CENTRAL VALLEY JOINT VENTURE 

TECHNICAL GUIDE TO BEST MANAGEMENT PRACTICES FOR

MOSQUITO CONTROL IN MANAGED WETLANDS 

Dean C. Kwasny1

, Mike Wolder2, and Craig R. Isola2

Photo: Jim Pickering, BLM 

1

California Department of Fish and Game, Lands and Facilities Branch, 1812 Ninth Street, Sacramento, CA

95814 

2

U.S. Fish and Wildlife Service, Sacramento National Wildlife Refuge Complex, 752 County Road 99W, 

Willows, CA 95988

June 2004 
TABLE OF CONTENTS

Page 

ACKNOWLEDGEMENTS................................................................. iii

LIST OF TABLES.......................................................................... iv 

LIST OF FIGURES......................................................................... iv 

INTRODUCTION........................................................................... 1

MANAGED WETLAND DESCRIPTION.............................................. 3

WETLAND HABITAT MANAGEMENT.............................................. 4 

Seasonal Wetlands.................................................................. 4 

Semi-permanent and Permanent Wetlands...................................... 5 

MOSQUITO BIOLOGY................................................................... 6 

Floodwater Mosquitoes............................................................ 6 

Ochlerotatus melanimon.................................................................. 6

Standing Water Mosquitoes........................................................ 6 

Culex tarsalis............................................................... 6

Culex erythrothorax....................................................... 7

Anopheles freeborni....................................................... 7

MOSQUITO ABUNDANCE IN WETLAND HABITATS........................... 9 

MONITORING MOSQUITO POPULATIONS........................................ 10 

Immature Stages.................................................................... 10 

Adult Stages......................................................................... 10 

BEST MANAGEMENT PRACTICES................................................... 11 

Water Management Practices ........................................... 11 

Vegetation Management Practices....................................... 17 

Wetland Infrastructure Maintenance.................................... 21 

Wetland Restoration and Enhancement Features..................... 23 

Biological Controls........................................................ 26 

COORDINATION WITH MOSQUITO AND VECTOR CONTROL 

DISTRICTS.................................................................................. 28 

REFERENCES AND LITERATURE CITED.......................................... 31 

APPENDICES............................................................................... 34

 

 ii 
ACKNOWLEDGEMENTS

This technical guide was prepared by the Central Valley Joint Venture’s 

Mosquito Working Group, which includes representatives from the California Waterfowl 

Association, California Department of Fish and Game, California Department of Health 

Services, Central Valley Joint Venture, Ducks Unlimited, Mosquito and Vector Control 

Association of California, Natural Resources Conservation Service, Suisun Resource 

Conservation District, U.C. Davis, U.S. Fish and Wildlife Service, and Wildlife 

Conservation Board. 

We are extremely grateful to the many individuals and groups that provided 

information on the mosquito Best Management Practices for the guide. Through the 

practical applications of these practices and continuing experimentation, creative 

solutions are being found to help reduce mosquito conflicts while providing quality 

wetland habitat in the Central Valley. Although much of this information can be found in 

other published literature, the focus of this guide is on techniques from the first hand 

experience of wetland managers and mosquito abatement technicians who spend much of 

their time working in wetland habitats. 

This guide would not have been possible without the encouragement and 

partnership of the Mosquito and Vector Control Association of California. This 

document exemplifies their commitment to Integrated Pest Management for mosquitoes 

and will be updated as additional information becomes available on habitat-based 

practices for mosquito control. 

The following individuals provided comments on the document: Donald Blake, 

Tom Blankinship, Dave Brown, Robert Capriola, Jim Camy, Steve Chappell, Steve 

Cordes, Craig Downs, Cliff Feldheim, Dave Feliz, Alan Forkey, Jessica Groves, Mike 

Higgins, Joe Hobbs, Rob Holbrook, Allan Inman, Gale Jirik, Sam Johnson, Vicki 

Kramer, Sharon Lawler, Greg Mensik, Ron McBride, Marco Metzger, Harold Novick,

Peter Perrine, Fritz Reid, Loren Ruport, Chadd Santerre, John Schulte, Jamesina Scott, 

Dave Smith, John Stroh, Joel Trumbo, Mike Womack, and William Walton. 

Special thanks to the staff at the Central Valley Joint Venture, including Rob 

Holbrook, Ruth Ostroff, and Bob Shaffer for their technical support. 

Document printed by U.S. Bureau of Reclamation. 

 iii 
LIST OF TABLES

Page

Table 1. Water Management Practices to Reduce Mosquito Production in 

Managed Wetlands.......................................................... 14 

Table 2. Vegetation Management Practices to Reduce Mosquito Production 

in Managed Wetlands....................................................... 20 

Table 3. Wetland Infrastructure Maintenance Activities to Reduce 

Mosquito Production in Managed Wetlands............................. 22 

Table 4. Wetland Restoration and Enhancement Features to Reduce 

Mosquito Production in Managed Wetlands............................. 24 

Table 5. Biological Controls......................................................... 27 

Table 6. Suggested Coordination Activities Between Wetland Managers 

and Mosquito and Vector Control Districts (MVCDs)................. 30 

LIST OF FIGURES

Page

Figure 1. Chronology of Wetland Loss in California.............................. 3 

Figure 2. Watergrass (Echinochloa crusgalli) and Wetland Irrigation......... 4 

Figure 3. Semi-permanent Wetland.................................................. 5 

Figure 4. Seasonal Wetland Flooding and Potential Time Periods for 

Significant Mosquito Production......................................... 9 

Figure 5. Rapid Flooding.............................................................. 12 

Figure 6. Vegetation Mowing......................................................... 17 

Figure 7. Vegetation Disking and Dense Bermuda Grass (Cynodon 

dactylon)..................................................................... 18 

Figure 8. Faulty Water Control Structure........................................... 21 

Figure 9. Wetland Swale............................................................... 23 

 iv 
INTRODUCTION

Public attitudes about wetlands have greatly changed in the last few decades. 

Historically, wetlands were regarded as impediments to economic progress and 

“reclaimed” for agricultural production and other commercial uses. Today, wetlands are 

valued for their habitat for fish and wildlife, recreational opportunities, and the benefits 

they provide in terms of flood control, water filtration, and ground water recharge. 

There is greater awareness of the significance of the loss of over 95% of the Central 

Valley’s historic wetlands, and a public desire to restore, protect, and manage these 

habitats (Central Valley Habitat Joint Venture 1990). 

With more public interest in restoration, protection, and management of wetland 

habitats, conservation projects have increased from a few small, site-specific efforts to

larger, cooperative programs. Partnerships have been forged between resource agencies, 

conservation groups, and private citizens to help further wetland restoration and 

protection goals. Several landscape scale restoration plans have been developed 

throughout California. However, as California becomes more urbanized and 

development encroaches into historically rural areas, conflicts can arise from public 

health concerns about mosquito production in wetlands, rice fields, or other rural sources. 

To address these concerns as they relate to wetlands, this technical guide has been 

developed to provide information on habitat management strategies to reduce mosquito 

production in managed wetlands, and to facilitate greater cooperation among wetland 

habitat mangers and Mosquito and Vector Control Districts (MVCDs). The term, “Best 

Management Practices (BMPs)” is used to describe habitat management strategies that 

are generally defined as a practice or combination of practices determined to be an 

effective and practical means for reducing mosquito populations, production rates, or the 

timing of hatch. These BMPs focus on exploiting the ecological relationships among

mosquitoes, their predators, and the wetland habitats they use for breeding. In many 

cases, these practices are also beneficial to the general management of wetlands and the 

wildlife they support. 

The BMPs identified in this guide are also an essential component of Integrated 

Pest Management (IPM) for mosquitoes, which incorporates knowledge of mosquito 

biology and the use of effective treatments to control mosquitoes while posing the least 

risk to people and the environment. IPM employs a variety of mosquito control methods 

that include habitat management, biological control agents, and pesticide application. 

Ideally, BMPs can be used to lower the production of mosquitoes and reduce the need for 

chemical treatment without significantly disrupting the ecological character, habitat 

function, or wildlife use of managed wetlands. 

The CVJV has prepared this guide to present a full range of BMP options specific 

to managed wetlands. The BMPs have been identified from the scientific literature, as

well as from practical applications by wetland managers and MVCDs. The extent of 

their use may be limited by cost and personnel constraints, physical limitations of certain 

wetlands, or specific habitat and wildlife management goals. In some instances, potential 

disadvantages need to be weighed before implementation of BMPs. BMPs to achieve

mosquito control should not greatly disrupt the ecological character or habitat function of 

the wetland site. Not all BMPs can be effectively implemented in every wetland 

environment. Some initial investigation will be required of wetland managers, in 

 1 
cooperation with MVCDs, to identify those BMPs most applicable to an individual site. 

Prior to the implementation of BMPs, consultation should be conducted with MVCDs 

and appropriate resource agencies to determine the suitability of BMPs, and to ensure 

compliance with State and Federal wetland regulations and conservation easements. 

The information in this guide is applicable to managed wetlands in the Central 

Valley of California, including the Sacramento and San Joaquin Valleys, and the DeltaSuisun region. It is intended to be a reference for wetland stewards including: the private 

wetland owner or caretaker, Refuge or Wildlife Area manager, wetland biologist, or 

mosquito and vector control technician, and should be useful when making habitatrelated decisions to reduce mosquito production. The guide is intended to be as 

comprehensive as possible and describe BMPs based on the best available information. 

