Permanent Link: http://ufdc.ufl.edu/IR00002789/00001
 Material Information
Title: Source Reduction
Physical Description: Fact Sheet
Creator: Carlson, D.B.
Publisher: University of Florida Cooperative Extension Service, Institute of Food and Agriculture Sciences, EDIS
Place of Publication: Gainesville, Fla.
Publication Date: 1995
Acquisition: Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Melanie Mercer.
Publication Status: Published
General Note: "ENY-700-4-1 was revised July 1995; reviewed March 2000."
General Note: "ENY-700-4-1"
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Source Institution: University of Florida Institutional Repository
Holding Location: University of Florida
Rights Management: All rights reserved by the submitter.
System ID: IR00002789:00001

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ENY70041 Source Reduction 1 D.B. Carlson, L.J. Salmela and J.R. David 2 1. This document is ENY-700-4-1, part of the Mosquito Control Handbook, published cooperatively by the Department of Entomology and Nematology, University of Florida; the Florida Medical Entomology Laboratory, Vero Beach; the Florida Department of Health and Rehabilitative Services, Office of Entomology Services (HRS-ES), Jacksonville; the Florida Mosquito Control Association (FMCS); the St. Lucie County Mosquito Control District; the Indian River Mosquito Control District; the Pasco County Mosquito Control District; Polk County Environmental Services; and the USDA Medical and Veterinary Entomology Research Laboratory, Gainesville, Florida. ENY-700-4-1 was revised July 1995; reviewed March 2000. The electronic edition of the Mosquito Control Handbook is provided by the Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611. Please visit the EDIS Web site at http://edis.ifas.ufl.edu 2. D.B. Carlson, Biologist with the Indian River Mosquito Control District, Vero Beach, Fla., since 1978. Oversees the source reduction and larviciding programs. He has been a member of the Florida Coordination Council on Mosquito Control's Subcommittee on Managed Marshes since its formation in 1983, and its chariman since 1984; L.J. Salmela, Director of the Brevard Mosquito Control District, Titusville, Fla., from 1959 to 1986. Was a leader in planning and constructing mosquito control impoundments along the Indian River lagoon. Member of the Subcommittee on Managed Marshes from its formation to his retirement; and J.R. David, Assistant director of the Saint Lucie County Mosquito control district, Fort Pierce, Fla., since 1982. Administers their innonvative impoundment management program and is a member of the Management Ccommittee of the Natinal Estuary Program, administered jointly by local, state and federal agencies. The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean. Throughout Florida, there are many sites capable of producing large numbers of mosquitoes that must be controlled for the health and well-being of both residents and tourists. Some examples of these mosquito-producing sites include marshy coastlines, freshwater permanent lakes and ponds, low-lying pastures, freshwater marshes and man-made containers. Ideally, the best method of controlling mosquitoes is to eliminate mosquito-producing sites altogether. This is called source reduction. Source reduction, also referred to as permanent control, physical control or water management, virtually eliminates the need to larvicide (use chemicals to kill larva) and greatly decreases the need to adulticide (use chemicals to kill adult mosquitoes). Source reduction is an important part of a complete pest management program. Unfortunately, it is often underused. Mosquito-producing Habitats There are several major categories of mosquito-producing sites that should be considered for permanent control measures. Freshwater lakes, ponds and retention areas. Although it is possible to fill small ponds that produce mosquitoes, this is not feasible for large, permanent bodies of water or for areas set aside for retaining stormwater or wastewater. In these situations, other effective options include minimizing emergent vegetation (plants with roots in the bottom of the pond and tops that protrude from the surface, e.g., cattails) and maintaining steep banks. Culex and Anopheles mosquitoes are frequently produced in these habitats. Freshwater swamps and marshes. Large inland permanent marshes can produce several different mosquito species: Culex, Culiseta, Psorophora, Mansonia, Coquillettidia and Anopheles Source reduction is difficult in these areas because


