosquitoes make up a very small percentage of their natural diet. In fact, the energy output by the bat or bird to catch a mosquito does not make it a worthwhile meal compared to larger insects such as moths, beetles and flies.
• Dragonflies and other insects such as Predaceous diving beetles and their larvae (Dytiscidae) and/or Water scavenger beetle larvae (Hydrophilidae) may have some impact in reducing mosquitoes. However, adult dragonflies feed during daylight hours when adult Culex mosquitoes are hiding in vegetation and are very territorial in that they chase other dragonflies from their hunting grounds. Mosquito larvae and pupae are consumed by the aquatic stage of the dragonfly although this stage also feeds on a host of other aquatic organisms. The release of these predators in sufficient numbers to effectively control mosquitoes in a SWM pond would require a commercially viable rearing system (similar to bait fish production) that is not currently available and consultation with the Ministry of Natural Resources.

10. The MOE following website provides additional information on pesticide licensing, the pesticides available for WNV control programs under an approved permit and the permit application requirements.