As additional monitoring and research is conducted, BMPs may be refined, new BMPs 

may be developed, and priorities for use will be determined. 

Additional copies of this technical guide can be obtained by contacting the 

Central Valley Joint Venture office at (916) 414-6460. 

 2 
MANAGED WETLAND DESCRIPTION

There are few wetland ecosystems in North America that have been more heavily 

modified than those in California. Massive flood control, water storage, and water 

conveyance projects have altered the natural hydrology that once supported over four 

million acres of wetlands in the Central Valley (Central Valley Habitat Joint Venture

1990). Today, most of the remaining five percent of historic wetlands in the Central 

Valley require intensive management, including “artificial” flooding. It is now the task 

of wetland managers to emulate natural hydrology and recreate a dynamic and productive 

wetland system through varied flooding regimes and periodic vegetation control. 

Managed wetlands are typically flooded using delivered canal water, water 

diverted from rivers or sloughs, or deep well pumping. Without the application of water, 

most managed wetlands would remain dry or experience only periodic flooding in the 

wettest of years. As a result, managed wetlands have levees, water control structures, and 

other features that allow for relatively intensive management, most notably the timing, 

depth, and duration of flooding. This infrastructure allows for habitat management 

practices to promote a variety of benefits to many species of wildlife (vegetation for 

food and cover, adequate water quality, breeding and resting sites). This capability can 

also lend itself to management practices that discourage mosquito production. 

The BMPs in this document were developed for use in managed wetlands, and are 

not applicable to wetland habitats where natural hydrology is still intact and human 

intervention is minimal or nonexistent. Some examples of wetland habitats where these 

BMPs are not applicable include vernal pools, tidal wetlands, natural seeps, and 

rivers/streams and their associated water bodies (oxbow lakes, sloughs, etc.). 

Chronology of Wetland Loss in California

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

3,000,000

3,500,000

4,000,000

4,500,000

5,000,000

1850 1875 1900 1925 1950 1975 2000

W

etland A

cre

s

Figure 1. Chronology of wetland loss in California. Information based on data provided in the 

Concept Plan for Waterfowl Wintering Habitat Preservation (USFWS 1978) and CVHJV 

Implementation Plan (1990).

 3 
WETLAND HABITAT MANAGEMENT

In the Central Valley, managed wetlands are described by three main flooding 

regimes; seasonal, semi-permanent, and permanent (Smith et al. 1995). These flooding 

regimes are distinguished by the timing and duration that water is present on the land. 

Seasonal Wetlands 

 Seasonal wetlands are by far the most abundant wetland habitat in the Central 

Valley, comprising about 85-95% of all managed wetlands (Central Valley Habitat Joint 

Venture 1990). They are initially flooded between August and October, remain flooded 

throughout the winter, and are drawn down in the spring, between March and May. 

Spring draw-down concentrates invertebrates in receding wetlands to provide forage for 

migratory waterfowl and shorebirds and exposes mudflats that germinate important seed 

producing plants called “moist-soil” plants. Following draw-down, most seasonal 

wetlands remain dry for the summer. Seasonal wetlands are the most diverse and 

productive type of managed wetland. They typically have the greatest diversity of 

vegetation and water depths, number of species (both plant and animal), and abundance 

of migratory birds and other wetland-dependent wildlife on an annual basis. 

Depending on location, conditions, or habitat objectives, seasonal wetlands may 

require irrigation one to two months following draw-down to bring moist-soil plants, 

such as swamp timothy (Heleochloa schenoides), watergrass (Echinochloa crusgalli), or 

smartweed (Polygonum lapathifolium), to maturity and enhance their production of seed 

for waterfowl and other wildlife (Naylor 2002). Irrigation may also be employed as a 

method to control undesirable plant species, such as cocklebur (Ducks Unlimited 1995a). 

Depending on the spring weather conditions, type of moist-soil vegetation that is 

being encouraged, or the need to discourage certain species, irrigation could occur 

Figure 2. Watergrass (Echinochloa crusgalli) 

and Wetland Irrigation. Wetlands are often 

irrigated to grow seed producing plants, such as 

watergrass, to meet the energetic requirements 

of migratory waterfowl and to reduce crop

depredation by attracting waterfowl to wetland

areas. Photos: Central Valley Joint Venture 

 4 
anytime from May through July, and could vary in both number and duration. Following 

irrigation, when soils dry sufficiently to support heavy equipment, vegetation 

management (disking and mowing) is conducted to limit the extent of undesirable 

vegetation and promote the growth of moist-soil plants. 

Additional information on seasonal wetland management can be found in A Guide 

to Wetland Habitat Management in the Central Valley (Smith et al. 1995) and 

Management of Seasonally Flooded Impoundments for Wildlife (Fredrickson and Taylor 

1982). 

Semi-permanent and Permanent Wetlands 

Semi-permanent and permanent wetlands comprise only 5-15% of the total 

managed wetlands in the Central Valley (Central Valley Habitat Joint Venture 1990). 

Semi-permanent wetlands are typically flooded from October through mid-July and 

permanent wetlands are flooded year-round. These habitat types are often characterized 

by a combination of open water, emergent vegetation (e.g., cattails (Typha spp.), tules 

(Scirpus acutus), and other bulrushes), and submergent aquatic vegetation (e.g., horned 

(Zannichellia palustris) and sago pondweed (Potamogeton pectinatus)). They provide 

habitat important to resident wildlife, and provide breeding and molting habitats for 

waterfowl at a time of year when most seasonal wetlands are dry (Ducks Unlimited 

1995b). Semi-permanent wetlands are drawn down after the breeding season, and 

measures such as disking, mowing, or burning are commonly used to manage vegetation 

growth. Permanent wetlands are typically drawn down every three to five years to 

recycle nutrients and increase productivity and, in some cases, control undesirable fish 

populations (e.g. carp). Similar to other managed wetland types, undesirable vegetation 

in permanent wetlands is typically controlled through disking, mowing, or burning. 

Figure 3. Semipermanent Wetlands.

Semi-permanent

wetlands provide 

critical habitat for 

resident wildlife at a 

time of year when 

most seasonal 

wetlands are dry. 

Photo: Jack Sparks, 

CDFG 

 5 
MOSQUITO BIOLOGY

Mosquitoes are dipteran insects with aquatic immature stages and an aerial adult 

stage. They have four aquatic larval stages (instars) plus an aquatic pupal stage. The adult 

emerges from the pupal stage onto the surface of the water, expands its wings, hardens its 

exoskeleton, and flies off. Depending on seasonal and environmental conditions and the 

particular mosquito species involved, it generally takes from three to 12 days for a mosquito 

to complete its life from developed egg to early adult stage. In general, as ambient 

temperature increases, the number of days required from hatching to emergence as an adult 

decreases. Although some species of mosquitoes (e.g., Culex tarsalis), are capable of long 

flights from the aquatic habitat, the mosquito problem created by a wetland will generally be 

proportional to the distance from concentrations of human and domestic animal populations. 

There are four primary species of mosquitoes (Ochlerotatus melanimon, Culex 

tarsalis, Culex erythrothorax, Anopheles freeborni) that can be produced in managed 

wetlands and surrounding agricultural lands that have been the subjects of control efforts by 

MVCDs in the Central Valley. These four species can be categorized by life history traits 

into two distinct groups (floodwater mosquitoes and standing water mosquitoes).

Floodwater Mosquitoes (Ochlerotatus melanimon) 

The life cycle of the floodwater mosquito begins with flooding of ground that has 

undergone a dry period. The summer dry cycle in seasonal or semi-permanent wetlands fits 

the criteria for this species’ habitat needs. Once flooded, eggs that were laid during the 

previous dry cycle hatch, pupate, and emerge as adults. Research conducted in Merced 

County found that Ochleratatus melanimon developed from first instar larvae to adult stage 

in eight to nine days in seasonal wetlands during the last half of September (Mortenson 

1963). Gravid females then return to lay their eggs singly on drying soil, in leaf litter, in 

cracks in the soil, or at the bases of grasses and other plants in areas that have been flooded 

previously. Each female lays approximately 150 eggs per ovarian cycle. These eggs are 

very drought resistant, allowing them to survive during the summer. 

Floodwater mosquitoes are often the most abundant mosquito produced by managed 

seasonal wetlands, especially during summer irrigations and/or fall flooding. Relative to 

other species, adult females are aggressive and feed primarily on mammals. During the day, 

females will bite if disturbed or if a host presents itself, but generally biting and swarming 

activities peak at dawn or dusk. Floodwater mosquitoes have been identified as a primary 

nuisance species and as secondary or “bridge” vectors for California encephalitis virus and 

western equine encephalitis, and are considered moderately effective as vectors of West Nile 

virus. 

 

Standing Water Mosquitoes (Culex tarsalis, Culex erythrothorax, Anopheles freeborni) 

Culex tarsalis 

Peak numbers of Cx. tarsalis occur in the Central Valley during the summer. 

Females lay their eggs on the water surface in bunches called rafts. Each raft contains

around 100-150 eggs, hatching about 24 hours after being laid. The immature stages can be 

 6 
found in almost any source of water except treeholes. During the summer, development 

from egg to adult takes about seven to nine days. Peak populations occur in late June or 

early July, but can continue into late summer. Adults can emerge continuously throughout 

the summer and fall in areas that have been flooded for an extended period of time, usually 

for more than 2-3 weeks (i.e. ricefields, poorly drained pastures, semi-permanent/permanent 

wetlands, seasonal wetlands flooded in August, sewer treatment plants, and dairy farms). 