Source Reduction 2 environmental laws greatly restrict what can be done to alter these environments. These changes are referred to as habitat manipulations. Temporarily flooded lands. Pastures and agricultural lands frequently generate huge numbers, or broods, of Aedes, Psorophora and Culex mosquitoes. An effective method of source reduction in these habitats is improved drainage by filling regrading or ditching. Containers. Containers such as flower pots, cans and tires are excellent habitats for several Aedes and Culex species. The container-breeding mosquito problem can be solved by properly disposing these containers or ensuring they do not collect water. Salt marshes. In Florida's not-so-distant past, extensive coastal salt marshes (flat land subject to flooding by salt water) produced enormous Aedes broods. This made coastal human habitation virtually impossible. Because of these problems, salt marshes have received considerable attention by mosquito control programs, often in the form of source reduction projects. To understand source reduction in salt marshes, it is important to know the different types of marshes. In Florida, daily tidal changes are small when compared with seasonal variations in tides. In other words, wind-generated water movements can be more important than lunar tides. The difference between low marshes and high marshes is that low marshes are flooded by daily tides and high marshes are flooded by large seasonal tides, strong wind tides or rainfall. In most locations, these tidal conditions produce more high marshes than low marshes. High marshes produce saltmarsh mosquitoes and, therefore, require physical control measures. Florida's salt marshes, which are inhabited by salt-tolerant vegetation, are broadly classified by Provost (1967) into three main vegetative types: Grass marshes are typically low marsh dominated by cordgrass ( Spartina alterniflora ) or black rush ( Juncus roemerianus ). Grass marshes in high marsh are usually vegetated by salt grass ( Distichlis spicata ) or salt marsh grass ( Spartina patens ) and can be prolific producers of Aedes sollicitans the eastern saltmarsh mosquito. In the Florida Panhandle, grass marshes dominate. Scrub marsh is typically high marsh dominated by saltwort ( Batis maritima ) and glasswort ( Salicornia spp.), but also includes mangrove. Scrub marsh dominates on the east coast from roughly St. Augustine to Palm Beach, and on the west coast from Tampa Bay to Naples. When highor low-marsh areas are dominated by mangrove trees, they are called mangrove swamps In low-marsh locations, red mangrove ( Rhizophora mangle ) dominates while in high-marsh locations, black mangrove ( Avicennia germinans ) dominates. Red mangroves, with their extensive prop-roots, protect the shoreline against erosion. Generally a berm forms behind this row of mangroves and restricts water movement between the marsh and estuary. Low-marsh and high-marsh vegetation differs from region to region. From Naples and Palm Beach southward, mangrove swamps dominate. Mangrove swamps in high-marsh locations most commonly produce Aedes taeniorhynchus The approximate northern limits of mangroves are shown in Figure 1 Figure 1 Uplands Adjacent to High Marshes Adjacent to high marshes are hardwood forests, or hammocks, consisting of oaks, palms and, in the low-lying areas, many tropical and subtropical plants. Palmettoes, Spanish bayonets, Cherokee beans, cocoplums, seagrapes and other plants dominate the drier, sandy high hammock areas that rarely produce mosquitoes. Along the area between the hammock and the high marsh is usually a fringe of white mangrove ( Laguncularia racemosa ) and buttonwood ( Conocarpus erectus ).


Source Reduction 3 Source Reduction for Saltmarsh Mosquito Control Historical Methods Several different source control efforts have greatly reduced, but not eliminated, saltmarsh mosquito production in these marshes. Deep ditching. Beginning in the late 1920s, high marshes were ditched by hand, with explosives or with heavy equipment, particularly draglines, to dewater the marsh within several days of flooding. This prevented sufficient time for adult mosquitoes to emerge and allowed larvae-eating fish access to larvae. This technique was largely unsuccessful: ditches were not always dug where they were needed most, many soon were obstructed, especially in the area between the ditch and the estuary, and fish were generally not present in sufficient numbers to provide appreciable control. Furthermore, the ditch banks made perfect breeding sites for biting sandflies ( Culicoides ), and this made the biting-insect problem even worse. Filling. In the past, mosquito-producing areas were sometimes filled as a method of source reduction. Because it was too slow and expensive, it was abandoned along Florida's east coast in the 1950s. Current environmental laws all but prevent wetland filling because it eliminates valuable wetland habitat. In some instances, filling has produced good sandfly breeding habitats. Fissures, or openings, that develop in the fill can also produce floodwater mosquitoes. Impounding. Mosquito control impoundments are high salt marshes that are surrounded by earthen dikes. Impoundments are generally flooded for mosquito control from May to October with water pumped from the estuary. Impounding and flooding eliminate oviposition sites for saltmarsh mosquitoes and have been shown to be both effective and economical in reducing their populations. However, some environmental problems have resulted from isolating and flooding these wetlands and have received considerable attention over the past decade. Years ago in a few locations, water was pumped out of impoundments as a source-reduction technique. It was unsuccessful because it was impossible to completely dewater the area before mosquitoes