Wetlands Development and Management Guidelines for Mosquito Control in Marin and Sonoma County

GUIDELINES, STANDARDS, AND CHECKLISTS

MARIN/SONOMA MOSQUITO AND VECTOR CONTROL DISTRICT

2000

INTRODUCTION

The purpose of this document is to serve as an aid to both professionals and laypersons in public health, land-use planning, environmental resources, agriculture and other disciplines involved with the development and/or management of wetlands in Marin and Sonoma County.

WETLANDS POLICY STATEMENT

The Marin/Sonoma Mosquito and Vector Control District (MSMVCD), as part of the Mosquito and Vector Control Association of California (MVCAC), supports the reasonable preservation, restoration, and creation of wetland habitats in Marin and Sonoma County, and acknowledges the importance of these habitats for wildlife, recreation, and other beneficial uses. At the same time however, MSMVCD and MVCAC recognized that wetlands can be a source of public health problems because they can provide an ideal habitat for the development of nuisance and disease-carrying mosquitoes and other insects of public health importance called "vectors". MSMVCD is concerned that the public health aspects of wetlands management frequently are being ignored, and that public health professionals only rarely are being included in the planning phases of wetland projects.

The formation of mosquito abatement and vector control districts was preceded by the passage of the Mosquito Abatement Act in 1915, which became part of the California Health and Safety Codes. This legislation was in response to vector-borne disease outbreaks and the exposure of growing communities to attack by mosquitoes. The first mosquito abatement districts in California were formed in the San Francisco Bay area in response to large numbers of pestiferous mosquitoes developing in coastal salt marshes and to eliminate malaria in the Central Valley.

There are now more than 50 such mosquito, pest and vector abatement districts in California. These agencies, as well as some counties, have joined together to form a professional organization, the MVCAC. It is the responsibility of these local agencies to protect the public from vector-borne diseases and pestiferous infestations by controlling mosquitoes and other pests. The present low levels of vector-borne disease in California can largely be attributed to the extensive network and effective operation of these vector control agencies.

The MSMVCD and the MVCAC is committed to working with public and private agencies to eliminate specific conditions that promote vector production in wetland habitats, and is eager to interact with other agencies to develop wetland management criteria conducive to achieving wildlife and recreational goals while minimizing public health problems. Interagency training is an essential element in achieving these mutual goals and optimizes the technical expertise of participating agencies. Public education regarding both the benefits of wetland and their potential for the production of vectors is also necessary. This requires collaboration among the many agencies involved in wetlands planning and management.

The MSMVCD prefers to mitigate vector problems associated with wetlands through cooperative planning and preventive measures. Comprehensive management programs for prevention of vector problems are more effective than emergency, one-time responses to acute problems. These management programs are tailored to the specific environmental characteristics of a given wetland and rely on sound ecological principals, based on more than 50 years of research and management experience. Once wetlands are established, management programs must involve surveillance of pest and vector species and their diseases to determine periods of greatest public health risk. If controlling vectors includes activities in environmentally sensitive areas, then the MSMVCD advocates prudent, ecologically sound intervention to suppress both the immature and adult stages of the vector.

In summary, it is a goal of the MSMVCD and the MVCAC to foster and promote close interagency cooperation in the planning, development and management of wetlands. The member agencies of MVCAC have considerable scientific expertise and have a strong desire to work towards enhancing wetland habitats while minimizing threats to public health. In order to achieve this goal the member agencies of MVCAC do hereby establish the following wetlands policy:

Be it therefore established that with regard to any and all aspects of wetlands, it shall be the policy of the MVCAC and its member agencies to:

• Participate in all levels of wetland planning in order to identify and minimize all real or potential public health impacts created by mosquitoes and other vectors.
• Educate planners, public and private, to the role wetlands play in mosquito and other vector production.
• Work cooperatively with all responsible participants on any wetlands project to achieve as many of the stated goals of the project as possible.
• Provide the necessary information to insure that any mosquito or other vector control and concomitant costs are provided for in the necessary Operation and Maintenance Plan of all wetland projects.
• It is the intent of this policy to insure that member agencies of MVCAC can effectively carry-out the legal mandate of the residents within and adjacent toheir district’s borders to minimize the threat of disease or infestation from mosquitoes and other vectors.

GUIDELINES

I. Mosquitoes

Mosquitoes are insects that evolve from aquatic sources. Mosquitoes are also vectors of diseases such as malaria and encephalitis. Their bites can also cause great discomfort and misery. Mosquitoes can breed wherever water collects, including constructed and natural wetlands.

There are 53 known species of mosquitoes in California, with 21 of them in Marin and Sonoma counties. Many of these species can inhabit stagnant water sources such as wetlands, marshes and other semi-permanent and permanent water sites.