Biting and swarming activities are typically at dawn or dusk. Adults spend daylight 

hours resting in secluded places such as animal burrows. Culex tarsalis primarily bite birds, 

but will bite humans, livestock, and other mammals if the opportunity presents itself. In

California, this species commonly feeds on song birds in spring and early summer, and 

switches to mammalian feeding in late summer and fall. This change in feeding habits from

birds to mammals, combined with large populations and the ability to travel long distances, 

makes Cx. tarsalis a potent vector of some of California’s arboviruses. Culex tarsalis is 

considered the primary vector for western equine encephalomyelitis virus (WEE) to humans 

and equines, and St. Louis encephalitis virus (SLE) to humans. Culex tarsalis has been 

identified as a primary vector of West Nile virus in the western United States. 

 

Culex erythrothorax. 

These mosquitoes prefer to deposit their egg rafts amid very thick aquatic vegetation 

in ponds and in the margins of lakes. The larvae can be difficult to sample because they are 

extremely sensitive to disturbances (e.g. vibrations from the collector’s footsteps or dipper), 

tend to remain submerged longer than other mosquito species after being disturbed, and 

prefer to stay amid the stems of vegetation where collection can be problematic. This larval 

habitat can also be difficult to treat with larvicides because the water is deep, creating a 

much greater volume to surface area ratio than commonly encountered for Cx. tarsalis. In 

addition, the thick vegetation can protect the water surface from liquid formulations of

larvicides. Fish are not always effective at controlling this species because they are visual 

predators and cannot find the larvae among thick growth of plants. Unlike most other Culex 

species, some individuals of Cx. erythrothorax bite (feed) during the day. Fortunately, this 

species does not migrate far (generally less than 1 mile) from the larval habitat. It is a major 

pest where wetlands with deep water and aquatic vegetation occur near human activity. 

Culex erythrothorax is highly susceptible to WNV infection and may act as a bridge vector 

of this virus in California (Goddard et al. 2002). 

Anopheles freeborni 

 Anopheles freeborni also occurs in the Central Valley and is numerous during the 

summer, peaking in late July or August. Ricefields, and semi-permanent and permanent 

wetlands are the primary production areas for this species, although the immature stages are 

also found in ditches, seepages, and sloughs. Females lay their eggs singly on the surface of 

the water where they hatch approximately 24 hours later. On the average, it takes about 9-

12 days for An. freeborni to develop from egg to adult. Like Cx. tarsalis, this species can 

produce a continuous supply of newly emerged adults under the right habitat conditions. 

Adults rest during the day and bite and swarm during dusk. In autumn, females enter a 

physiological state called diapause, during which reproduction is suspended and activity is 

 7 
diminished. They over-winter until January, February, or March when they come out of

diapause and seek blood meals on warm days. After obtaining a blood meal, many females 

resume their over-wintering state until April or May when they begin laying eggs once 

more. The females will readily bite humans and livestock. This species can be a vector of

malaria in the western United States. Malaria was a major public health problem in 

California through the early 1900s. A combination of case detection and mosquito control 

reduced transmission to very low levels by the 1920s. Three major outbreaks in the last 40 

years have served as reminders that the introduction of the malaria parasite (Plasmodium 

falciparum and P. vivax), combined with presence of the Anopheles mosquito vectors, can 

result in transmission. 

 8 
MOSQUITO ABUNDANCE IN WETLAND HABITATS

Mosquito abundance reaches its peak during the flood-up of seasonal wetlands 

during late summer and early fall (Figure 4). Because hundreds of acres of seasonal 

wetlands are flooded per week, there can be a constant influx of new mosquito cohorts, 

resulting in a large sustained population of mosquitoes over the flood-up period. As each 

wetland floods, floodwater mosquitoes (Oc. melanimon) can be produced initially (on 

average within seven to ten days), followed by Cx. tarsalis after approximately two weeks 

of inundation. There is a second smaller and much shorter-lived peak that is often observed 

during the spring/summer irrigation of seasonal wetlands. This involves less acreage and is 

not sustained since the irrigations are usually completed in seven to ten days. The irrigated 

wetlands then revert back to a dry period until flood-up in the fall. 

J F M A M J J A S O N D

Month

Mosquito production

potential

Irrigations

Fall Flood-up

Acres of Flooded Managed Seasonal Wetland

Figure 4. Seasonal wetland flooding and potential time periods for significant mosquito production.

 

Semi-permanent and permanent wetlands can produce An. freeborni and Cx. tarsalis, 

but because of their limited acreage, stable water levels, and abundance of mosquito 

predators (fish, dragonflies, and other predatory invertebrates) they are typically not 

considered “problem” production areas requiring additional control measures. However, 

they can still be managed to minimize mosquito populations by properly managing water 

and vegetation (Sacramento National Wildlife Refuge Complex 2000). If water levels are 

allowed to fluctuate, these wetlands can produce floodwater mosquitoes that may become a 

concern. 

 9 
MONITORING MOSQUITO POPULATIONS

MVCDs determine mosquito abundance in wetlands using a variety of sampling 

techniques. Potential breeding sites and surrounding areas are sampled to assess mosquito 

populations and determine the need for control efforts. Treatment thresholds are typically 

determined based on monitoring both aquatic and aerial life stages of mosquitoes. 

 

Immature Stages (aquatic) 

 

Larvae and/or pupae are typically sampled using a “dipper”, which is a 14-ounce 

white cup attached to a long wooden or metal handle. The dipper is used to determine the 

relative density of larvae by taking standard samples of water from a potential mosquito

source. The contents are examined, recording mosquito numbers and species, resulting in a 

“number per dip” index. Sampling immature life stages of mosquitoes serves to identify 

significant production sites and can help determine the potential need for treatment. This is 

done based on the quantity and species of larvae found (per dip), identification of the larval 

instar stage, number of days required for the larvae to pupate and emerge, and proximity to 

urban areas. Monitoring larval mosquito production throughout a wetland can help identify 

problem areas where certain BMPs can be employed to reduce future production.

When aquatic vegetation is abundant, some species of mosquitoes are difficult to 

sample in their immature stage. Larval traps have been developed, but they are not widely 

used. Column sampling devices have been used successfully by quickly inserting a wide 

pipe into the substrate and then exhaustively sampling the water within. 

Adult Stage (flying) 

The relative abundance of adult mosquitoes is estimated using light traps, carbon 

dioxide-baited traps, or landing counts, depending on the individual MVCD’s protocol. 

Traps are hung from trees, buildings, or special stands and usually remain in the same place 

throughout the season and from year to year. Light trap contents are collected weekly 

during the mosquito season. The total number of adult females, by species, is divided by the 

number of nights the trap was activated, and the index value is reported as adult females per 

trap night. 

With landing counts, an observer stands within the sample site and faces away from

the wind and counts the number of mosquitoes that land on his pant legs over a period of 

one minute. The count is divided by two for a “leg count” index. This procedure is usually 

done early in the morning or at dusk when temperatures are cool and mosquitoes are most 

active and is repeated three or four times per site.

 10 
BEST MANAGEMENT PRACTICES 

The BMPs in this document are habitat-based strategies that can be implemented 

when needed for mosquito control in managed wetlands. These strategies represent a range 

of practices that wetland managers can incorporate into existing habitat management plans 

or in the design of new wetland restoration or enhancement projects. Ideally, BMPs can be 

used to decrease the production of mosquitoes and reduce the need for chemical treatment 

without significantly disrupting the ecological character, habitat function, or wildlife use in 

managed wetlands. It should be recognized that BMPs function as a first line of defense in 

deterring mosquito production and can be used in combination with other Integrated Pest 

Management (IPM) tools such as, biological controls, larvicides (Appendix A), and 

adulticides (Appendix B) when necessary. 

In many cases, BMPs overlap with commonly used habitat management practices to 

conserve water and manage wetland vegetation for wildlife (Batzer and Resh 1992a, Batzer 

and Resh 1992b, Resh and Schlossberg 1996). Not all BMPs will be appropriate for a given 

wetland location or set of circumstances. Therefore, habitat managers are encouraged to 

work closely with both their local MVCD and agency biologists to select BMPs based on 

their potential effectiveness for regional or site specific conditions, and habitat management 

strategies. The implementation of BMPs will likely be limited by cost and personnel 

constraints, potential impacts on wetland habitat, and wildlife response to these measures. 

In the following section, BMPs have been classified into five categories. These 

categories are not listed in order of importance and may be used in combination. 

• Water Management Practices 

• Vegetation Management Practices 

• Wetland Infrastructure Maintenance 

• Wetland Restoration and Enhancement Features 

• Biological Controls 

Following each category is a table summarizing the BMPs that outlines strategies, mosquito 

control objectives, advantages, and disadvantages (Tables 1 through 6). 

Water Management Practices 

Water management is one of the wetland manager’s greatest tools for reducing 

mosquito populations (Table 1). However, it requires that water is readily available, of

sufficient quantity and quality, and that the conveyance infrastructure is adequate to permit 

rapid flooding or drainage. In some instances, circumstances outside the control of wetland 

managers may limit the ability to implement water management BMPs. Such circumstances 

may include when agriculture drain water or delivered water is available for flooding, 

limited water quantity or poor water quality, and undersized water delivery or drainage 

infrastructure. In managed wetlands where these limitations are not an issue, the following 

water management practices should be considered. 