were produced. Modern Methods Environmental considerations. Before the 1970s, when the majority of the ditching, filling and impounding construction was completed, only mosquito control was considered. Little concern was given to environmental issues. Today, when source reduction projects are designed, close attention must be given to minimizing negative effects on the salt marshes. In 1986, the Subcommittee on Managed Marshes (SOMM) was formed (Florida Statutes Chapter 388.46), highlighting the importance of mosquito control as well as environmental issues. SOMM, a subcommittee of the Florida Coordinating Council on Mosquito Control, has developed guidelines for impoundment and mosquito-control ditching management plans. These guidelines state that management plans should incorporate the mutual objectives of mosquito control, fish and wildlife resources and water-quality enhancement. The most desirable environmental benefits appear to be those that attempt to recreate natural marsh functions while controlling mosquitoes. Reducing insecticide use is also an important goal. Management plan development. When developing the best management plan for a marsh, one must carefully consider the impoundment's biological, topographical and hydrological characteristics. The following are considered to be important management objectives: effective mosquito control minimizing the use of insecticides; optimal interchange of nutrients and organisms between the marsh and lagoon; effective water circulation within the marsh; water level controls that will enhance desirable marsh vegetation; and protection or enhancement of water quality both within and out side the marsh.


Source Reduction 4 The following is a list of information required to develop a plan that achieves the above-listed goals. Breeding history. Historical and current larval mosquito and (if available) sandfly production data. Topography. Topographical survey of representative marsh locations. Vegetation. A survey of major vegetation species. Water quality. Representative water-quality measurements (dissolved oxygen and salinity, or salt content). Water-control structures. Determination of best location and number of water-control structures (e.g., pump stations, culverts, ditch sills). Determination should consider acreage, marsh surface elevations and tidal exchange data (representative data are available at the St. Lucie County Mosquito Control District for 30 inch diameter culvert exchanges under various scenarios). Management schedule. For impoundments, the culvert opening and closing schedule attempts to minimize the closed period. Closure times are based on historical mosquito and sandfly production patterns, rainfall and expected tidal inundations. Flooding elevation should be minimized to protect vegetation while preventing the vast majority of mosquito production. Rotary ditching Ditching can be used in both salt marsh or freshwater locations to improve water management generally by enhancing drainage, which eliminates mosquito-producing sites. Candidates for rotary ditching include marshes, dredge spoils, temporary grassy ponds and savannas when vegetated with grasses and small scrubs if they are not rocky. Ditch cleaning or new construction is possible in areas of woody vegetation if planned carefully. Experience has shown that poorly engineered ditches can produce more mosquitoes than the area did before construction. In Florida the construction of shallow ditches, using high-speed rotary equipment that scatters spoil evenly over the marsh surface, is used in two management techniques: 1. in impounded marshes connected to the estuary by breaches or culverts; and, 2. in unimpounded marshes using Open Marsh Water Management (OMWM) techniques. A ditching network frequently connects shallow ditches to permanent water bodies (ponds or canals). Where it is impossible or impractical to connect to major waterways, it is possible to dig a pond deep enough to hold water throughout the year. Radial ditches are then connected to mosquito-producing locations. The pond should be stocked with larvae-eating fish that can provide some control when rainfall allows these fish access to mosquito-breeding sites. Over the past 15 years OMWM, using ponds, rotary ditches and ditch sills, has been implemented in many coastal locations throughout the United States (see references). Research in Florida on the total ecosystem effects of rotary ditching are incomplete. However, management experience indicates that rotary ditching can reduce the need for larviciding, while increasing the hydrological connection between a marsh and estuary. Rotary ditching is generally considered more environmentally acceptable than deep ditching because spoil material from these shallow ditches is evenly distributed over the marsh surface. This eliminates the problem of the marsh accumulating spoil material and the resulting invasion of exotic vegetation. As with any environmental manipulation, there are advantages and disadvantages to rotary ditching. A study is currently under way in Charlotte County to determine ecosystem effects of this source-reduction technique. (For the latest results, contact Robert Ward, Charlotte County MCD.) Advantages of Rotary Ditching Time-tested equipment. Rotary ditchers are connected to suitable tractor/carriers. They have been used for years in agriculture and therefore are time-tested. The tractors are usually modifications of