Wetland Mosquito Species

Listed below are several of the most common species of mosquitoes found in wetland in Marin and Sonoma counties:

• Anopheles freeborni (Aitken), referred to as the western malaria mosquito, is found throughout most of California. Spring rain pools, river seepage areas, marshes, swamps and semi-permanent and permanent wetlands are the preferred breeding habitat. Anopheles freeborni is an efficient malaria vector, and has been responsible for the transmission of this parasite throughout much of Northern and Central California.
• Aedes dorsalis (Meigen) is considered a major pest species and is associated with coastal salt marsh. This species is found during spring through fall months. Aedes dorsalis is a major pest species and is capable of migrating some distance from their breeding grounds.
• Aedes squamiger (Coquillett), sometimes referred to as the California salt marsh mosquito, is closely associated with tidal and reclaimed marshes of the Pacific Coast from Baja California to Bodega Bay.
• Culex tarsalis (Coquillet), the western encephalitis mosquito, is the primary vector of western equine encephalitis and St. Louis encephalitis in California. It is associated with floodwaters, rain pools, and semi-permanent and permanent water sources.

Wetland managers should become familiar with the different characteristics and breeding habitats of the mosquito specie(s) in their area.

Wetland Development Reports

Prior to the development of a new wetland, or the resurrection of an existing wetland, the following reports should be developed by the wetland manager or owner.

A. A project report should address any significant mosquito history in the area, especially if the wetland is near a human population center or rural-residential zone. Past and present mosquito population data should be solicited from the local vector control agency or public health department for use in developing the project report. If no data is available, the local agency should be contacted to initiate a study for use in the report.

B. A project report for any wetland, which has the potential for producing mosquitoes, should include appropriate mitigative measures. Mitigation measures should be in the form of an integrated pest management (IPM) plan for all species of mosquitoes found or anticipated in the proposed wetland area. A comprehensive IPM plan for mosquitoes in wetland habitats would include the following components:

• Mosquito identification and biology
• For all species associated with similar-type wetlands
• Mosquito monitoring and thresholds
• Establishing larval and adult mosquito populations that would trigger implementation of a control program.
• Non-chemical control of mosquitoes
• Physical control techniques, including water control
• Biological control techniques
• Chemical control of mosquitoes
• Adulticides, larvicides
• Pesticide application equipment and techniques
• A written plan that provides direction for the implementation of the IPM plan and coordination and communication with the local vector control agency.

Most local vector control agencies are available to provide some or all of the information required to develop the above-described reports. By working with one of California’s local vector control agencies from the very beginning of a new or resurrected wetland, the wetland manger or owner can be assured of proper communications regarding local vector control agency policies, mosquito control programs, etc.

STANDARDS

The following is an overview of standards commonly required for the prevention of mosquitoes in developed and managed wetlands. These standards and other requirements can vary significantly for different areas of California, depending on climate, mosquito activity, and severity of effect on human and domestic animal populations. Most vector control agencies responsible with mosquito prevention have developed specific criteria designed to meet the needs of their own geographical areas.

MOSQUITO PREVENTION STANDARDS

I. Wetland Design Criteria

A. Fresh Water Wetland Development and Restoration

1. Land Preparation

Levees

Levees and levee construction, are important aspects of wetland development. They are an integral component of the water control system and the land’s intended use.

Levees are generally designed to be either permanent or temporary, with permanent levees preferred due to their ability to hold up under adverse weather conditions and their ability to be maintained at a lower cost. Following are recommendations for levee construction for wetlands in California:

• Use clay or silt clay soils due to their ability to be compacted; sandy or organic soils erode and do not compact well.
• Levees should be large enough to support equipment for maintenance and vector control vehicles (12’ minimum).
• Side slopes of 4:1 generally suffice and deter damage from burrowing mammals such as muskrat, beaver and ground squirrel.
• Levee elevation should be a minimum of one-foot (1’) above the maximum planned flooding depth.

Water Control Structures

Water control is an important aspect of wetland management. Effective wetland management requires the water depth to be precisely controlled. There are two (2) types of water control structures necessary in a developed wetland. The first is a structure to de-water the wetland, and it should be designed and installed at the lowest elevation possible to facilitate draining of the entire impoundment and borrow ditches. Drainage structures should be of adequate size and type to allow for rapid drainage (drawdown). Drainage of impoundments should be completed in 1-3 days to discourage mosquito development in vegetated areas and to reduce the incidence of salinity build-up in the soil.

One of the most efficient types of water control structures is the stoplog structure.

Typical stoplog water control structure. Source: Natural Resource Conservation Service

Other types of structures include screw gates, siphons, electric and diesel-driven pumps, weirs and tide gates. All structures used in water control require periodic maintenance and constant monitoring during use.

Ditches, both lined and unlined, should be constructed with adequate depth and grade to insure they convey water as efficiently as possible. Head, or supply ditches should be designed to allow for fast floodup, and drainage ditches should allow for quick and complete de-watering of the entire impoundment.

Water Supply and Distribution

Standing water is the breeding ground for mosquitoes. Water applied slowly or to shallow depths can produce more mosquitoes than water applied rapidly and deep. Wetland managers should have a good understanding about flooding land prior to developing a wetland.

In order to effectively apply and distribute water, it is necessary to understand the variables involved and be able, before the flooding is done, to determine how much water is needed, how long flooding should take, etc.