Timing of Flooding: The timing of wetland flooding can greatly influence 

mosquito production (Fanara and Mulla 1974, Batzer and Resh 1992a). Delayed flooding 

may reduce mosquito production by shifting flooding schedules later in the year, when 

 11 
temperatures are cooler and mosquito production is less of a problem. Delayed flooding 

should be considered for wetlands with historic mosquito problems and those in close 

proximity to urban areas. However, delayed flooding means that less wetland habitat is 

available for wildlife during times of the year such as August and September when wetlands 

are particularly limited. Delayed flooding may also have limited applicability for some

properties that are required to take water on a “when available” schedule and have little 

control over the timing of flooding. Delayed flooding may be especially difficult for State 

and Federal areas that are obligated to provide “early” habitat to reduce crop depredation by 

waterfowl. 

Given the limited feasibility of delayed flooding on some properties, phased flooding 

of wetlands may be useful to allow habitat managers to provide some level of early flooded 

habitat while delaying flooding on a portion of a property. Phased flooding involves 

flooding habitat throughout the fall and winter in proportion to wildlife need and takes into 

consideration other wetland habitat that may be available in surrounding areas. 

For wetlands that are flooded early (August - early September) or in close proximity 

to urban areas, the use of vegetation and water management BMPs should be a high priority 

(Tables 1 and 2). 

BMPs: Delayed or phased fall flooding, Early fall flood-up planning (see Table 1 for 

additional explanation) 

Speed of Wetland Flooding: As a general rule, the faster water can be applied 

during fall flooding and spring/summer irrigation, the fewer generations of mosquitoes will 

be hatched. Slow feather-edge flooding, although beneficial to foraging waterbirds, can 

produce multiple, staggered hatches of floodwater mosquitoes and, if treatment is necessary, 

often requires MVCDs to visit wetlands over a number of days for control activities (Garcia 

and Des Rochers 1983). Such an intensive treatment effort is expensive and results in 

additional disturbance to wildlife. 

BMPs: Rapid fall flooding, Rapid irrigation (see Table 1 for additional explanation)

Figure 5. Rapid 

Flooding. Rapid 

flooding should be 

used to reduce the 

potential for 

multiple hatches of 

mosquitoes caused

by slow feather-edge

flooding. Photo:

USFWS 

Water Control: Once wetlands have been flooded, it is important for wetland 

managers to ensure that pond elevations do not fluctuate except during planned draw-down 

or periods of low mosquito production (i.e. winter months). Fluctuating water levels tend to 

 12 
expose wetland edges to drying and provide suitable habitat for floodwater mosquitoes to 

lay eggs (Garcia and Des Rochers 1983). When water levels are subsequently raised, a new 

cohort of mosquitoes may be hatched. Water levels should be maintained by checking water 

levels frequently, and adding water to offset any losses. A constant maintenance flow of 

water will also help maintain steady water levels, improve water quality, and reduce 

stagnation. 

If possible, wetlands can be flooded to deeper water depths during the fall and 

allowed to recede during the cooler winter months to provide shallow water depths for 

foraging waterbirds. Deeper water depths (24 inches) at initial flooding have been shown to 

significantly reduce mosquito densities at Grizzly Island Wildlife Area (Batzer and Resh 

1992a, b). 

When flooding wetlands, water sources containing mosquito predators should be 

used to help colonize wetlands with predacious insects or mosquitofish that are passively 

transported by water from upstream locations (Collins and Resh 1989). Predator 

populations can be maintained in permanent waterways used to flood seasonal wetlands. In 

the Suisun Marsh, where water is readily available for flooding, seasonal wetlands are often 

initially flooded, and if mosquitoes become abundant, water levels are drawn down to 

concentrate mosquito larvae in ditches for biological control, larvicide treatment, or to 

drown larvae through turbulent water movement (Chappell pers. comm). Following this 

action, wetlands are immediately re-flooded. 

BMPs: Maintain stable water levels, Circulate water, Use deep initial flooding, 

Subsurface irrigate, Utilize water sources with mosquito predators for flooding, Flood and 

drain wetland (see Table 1 for additional explanation)

 Frequency and Duration of Irrigation: Spring and summer irrigation is a common

wetland management practice used to increase seed production and biomass of moist-soil 

plants (Naylor 2002), and reduce competition from undesirable plants in seasonal wetlands. 

The need to irrigate seasonal wetlands should be assessed closely by wetland managers. 

During years with above average spring precipitation, irrigations may not be necessary to 

maximize moist-soil plant production. When possible, managers should shorten the duration 

of irrigation to 4 to 10 days to reduce the likelihood of hatching floodwater mosquitoes and 

eliminate the possibility of creating habitat for standing water mosquitoes. However, shorter 

irrigations may not always be feasible, especially when growing more water intensive plants 

such as watergrass and smartweed, or when conducting flooding to control undesirable plant 

species. In the case of weed control, plants should be monitored and water held only long 

enough to eliminate weeds. The necessary timing can be determined when weeds have 

turned black or have disintegrated. Finally, following wetland irrigations, water should be 

drawn down into waterways containing mosquito predators that can consume any mosquito 

larvae which may have hatched. 

BMPs: Reduce number of irrigations, Use rapid irrigation, Draw down and irrigate 

in early spring, Irrigate prior to field completely drying, Drain irrigation water into ditches 

or other water sources with mosquito predators, Use subsurface irrigation (see Table 1 for 

additional explanation) 

 13 
Table 1. Water Management Practices to reduce mosquito production in managed wetlands. 

Best Management Practice 

Strategies Mosquito Control Objective Advantages D

Delayed or phased fall flooding 

Delay flooding of some wetlandunits until later in the fall. Delay flooding units with greatest historical mosquito productionand/or those closest to urbanareas. 

To delay initiation of floodwater mosquito production in seasonal wetlands by reducing the amount of mosquito habitat available during optimal breeding conditions (warm summer/early fall weather). Reduce the time available for standing water mosquito production in seasonal wetlands. 

Depending on flood date, canreduce the need or amount ofadditional treatment. Delayed flooding can provide “new” foodresources for wildlife later in the season when wetlands are finally flooded. 

Rfowmwagriby coDpryele

Early fall flood-upplanning

Apply BMPs to wetlands identified for early flooding. To the extent possible, areas targeted for early fall flooding should not be near urban centers and should not have a history of heavymosquito production.

To reduce the early season production of mosquitoes or to reduce their encroachment on urban areas. 

Allows for the provision of early flooded habitat while minimizing mosquito production and conflicts with urban areas. 

Sto arof ar

Rapid fall flooding Flood wetland unit as fast as

possible. Coordinate floodingwith neighbors or water district to maximize flood-up rate. 

To minimize number of mosquito cohorts hatching on a given area. 

Reduces the need for multiple treatments needed bysynchronizing larval development and adult emergence. In turn, reduces

wildlife disturbance by MVCDs.

Rto fehe

Rapid irrigation 4-10 day irrigation (from time

water enters the pond to complete draw-down). 

Shorten irrigation period toreduce time available for mosquitoes (especially Culex tarsalis and Anopheles freeborni) 

to complete lifecycle. 

Provides some level of wetland irrigation while reducing the timeavailable for mosquitoes to complete lifecycle. 

R& usir
Table 1. Water Management Practices to reduce mosquito production in managed wetlands, Continued 

Best Management Practice 

Strategies Mosquito Control Objective Advantages D

Maintain stable water level (summer and early fall flooding)

Ensure constant flow of waterinto pond to reduce water fluctuation due to evaporation, transpiration, outflow, and seepage. 

To reduce conditions for additional floodwater mosquito production in summer and fall. 

Provides a stable wetland environment for breedingwildlife during spring and summer. Discourages undesired excessive vegetative growth which could also becomeadditional mosquito breeding substrate. 

Rad

stwor hash

Water circulation Provide a constant flow of water 

equal to discharge at drain structure. 

To keep water fresh and movingto deter stagnant conditions formosquito production; reduceswater level fluctuation and potential production offloodwater mosquitoes. 

Discourages warm water conditions associated with avian botulism outbreaks. 

RadMavun

Deep initial flooding (18-24”)

Flood wetland as deep aspossible at initial flood-up. 

To reduce shallow water habitat for mosquito breeding. May provide more open water byover-topping vegetation, therebyfacilitating mosquito predation or wind action that drowns larvae. 

Potentially slows mosquito development by eliminating warm, shallow water habitat. 

Rindefoan

Utilize water sources with mosquito predators for flooding wetlands 

Flood wetlands with water sources containing mosquito fish or other invertebrate predators. Water from permanent ponds can be used to passively introduce mosquito predators. 

To inoculate newly flooded wetlands with mosquito predators. 

May establish mosquito predators faster than natural colonization. 

Ralprww

Drain irrigation water into ditches or otherwater bodies with abundant mosquitopredators 

Drain irrigation water into locations with mosquito predators as opposed to adjacent seasonal wetland or dry fields. 

To reduce the amount of larvae through natural predation and minimize the number of adultsthat emerge. 

Already a common wetlandmanagement practice. 

Mwpo

dr

Flood & drain wetland Flood wetland and hatch larvae

in pond. Drain wetland to borrow or other ditch where larvae can be easily treated, drowned in moving water, orconsumed by predators. Immediately reflood wetland. 

Hatches mosquito larvae and moves them to a smaller area for treatment before they can emergeas adults. 

Can eliminate or reduce the need for additional mosquito control efforts. 

Ato cris 
Table 1. Water Management Practices to reduce mosquito production in managed wetlands, Continued 

Best Management Practice 

Strategies Mosquito Control Objective Advantages D

Reduce number ofirrigations 

Evaluate necessity of irrigation, especially multiple irrigations, based on spring habitat conditions and plant growth. Eliminate irrigations when feasible. 

To eliminate unneeded additional irrigations which could provide potential habitat for mosquitoes. 

Reduces potential need for additional mosquito control. Saves water and manpower costs. Discourages excessive growth ofundesirable vegetation (i.e. joint and bermuda grass) 

Mpl

Early spring drawdown and irrigation Draw-down wetland in late March or early April. Irrigate in 

late April or early May when weather is cooler and mosquitoes are less of a problem. 

To reduce need for irrigation in June, July, and August, whenpotential for mosquito production would be higher. 

Wetland irrigation can be accomplished without creating potential mosquito problems. May allow moist-soil plants to take advantage of natural rainfall during the spring. 

RfowduNmcogeun

Don’t let field completely dry and crack between spring draw-down andirrigation 

Irrigate wetland before soil completely dries. 

To eliminate necessary drying period for floodwater mosquito to lay eggs.

May reduce mosquitoes produced from irrigation

Rmir

Subsurface irrigation Maintain high ground water 

levels by keeping boat channels or deep swales permanently flooded. 

To reduce amount of irrigation water during mosquito breeding season. 

Reduce need for surfaceirrigation while maintaining soil moisture to promote moist-soil plant production. 

Rchadeqprif Rmswveprpl
Vegetation Management Practices 

 

Wetland managers commonly use vegetation control to alter plant species

composition and abundance to influence wildlife use (Smith et al. 1995). As the vegetative 

community in a wetland changes through time, undesirable species inevitably encroach into 

wetland habitats. Some species may be relatively benign, while others may be problematic, 

reducing habitat diversity or crowding out plants important as wildlife forage or cover. 

Vegetation is also an important habitat requirement for mosquitoes and can improve 

mosquito survivorship by providing refuge from predators (Walton and Mulla 1989) and 

abundant food resources for larvae. Vegetation can also increase mosquito developmental 

rates by raising water temperatures (Collins and Resh 1989). To manage the vegetative 

community to benefit wildlife, wetland managers use a number of techniques, including 

mowing, burning, disking, and grazing. These habitat management practices can be used 

alone or in combination and can also be used to reduce mosquito production (Table 2). 

Any management action that alters the composition of wetland vegetation may create 

either benefits or detriments to wildlife. The decision to conduct such operations should be 

determined by the management objectives of an individual habitat manager. Site specific 

characteristics, habitat management objectives, cost, and recreational use (e.g., hunter 

access) all have to be carefully considered in vegetation management plans. Typically, any 

vegetation control measure will result in the short-term loss of cover for wildlife. Such 

compromises have to be weighed in terms of the long-term benefits they provide for wildlife 

resources versus the ancillary benefits they provide for mosquito control.

Mowing: Mowing is commonly used to create open water habitat for shorebirds and 

waterfowl prior to flooding seasonal wetlands. Mowing to create open water provides 

opportunities for the biological control of mosquitoes and enhances the effectiveness of 

pesticides by allowing greater saturation of mosquito habitats. Experimental mowing of 

approximately 50% of a wetland has been shown to reduce the density of mosquitoes and 

concentrate their distribution while increasing densities of invertebrates that are consumed 

by waterfowl (Batzer and Resh 1992a). Similarly, Garcia and Des Rochers (1984) found 

Figure 6. Vegetation

Mowing. Mowing

can be used to create 

open water habitat

and reduce mosquito

densities. If mosquito 

control is necessary, 

mowing can 

concentrate 

mosquitoes and 

enhance pesticide 

application by

allowing better 

penetration of

vegetation. Photo:

Jack Sparks, CDFG 

 17 
that seasonal wetlands mowed with a 50% plant-cover ratio enhanced wind action that 

moved mosquito larvae to wetland edges where treatment efforts could then be concentrated. 

The benefits of mowing, unlike burning or disking, tend to be short term and require 

that the practice be implemented on an annual basis. Mowing may also leave residual 

matter that, when flooded, provides habitat for mosquitoes (Brown pers. comm). 

Burning: Controlled burning, where and when feasible, can effectively control 

vegetation. Burning, especially when immediately followed with disking, can offer multiple 

year control of mosquitoes, reduces vegetation used by mosquitoes for breeding activities, 

and also directly results in the kill of mosquito eggs due to high temperatures associated 

with fire (Resh and Schlossberg 1996, Whittle et al. 1993). Burning releases nutrients stored 

in plant materials and makes them available to benefit plant and invertebrate production 

during the following flooding cycle. However, due to liability issues, difficulty with 

permitting, air quality, and the coordination that must take place with multiple agencies prior 

to its implementation, burning has limited applicability for Central Valley wetlands, 

especially those on private lands. 

Disking: Disking is commonly used by habitat managers to reduce dense stands of 

emergent or undesirable vegetation, and provide favorable conditions for the establishment 

of moist-soil plants consumed by waterfowl. Disking, unlike mowing, tends to change the 

vegetation composition of a wetland and provides a more permanent means of controlling 

vegetation. Disking has also been shown to significantly reduce densities of mosquitoes 

over multiple years, increase the densities of macroinvertebrates important in the diets of

waterfowl, and encourage the replacement of less desirable vegetation by moist-soil plants 

(Resh and Schlossberg 1996). The benefit of disking can often be enhanced by first 

mowing or burning vegetation targeted for control. 

 

 

 

Haying and Grazing: Agricultural practices such as haying and grazing may also 

be useful to control wetland vegetation. However, little information is available on their 

effectiveness as mosquito control measures. Haying, while functionally similar to mowing, 

may provide the same mosquito control benefits. Haying has the potential added benefit of 

removing cut plant material that may decay and negatively affect water quality, thereby 

increasing mosquito production (Brown, pers. comm). However, haying removes valuable 

Figure 7. Vegetation Disking and Dense Bermuda Grass (Cynodon dactylon). Dense stands of bermuda 

grass are known to harbor mosquitoes (Garcia and Des Rochers 1983). Bermuda grass also tends to limit

habitat diversity in seasonal wetlands and is targeted for control by wetland manager using disking. Photo:

Jack Sparks, CDFG and USFWS

 18 
seed resources consumed by wildlife and interrupts the cycle of nutrient release back into 

wetland environments. In addition, haying has limited applicability in most wetland 

environments because of the relative remoteness of wetlands from agricultural operations, 

difficulty of running haying equipment in unleveled wetland terrain, and low palatability of 

wetland vegetation as livestock forage. 

Grazing is frequently used to reduce plant biomass in wetlands and provide short 

grass and open water habitats for shorebirds and waterfowl. However, grazing animals 

require sources of water for drinking and often irrigated grasslands for forage. Water used 

to meet these needs may provide additional habitats for mosquitoes. Furthermore, grazing 

animals can create wallows or depressions in moist wetland terrain that trap water and create 

additional microhabitats for mosquitoes (Brown, pers. comm.). Additional research needs to 

be conducted to determine the usefulness of grazing as a mosquito reduction tool. 

 19 
Table 2. Vegetation management practices to reduce mosquito production in managed wetlands.

Best Management Practice 

Strategies Mosquito Control Objective Advantages D

Mowing Mow undesirable or overgrown 

vegetation that serves asmosquito breeding substrate prior 

to flooding. 

To reduce standing vegetation that mosquitoes can use for egg laying and larval development. To create open water habitat that allows mosquito predators (fish, invertebrates, birds) better access to larvae and potentially more wave action to drown mosquito larvae. 

Improves wildlife habitat by providing open-water. 

EmOsotavemdequ

Burning Controlled burn of undesirable or

overgrown vegetation that mayprovide mosquito breeding substrate. 

See mowing. Can also kill mosquito eggs.

See mowing. R

conocoSptawcospin

Disking Disc undesirable or overgrown

vegetation that may provide mosquito breeding substrate.

See mowing. See mowing. Can provide 

longer-term control of undesirable vegetation by itself or in conjunction with other management practices. 

Chudewin

Haying Mow and bale undesirable or

overgrown vegetation that mayprovide mosquito breeding substrate. 

See mowing. Also removes vegetation after cutting. 

Dual benefits of improvinghabitat and reducing mosquito breeding substrate. Removal ofmowed vegetation further decreases mosquito breeding substrate and may improve waterquality. 

Osotasefodian

Selective Grazing Summer-Fall grazing. Short 

duration, high intensity grazing. 

To reduce standing vegetation that provides habitat for mosquitoes. 

Relatively inexpensive. Ir

livexLplwda
Wetland Infrastructure Maintenance 

Wetland infrastructure is the foundation for habitat management. A properly 

functioning water delivery and drainage system, well maintained levees, correctly operating 

water control structures, and efficient pumps are key to avoiding the unnecessary production 

of mosquitoes through simple neglect (Table 3). Time and money invested in these 

proactive maintenance activities will reduce mosquito production and help landowners avoid 

additional costs of controlling mosquitoes and unwanted vegetation when fall flooding or 

irrigating wetlands. 