The variables involved with distributing water are the:

• Area to be flooded
• Depth of water applied
• Time interval over which water is applied

These variables are related in the following formula:

D x A = Q x T

Where

            D = depth of water applied, in inches
            A = area flooded, in acres*
            Q = water supply stream flow, in cubic feet per second**
            T = length of time water is applied, in hours
            
            To solve for Q:  Q = (D x A) ¸ T
            To solve for D:  D = (Q x T) ¸ A
            To solve for T:  T = (D x A) ¸ Q
            To solve for A:  A = (Q x T) ¸ D
            
            *one (1) acre equals 43,560 square feet
            **one (1) cubic foot per second (cfs) equals 450 gallons per minute (gpm)
			

Vegetation Management

Most wetlands are designed and managed to provide a mixture of vegetation and water. Some wetlands utilize existing or native plants, while others use volunteer weeds or crop residue.

Mosquitoes prefer wetland habitat that has a large percentage of emergent vegetation and plant cover. By reducing the overall amount of emergent vegetation, the wetland is less desirable for female mosquitoes to lay their eggs, and ultimately the wetland is less likely to produce large populations of adult mosquitoes.

Emergent vegetation within a wetland should be managed in a checker board manner, sometimes referred to as a hemi-marsh, with small islands of vegetation surrounded by areas of open water. Ideally, emergent vegetation within a wetland’s flooded basin(s) should be manged in small (<300-sq. ft.) islands. Small islands of vegetation surrounded by open water allow for the natural forces of wind and wave action to limit mosquito development, and also encourage the development of beneficial aquatic invertebrates.

Mosquito densities are highest in areas with shallow water and dense emergent vegetation, which generally occurs along the margins of wetlands. Vegetation should be managed so that emergent vegetation exists only in the deepest areas of the wetland.

Vegetation management can be accomplished by disking, mowing, grazing, burning and herbicide application. Following are options for control of several problem wetland weed species:

• Cattail and bulrush can be controlled in wetlands by drawing down the water level in March keeping the area dry through September. In late July, the dried tulles can be disked with a tractor and disc implement, allowing several months for the tulle roots to dry in the summer heat. Do not flood-up until October.
• Johnson grass can be controlled by using the herbicide glyphosate during times of the year that the plant is actively growing.
• Common reed can be controlled by burning and disking in late summer or early fall, with treatments performed over several years for good control.

With regards to crop residue such as corn, wheat and safflower stubble, disking or mowing of at least 50% of the proposed wetland area to be flooded is preferred. Stubble should be mowed or disked to a height that will be less than the anticipated water level of the wetland.

Rice fields that will be flooded post-harvest can be managed by incorporating the rice straw into shallow-flooded paddies with large rollers. This practice eliminates some of the emergent vegetation and promotes the development of beneficial aquatic insects.

II. Operations and Maintenance Criteria

• Programs and responsibility assignments for maintenance of wetlands should be included in any wetland development proposal.
• Water control structures, including ditches, levees, pumps and siphons should be maintained in such a way that small water flows are not blocked by sand, silt, vegetation, or debris.
• Water conveyance and storage structures should include maintenance procedures for prompt attention to structure leakage or seepage to prevent water from entering non-wetland areas.
• Semi-permanent and permanent wetlands should be designed and managed to allow for stocking with appropriate fish-types that will prey on mosquito larvae, i.e., Gambusia affinis, the mosquitofish.
• Mosquito prevention requirements for wetland developments should be approved by the local mosquito and/or vector control agency.
• Mosquito control costs directly attributable to any land use are the responsibility of the wetland developer, owner or operator.

WETLAND DEVELOPMENT AND MANAGEMENT QUESTIONAIRE

By answering the following questions, wetland developers and managers will have a better understanding of the concerns raised by local mosquito/vector control agencies when new or resurrected wetlands are proposed.

• Is adequate wetland drainage provided for?
• Are drainage facilities designed to drain both major and minor wetland areas and prevent ponding?
• Is the design of wetland basins adequate to minimize mosquito production?
• Are wetland management and maintenance provisions adequate?
• Is the probability of leakage or seepage from water conveyance systems and wetland basins considered?
• Will any abandoned wetland that could retain water be removed or reworked to prevent mosquito development satisfactorily?
• Are nearby industrial, commercial, suburban or urban operations identified?
• Is there adequate access and clearance for motorized mosquito control and wetland maintenance equipment?
• Do individual wetland basins have a drain to completely empty the structure, or can it be pumped dry adequately?
• If the wetland will hold waste water that is high in organic nutrients, i.e. animal or municipal waste water, has the greater potential for breeding mosquitoes been considered?
• If the project restores wetland habitat, has the probability of increased mosquito breeding (and higher adult mosquito populations) been considered?
• If the wetland will support mosquitofish for the control of mosquitoes, will the proper agency be notified prior to flooding of the wetland to ensure stocking of the fish?
• Have additional funds been reserved for the continual monitoring and control of mosquitoes if prevention standards are not heeded?