Levee and Water Control Structure Inspection and Repair: Levees and water 

control structures should be inspected on an annual basis to identify problem areas that may 

inadvertently leak water and produce mosquitoes. This includes identifying weak spots or 

rodent damage in levees that may seep water during flooding. Water control structures 

should be water-tight and properly sealed to prevent seepage. 

Ditch and Swale Cleaning: Vegetation in water delivery ditches and swales can be 

problematic by creating habitat for mosquitoes or by simply impeding the flow of water that 

facilitates rapid flooding or drainage. Typical maintenance activities of water delivery and 

drainage ditches include the use of herbicides or periodic dredging to remove problem

vegetation that inhibits water flow. Ditches and swales should be cut to grade to prevent the 

unintentional trapping of water. Likewise, silt that accumulates in front of outlet structures 

should be removed so it does not trap water in drainage swales. 

Figure 8. Faulty Water 

Control Structure. 

Defective water control 

structures should be

replaced with structures 

that can be completely 

sealed to prevent water 

seepage. Photo: USFWS

Pump Tests and Repair: If wetland managers use pumps for flooding, periodic 

pump testing should be conducted to make sure pumps are operating at optimum efficiency. 

This will ensure that pumps are providing maximum output, and will facilitate rapid 

flooding.

 

 21 
Table 3. Wetland infrastructure maintenance activities used to reduce mosquito production in managed w

Best Management Practice 

Strategies Mosquito Control Objective Advantages D

Levee Inspection &Repair 

Walk or drive levees, flag problem spots, repair as needed. Consider design elements to improve integrity of levee (see levee design in Table 4). 

To reduce mosquito habitat/production caused byseepage into adjacent fields or dry ponds. 

Allows for early identification of problem spots. Helps conservewater and reduces growth ofunwanted vegetation. 

Rfu

Water Control Structure Inspection, Repair, & Cleaning

Inspect structures and repair or replace as needed. Remove silt and vegetation build-up in front of structures. Adequately close, board or mud-up controls. 

To reduce mosquito habitat/production caused byseepage into adjacent ponds or drainage ditches. Remove silt blockages that may trap waterand impede drainage. 

Enhances water management capabilities and limits unwanted vegetation or standing water.

Rfu

Ditch Cleaning Periodically remove silt or 

vegetation from ditches to maintain efficient water delivery and drainage. 

To allow for rapid flooding/drainage & reduce vegetation substrate for breeding mosquitoes. 

Enhances water management capabilities and limits unwanted vegetation or standing water.

Rclrein bi

Pump Tests & Repair Test pump efficiency and make 

any necessary repairs to maximize output. 

Could identify output problemsand if corrected, allow managers to flood more rapidly.

May promote faster irrigation and flood-up if output can be improved. 

Rcopu
Wetland Restoration and Enhancement Features 

All well planned wetland restoration and enhancement projects begin with an initial 

survey and design phase. It is during this phase that landowners and restoration biologists 

have the opportunity to discuss design features with MVCDs and incorporate BMPs to

reduce mosquito production. Time spent at the design stage can save thousands of dollars in 

annual operation and maintenance costs and prevents problems resulting from poor water 

management and unintended mosquito production. Wetland design typically focuses on 

aspects of water control that promote vegetation beneficial to wildlife, conserve water, and 

allow for periodic vegetation control. In turn, water control is also an important mosquito 

BMP (Sacramento-Yolo Mosquito and Vector Control District 1995, Contra Costa Mosquito 

and Vector Control District 2001). 

Wetland design features to reduce mosquito production: Wetland design features 

that reduce mosquito production include independent flooding and drainage capabilities of 

wetland units, size considerations in the design of wetland units to facilitate rapid flooding, 

and the incorporation of design features that promote habitats for mosquito predators and 

allow those predators access to mosquitoes (Table 4). Water delivery ditches, water control 

structures, and levees should be designed and built to specifications that prevent wind and 

water erosion, provide equipment access for maintenance activities, and reduce damage 

caused by burrowing animals (Table 4). These design features will facilitate other mosquito 

BMPs such as water and vegetation management practices, infrastructure maintenance, and 

natural mosquito predation. 

BMPs: Independent water management, Adequately sized water control 

structures, Swale construction, Wetland size consideration, Ditch design, Levee design & 

compaction, Deep channels or basins constructed in seasonal wetlands, Permanent water 

reservoir that floods into seasonal wetlands (for additional explanation see Table 4) 

Figure 9. Wetland

Swale. Swales can 

be designed into

restoration or

enhancement 

projects to 

facilitate rapid

flooding and 

remove standing

water during spring 

draw-down. Photo:

Chadd Santerre, 

CWA 

 23 
Table 4. Wetland restoration and enhancement features to reduce mosquito production in managed wetlan

Best Management Practice 

Str

ategies Mosquit

o C

o

ntrol Objective Advantages D

Indepen

dent w

ater 

ma

nagement 

To the e

xtent 

possible, desig

n

wetland projects to include independent inlets and outlets for each wetland unit. 

To reduce the need to movewater through 

multiple wetland 

units 

w

hen flooding or irrigating 

target areas. T

his can reduce the 

number of

mosquitoes produced 

per flood event.

Creates wetland units that arehy

drologically distinct from o

ne 

another allowing for diversewetland management. 

Mcobe Inco

Adequately sized w

ater 

control structures 

Increase size and number of water control structures. Whe

n

installing, set to 

proper grade to

allow for complete draw-down.

To improve ability to imple

ment 

rapid flooding/irrigation B

M

Ps

(Table 1). 

See rapid flooding/irrigationBMPs (Table 1). 

Inwinm

Swale construction (sloped from i

ntake to 

drain)

Construct or enhance swales so they are sloped from inlet to outlet and allow the majority of the wetland to

be drawndown. 

To improve ability to imple

ment 

rapid flooding/irrigation B

M

Ps

(Table 1). Creates a means to move water through wetlands without flooding entire wetland basin. Reduces mosquito habitat by allowing isolated sections of habitat to drain. Provi

des 

mosquito pre

dators with access 

to all portions of wetland. 

See rapid flooding and irrigatio

n

B

MPs (Table 1). Provides habitat 

diversity and enhancescapabilities to i

mple

ment 

moistsoil management. Provides a 

more cost-effective and wildlife friendly alternative to laserleveling to create drainage. 

SeBstofwadswpedevebe ar

Wetland size considerations 

Install cross-levees to facilitate more rapid irrigation and floodu

p (

Table 1). Build “under

water”

levees that isolate irrigation water during the spring, but canbe overto

p

ped during fall and 

winter flooding. 

To improve ability to imple

ment 

rapid flooding/irrigation B

M

Ps

(Table 1). 

Assists with faster flooding a

nd 

drainage. Cross levees (checks) can provide loafing habitat for waterfowl and shorebirds. 

Adediadw

Ditch desig

n (2:1

slopes & minimum 4 foot bottom)* *co

nsider 3:1 slope or

gre

ater to discoura

ge 

burrowing a

nimal 

damage and potential seepage problems 

Construct or improve ditches to quality standard that prevents unwanted vegetation growth orunnecessary seepage.

Reduces likelihood of vegetation growi

ng alo

ng ditch ban

ks.

Excessive vegetation slo

ws water 

flow, traps silt, and can be used as substrate for mosquito eggs. 

Improves water flow and decreases maintenance of vegetation that grows along canal ban

ks.

Mdedehaas slan
Table 4. Wetland restoration and enhancement features to reduce mosquito production in managed wetlan

Best Management Practice 

Strategies Mosquito Control Objective Advantages D

Levee design &compaction (>3:1 

slopes & >80%compaction)* *consider5:1 slope orgreater in areas prone to over-land floodingand levee erosion. 

Construct or improve levees to quality standard that ensures stability and prevents unwanted seepage. 

To reduce mosquito habitat caused by seepage into adjacent fields or dry ponds. 

Properly constructed levees prevent seepage from erosion orrodent damage, and reduce need for annual maintenance. 

Abupr

Deep channels or basins constructed in seasonal wetlands

Excavate deep channels or basins to maintain permanent water areas (> 2.5 feet deep) within a portion of seasonal wetlands.Provides year-round habitat formosquito predators which can inoculate seasonal wetlands when they are irrigated or flooded. 

To reduce mosquito larvae through predation. 

Provides on-site source ofmosquitofish and other mosquito predators to seasonal wetlands. Increases overall habitat diversity. 

EmPveha

Permanent water reservoir that floods into seasonal wetlands 

Maintain separate permanent water reservoir that conveys water to seasonal wetlands.Provides year-round habitat formosquito predators which can inoculate seasonal wetlands when they are irrigated or flooded. 

To reduce mosquito larvae through predation. 

Provides on-site source ofmosquitofish and other mosquito predators to seasonal wetlands. Increases overall habitat diversity. 

Areseto 
Biological Controls 

Naturally occurring predators, such as fish, dragonflies, aquatic macroinvertebrates, 

and certain bird and bats, can contribute to the reduction of mosquitoes (Table 5). However, 

it often takes time before predator populations become established and have an effect on 

mosquito numbers, because mosquitoes frequently exploit ephemeral habitats and have 

relatively short lifecycles. Certain predator populations can be supplemented, for example, 

through the stocking of mosquitofish (Gambusia affinis), but in most instances a habitat 

manager’s best strategy is to maintain natural predator populations on-site. For biological 

controls to be effective, wetland managers need to create an environment that is both 

conducive to maintaining predator populations and providing predators with access to 

mosquito prey. 

Encourage on-site predator populations: Management practices have been 

developed that incorporate permanent water within seasonal wetlands to “inoculate” 

newly flooded habitats with an on-site predator source. This can be accomplished by 

maintaining permanent water in swales and deep borrow ditches, or by flooding wetlands 

with water from nearby permanent wetlands. Permanent water sources will need to be 

maintained when seasonal wetlands are dry, if wild populations of mosquitofish are to be 

sustained on-site. Such “dry season” predator reservoirs should be 18 inches or greater in 

depth to reduce predation of mosquitofish by herons and egrets (Collins and Resh 1989). 

Dry season reservoirs should be interconnected to seasonal wetlands through swales or 

ditches to allow mosquitofish to seek habitat as seasonal wetlands are drawn down and, 

conversely, to allow mosquitofish to disperse through seasonal wetlands as they are reflooded in the fall. 

 It is critical to the success of biological control to limit the use, when possible, of 

broad spectrum insecticides that not only kill mosquitoes, but also eliminate their natural 

predators. Control programs that combine biological controls with chemical insecticides 

often result in suboptimal results because the predators take longer to recover from

insecticide induced mortality than mosquito populations (Walton et al. 1990). 

Provide predator access to mosquitoes: The extent of wetland vegetation may limit 

predator access to mosquitoes (Walton and Mulla 1989, Collins and Resh 1989). Wetlands 

with significantly dense vegetation provide an abundance of hiding places for mosquitoes 

and can limit aquatic predator dispersion. Vegetation management BMPs can be used to 

reduce dense stands of cattail, tules, or other emergent vegetation and provide predators 

access to mosquito prey. Isolated basins that do not interconnect to the main water body of 

a wetland will also limit an aquatic predator’s access to mosquito prey. To encourage 

mosquito predation, wetland swales can be constructed to connect isolated basin with deep 

water areas containing aquatic mosquito predators. 

 

26 
Table 5. Biological Controls 

Best Management Practice 

Strategies Mosquito Control Objective Advantages D

Encourage or stock Mosquitofish (Gambusia affinis) 

Stock managed wetlands with mosquitofish or encourage habitats for naturalized populations. Utilize water sources with mosquitofish to passively transport predators tonewly flooded habitats.

To supplement mosquito predator population. 

Provides a non-chemical control of mosquito larvae. Mosquito fish are often available free of charge to landowners from their local district. 

Mpootaphase

Encourage invertebrate predators 

Maintain permanent or semipermanent water where mosquito predators can develop and bemaintained. Discourage use of broad spectrum pesticides. 

To reduce mosquito populations through predation. 

Provides biological control of mosquito larvae and adults. 

N

Encourage swallowcolonies and other insectivorous birds 

Do not discourage nesting swallows. 

To reduce mosquito populations through predation. 

Provides biological control of adult mosquitoes. 

Ginad

Encourage Bats Encourage bat colonies (e.g. 

build bat boxes) 

To reduce mosquito populations through predation. 

Provides biological control of adult mosquitoes. 

Pfovano
COORDINATION WITH MOSQUITO AND VECTOR CONTROL DISTRICTS

The responsibilities of MVCDs and wetland managers have some inherent conflicts. 

MVCDs have a responsibility to reduce vectors that may transmit disease to humans or

cause a significant nuisance. Wetland managers have a responsibility to maintain and 

enhance wetlands; a public trust resource that provides habitat for migratory birds, 

threatened and endangered species, and helps to reduce depredation from waterfowl on 

agricultural lands. Public resource agencies are increasingly being asked to provide 

wetland habitat to meet the recreational demands of hunters, anglers, bird watchers, 

photographers, and hikers. Wetland managers recognize that mosquito production may be a 

by-product of wetland management and in some cases may contribute to problems requiring 

mosquito control. However, in order for both MVCDs and wetland habitat managers to 

accomplish their objectives, cooperation and coordination is essential. 

MVCDs and wetland managers have a history of conflict going back over 30 years in 

the Central Valley (Lusk 1979). Such conflict was largely the result of MVCDs being 

perceived as relying solely on the use of chemical pesticides for control activities and 

concerns regarding the potential effects on wildlife and food-chain resources such as 

invertebrates. However, MVCD use of Integrated Pest Management (IPM) has helped to 

address this concern by employing a variety of control methods that include habitat 

management, biological control, and the use of more target-specific pesticide products. The 

increasing costs of pesticides and concerns over pesticide resistance have also spurred a 

more holistic approach to mosquito management. 

The success of IPM depends on cooperation and sharing information on habitat and 

water management schedules, collaborating on the identification of problem areas, 

monitoring the effectiveness of BMPs, and coordinating on wetland restoration and 

enhancement projects (Table 6). 

Coordinate habitat management and flooding schedules: By providing advanced 

information regarding habitat and water management schedules, wetland managers enable 

MVCDs to make suggestions regarding BMPs, schedule monitoring efforts, and, if 

necessary, control mosquitoes as efficiently as possible. In return, MVCDs can provide

landowners with useful input and feedback on how to reduce mosquito production and 

potentially reduce their abatement costs. 

Identify problem areas for mosquito production and target implementation of 

BMPs: Coordination is also required on the part of MVCDs to identify the highest priority 

wetlands for the implementation of BMPs. For example, Garcia and Des Rochers (1983) 

found that Ochlerotatus melanimon larvae were largely restricted to specific areas in a few 

fields on Gray Lodge Wildlife Area. As a result, limited resources could be focused on 

these problem areas.

 Coordinate wetland habitat restoration and enhancement project design: Prior 

to enhancement or restoration projects, MVCDs should be consulted to determine if design 

features to reduce mosquitoes can be incorporated. By involving MVCDs early in the 

process, problems associated with design features or poor engineering that may encourage 

mosquito production can be avoided. 

Coordinate Pest Control Activities: To facilitate the timely and effective use of

pesticides in wetland areas, MVCDs should inform wetland managers about proposed 

pesticide applications. This can be accomplished by providing a brief, annual plan that 

 28 
identifies the pesticide products, approximate use rates and application methods that may be 

used during the year. Whenever possible, MVCDs should also provide wetland managers 

with information about the specific locations where pesticide products will be used. An 

annual summary report of pesticide use should be provided to the wetland manager after the 

conclusion of the application season. 

Coordinate monitoring activities: Determining the success of BMPs will be largely 

based on the monitoring of mosquito production following their implementation. This effort 

will require wetland managers to work directly with MVCDs to facilitate monitoring and 

will also provide an opportunity for wetland managers to familiarize themselves with the 

methods used for monitoring and thresholds for treatment. The monitoring process will 

provide valuable information to refine BMPs and determine their future priority for use. 

Ultimately, this process will provide a unique opportunity to develop and use new 

techniques to reduce mosquito production in managed wetlands. 

 

 

 29 
Table 6. Suggested coordination activities between wetland managers and Mosquito and Vector Control D

Best Management Practice 

Strategies Mosquito Control Objective Advantages D

Habitat management and flooding schedule coordination 

Consult with MVCDs on agencysponsored habitat management 

plans on private lands (i.e. Presley Program). Consult with Districts on the timing of wetland flooding on public lands – urge private landowners to do the same. 

Allows MVCDs the opportunity to provide input on habitat management and recommend BMPs to reduce mosquitoes. 

Reduces potential conflicts between MVCDs, landowners, and agencies/NGOs whenmanaging or flooding wetlands. Provides information exchange. 

Rfragco

Identify problem areasfor mosquito production and target for implementation of BMPs 

Identify problem locations for mosquito production with local MVCDs and work to implement mosquito BMPs. Identify potential cost-share opportunities to implement BMPs. 

Work to reduce mosquito production through BMPs onproperties that are most problematic. 

Allows limited resources fromMVCDs and agencies/NGO’s to be targeted towards problem areas. Provides opportunities for monitoring the effectiveness of BMPs. 

N

Wetland HabitatRestoration andenhancement project design & coordination 

Consult with local MVCDs on the design of restoration and enhancement projects.

To determine where features to discourage mosquito productioncan be incorporated into wetland habitat restoration and enhancement projects where feasible. 

Reduces potential conflicts between MVCDs, landowners, and agencies/NGOs whenrestoring or enhancing wetlands. Provides a priori consultation for MVCDs on wetland projects.

RMagprin

Coordinate Pest Control Activities 

Work with local MVCDs tounderstand pesticides used formosquito treatment, and their costs and environmental impacts. 

To assure the use of mosquito control agents with the greatest efficacy and environmental safety. 

Reduces potential conflicts between districts, landowners,and agencies/NGOs regarding pesticides used for mosquito treatment. 

McoMag

Coordinate MonitoringActivities 

Facilitate monitoring mosquito populations of larval and adult stages before and after implementation of BMPs. 

Determine the effectiveness of BMPs to refine and prioritize their future use. 

Provides a means to evaluate and document effectiveness of BMPs. 

Rac
REFERENCES AND LITERATURE CITED 

Batzer, D. P. and V. H. Resh. 1992a. Recommendations for managing wetlands to 

concurrently achieve waterfowl habitat enhancement and mosquito control. 

Proceedings California Mosquito and Vector Control Association. 60:202-206. 

Batzer, D. P. and V. H. Resh. 1992b. Wetland management strategies that enhance 

waterfowl habitats can also control mosquitoes. Journal of the American Mosquito 

Control Association. 8(2):117-125. 

Brown, Dave. Personal communication. Manager, Sacramento-Yolo Mosquito and Vector 

Control District. Elk Grove, CA. 

Central Valley Habitat Joint Venture. 1990. CVHJV Implementation Plan. Sacramento, CA. 

 102 pp. 

Chappell, Steven. Personal communication. Executive Director, Suisun Resource 

Conservation District. Suisun, CA. 

Collins, J. N. and V. H. Resh. 1989. Guidelines for the ecological control of mosquitoes in 

non-tidal wetlands of the San Francisco Bay Area. CA Mosquito and Vector Control 

Association, Inc. and University of California Mosquito Research Program. 93 pp. 

Contra Costa Mosquito and Vector Control District. 2001. Guidelines for creating and 

maintaining waterfowl habitats. Contra Costa Mosquito and Vector Control District. 

Concord, CA. 4 pp. 

Ducks Unlimited, Inc. 1995a. Managing problem vegetation. Ducks Unlimited, Inc. Valley 

Habitats Number 7. Sacramento, CA. 8 pp. 

Ducks Unlimited, Inc. 1995b. Management of spring and summer brood water wetlands in 

the Central Valley. Ducks Unlimited, Inc. Valley Habitats Number 2. Sacramento, 

CA. 8 pp. 

Fanara, D. M., and M. S. Mulla. 1974. Population dynamics of larvae of Culex tarsalis 

(Coquillett) and Culiseta inornata (Williston) as related to flooding and temperatures 

of ponds. Mosquito News. 34(1):98-104. 

Fredrickson, L. H. and T. S. Taylor. 1982. Management of seasonally flooded 

impoundments for wildlife. U.S. Department of Interior, Fish and Wildlife Service. 

Resource Publication 148, Washington DC. 29 pp. 

Garcia, R. and B. Des Rochers. 1983. Towards an integrated mosquito control strategy for 

Gray Lodge Wildlife Refuge with emphasis on the floodwater species: Aedes 

melanimon and Ae. nigromaculus. Proceedings California Mosquito and Vector 

Control Association. 52:173-180. 

 31 
Garcia, R. and B. Des Rochers. 1984. Studies on the biology and ecology of mosquitoes in 

relation to the development of integrated control measures at Gray Lodge Wildlife 

Refuge, Butte County, California. University of California Mosquito Control 

Research Program, Davis, CA. Miscellaneous publication. 42 pp. 

Goddard, L. B., A. E. Roth, W. K. Reisen, and T. W. Scott. 2002. Vector competence of 

California mosquitoes for West Nile virus. Emerging Infection Diseases 8(12): 

1385-91. Erratum in: Emerg Infect Dis. 2003 Mar; 9(3):406. 

Lusk, E. E. 1979. Mosquito control problems on Wildlife Areas – A case history: Gray 

Lodge, Butte County, California. Proceedings California Mosquito and Vector 

Control Association. 47:69-70. 

Mortenson, E. W. 1963. Mosquito occurrence in a seasonally flooded waterfowl area, 

Merced County, California. Mosquito News. 23(2):89-96. 

Naylor, L. W., 2002. Evaluating moist-soil seed production and management in Central 

Valley wetlands to determine habitat needs for waterfowl. M. S. Thesis. 

University California Davis. Davis, CA. 22 pp. 

Resh, V. H. and E. B. Schlossberg. 1996. Large-scale, long-term control of mosquitoes and 

enhancement of waterfowl habitat at Grizzly Island Wildlife Area, Suisun Marsh. 

Mosquito Control Research Annual Report 1996. Univ. Calif. Division Agric. Nat. 

Resources. pp. 41-45. 

Sacramento National Wildlife Refuge Complex. 2000. Draft integrated pest management 

plan for mosquito abatement on the Sacramento National Wildlife Refuge Complex. 

Unpublished. 40 pp. 

Sacramento-Yolo Mosquito and Vector Control District. 1992. Wetland development and 

management guidelines for the control of mosquitoes. Sacramento-Yolo Mosquito 

and Vector Control District. Elk Grove, CA. 

Smith, W. D., Rollins, G. L., and R. L. Shinn. 1995. A guide to wetland habitat management 

in the Central Valley. CA Department of Fish and Game. Sacramento, CA. 34 pp. 

U.S. Fish and Wildlife Service. 1978. Concept plan for waterfowl wintering habitat 

preservation, Central Valley California. U.S. Fish and Wildlife Service. Portland, 

OR. 116 pp. 

Walton, W. E. and M. S. Mulla. 1989. The influence of vegetation and mosquitofish on 

Culex tarsalis abundance in duck club ponds in southern California. Proceedings 

California Mosquito and Vector Control Association. 57:114-121. 

Walton, W. E., Mulla, M. S., Wargo, M. J., and S. L. Durso. 1990. Efficacy of a microbial 

insecticide and larvivorous fish against Culex tarsalis in duck club ponds in southern 

California. Proceedings California Mosquito and Vector Control Association. 

 58:148-156.

 32 
Whittle, R. K., Linthicum, K. J., Thande, P. C., Kamau, C. M. and C. R. Roberts. 1993. 

Effect of controlled burning on the survival of floodwater Aedes eggs in Kenya. 

Journal of the American Mosquito Control Association. 9(1):72-77. 

 33 
Appendix A. Comparison of potential mosquito larvicides used in managed wetland 

environments. Information provided by the Pesticides Investigation Unit of the Department 

of Fish and Game. 

Pesticide Surface Oil Surface Film Methoprene Dimilin Bacillus

thuringiensis 

israelensis (Bti) 

Bacillus 

sphaericus (Bs)

Trade Name Golden 

Bear Oil 

Agnique MMF Altosid Dimilin 25W Vectobac 

Teknar 

Aquabac 

Vectolex 

Mode of

Action 

Suffocation Suffocation Growth

regulator 

Chitin inhibitor Stomach poison Stomach poison

Toxicological

Impacts

1

: 

Fish & Frogs 

Unknown Unknown Low Practically non

toxic 

Practically non 

toxic 

Practically non 

toxic 

Toxicological

Impacts

1

: 

Birds 

Low; at cold

temperatures 

may impact 

young 

waterfowl 

Unknown Very low Practically non 

toxic 

Practically non 

toxic 

Practically non 

toxic 

Potential Impacts

to invertebrate

forage base 

for wildlife2

Moderate to 

high; broad 

spectrum

compound 

Moderate 

to high; 

broad spectrum

compound 

Probably 

low; fairly 

narrow 

spectrum

Moderate 

to high; 

broad spectrum

compound 

Some species 

of midge larvae

subfamily 

Chironominae;

potentially 

other primitive 

dipteran larvae 

Probably 

low; fairly 

narrow 

spectrum

compound 

Comments Petroleum

distillate; 

applied over

surface of 

water; may be 

effective for 2-3

days

Reduces water 

surface tension

Juvenile 

hormone 

mimic; 

effective on 

later stage 

mosquitoes 

(instars 3-4) 

Naturally

occurring 

soil bacterium;

most effective 

on early stage

larvae (instars 

1-3);effective 

for only 1-2 

days; not 

effective in 

water with high

organic content

Can be effective

for weeks; used

primarily for 

Culex spp. in 

water with high

organic content;

not effective on 

all species of 

mosquito

1

Actual hazard is dependent on exposure, as well as toxicity. 2

Very difficult to assess. Dependent on numerous factors such as the total area treated, frequency of treatment,

and season of treatment. 

 34 
Appendix B. Comparison of potential mosquito adulticides used in managed wetland 

environments. Information provided by the Pesticides Investigation Unit of the Department 

of Fish and Game. 

Malathion Naled Pyrethrin Permethrin Resmethrin Sumithrin 

Trade Name Fyfanon Trumpet

Dibrom

Pyranone 

Pyrocide 

Aqua Reslin

Biomist 

Permanone 

Scourge Anvil

Mode of

Action 

Central nervous

system (CNS) 

inhibitor 

CNS inhibitor Contact Contact Contact Systemic

Toxicological

Impacts

1

: Fish 

and Frogs 

Variable; some

fish species are

highly 

susceptible, 

as are many 

amphibians 

Moderately to

highly toxic 

Highly toxic Highly toxic Highly toxic May be less 

toxic to some

aquatic species

than other 

pyrethrins 

Toxicological

Impacts

1

: Birds 

Moderately 

toxic 

Moderately to

highly toxic 

Moderately 

toxic 

Practically non 

toxic 

Practically non 

toxic 

Practically non 

toxic 

Potential Impacts

to invertebrate 

forage base 

for wildlife2

Moderate to 

highly toxic; 

broad spectrum

insecticide 

Moderate to 

highly toxic; 

broad spectrum

insecticide 

Moderate to 

highly toxic; 

broad spectrum

insecticide 

Moderate to 

highly toxic; 

broad spectrum

insecticide 

Moderate to 

highly toxic; 

broad spectrum

insecticide 

Moderate to 

highly toxic; 

broad spectrum

insecticide 

Comments Organophosphate 

insecticide 

Organophosphate

insecticide 

Botanical 

pesticide 

Synthetic 

pyrethrin 

Synthetic 

pyrethrin 

Synthetic 

pyrethrin 

1

Actual hazard is dependent on exposure, as well as toxicity. 2

Very difficult to assess. Dependent on numerous factors such as the total area treated, frequency of treatment,

and season of treatment. 

 35