Source Reduction 5 existing equipment. The equipment can be mounted on carriers that have lighter ground-bearing pressures than standard heavy excavation equipment. When mounted on amphibious carriers, the total package can be much smaller and less encumbered. Efficiency. Rotary ditchers are more efficient than conventional methods; they cut, clean and shape in one continuous action and therefore can handle all types of soil and small roots. Trapezoidal-shaped ditches 2 feet deep and 3 feet wide at the top, can be cut at speeds up to 5,000 feet per hour ( Figure 2 ). Rotary ditchers work well but are slower in hard-packed dry clay and when small stones are present. With a narrow tractor width, rotary ditchers can operate in wooded habitats and maneuver around trees. Figure 2 Broad spoil dispersal. Excavated spoil is finely scattered over a broad, continuous area adjacent to the ditch, resulting in spoil less than 3 inches deep. This even distribution avoids the problems associated with spoil pile and ditch berms created with conventional excavators. That is, rotary ditching does not create raised areas that have two negative affects: eliminating marsh habitat and providing habitat for undesirable vegetation, such as Brazilian pepper ( Schinur terebinthifolius ) and punk trees ( Melaleuca quinquenervia ). The raised areas may also affect drainage patterns. Vegetation impact. Impact to vegetation is usually limited to the ditch itself, as the tractor will climb over most vegetation, allowing it to spring back, causing little damage. Marsh ditching seems to affect vegetation as only a top-dressing of soil might affect a lawn. Because rotary ditchers distribute material evenly and do not form spoil piles, they should receive serious consideration for any mosquito-control ditch construction. Environmental regulatory agencies generally look favorably on rotary ditching of impoundments if it allows keeping an impoundment open permanently for a greater portion of the year. Disadvantages of Rotary Ditching Indiscriminate spoil placement. Rotary ditchers can throw debris up to several hundred feet from the edge of the created ditch, depending on soil characteristics. Therefore, care is necessary when working in congested areas. Deflectors, which can help control the dispersal of excavated spoil, are standard on all machines. Ditch stability. In loose soils, the size and shape of the finished ditch cannot be maintained due to erosion. The width of the ditch is fixed by the depth. Therefore, a shallow ditch must also be narrow. Pond construction limitations. Rotary equipment can construct small ponds, but is limited as to depth and shape. It generally cannot dig below 4 feet on a single pass. Gearing considerations. The tractor/carrier requires a hydrostatic drive that matches ground speed to the cutter while still supplying maximum power to the ditcher. Hydrostatic systems are expensive. Although very low-geared tractors can be used, they will never be as efficient as hydrostatically equipped units. In data sheets, rotary ditcher manufacturers are typically conservative regarding their equipment's true horsepower requirements. Because the unit will generally be operated at maximum speed, an overrated power unit should be specified. Waterproofing equipment. Unlike most excavation equipment, rotary ditchers often have high-precision parts running under water. Therefore, waterproofing the gearbox may be a problem. Parts and service for rotary ditchers can be difficult to obtain, especially for foreign-made equipment. Safety Precautions Because rotary ditchers can throw spoil several hundred feet to either side, the only safe locations are directly in front of and behind it. The operator's station should have a steel mesh shield and clear safety panels (e.g., Lexan, acrylic). Deflection shields are installed for protection and should be kept intact and


Source Reduction 6 securely fastened. Additional mud flaps may be needed and can be fabricated from rubber conveyor belting or truck flaps. If the ditcher is hydraulic, the bypass valves must be set within the motor manufacturer's limits. On PTO-driven equipment, the slip-clutch, a safety device, must be checked to ensure that it is not stuck. Equipment maintenance The rotary ditcher should be equipped with swamp type seals. Some agricultural units may not have this outboard seal on the rotor shaft, thus allowing mud and water into the outboard bearings. Because water and grit cause excessive wear on bearings or gearsets, the gearbox oil level and condition must be checked daily. On some models, the rotor shaft bearings are separate from the gearbox and should be greased several times daily. The universal joints of PTO-driven units should be greased daily. Bolts holding the cutting edges and main rotor can loosen and should be checked frequently. For more information about rotary ditching and ditching equipment, contact William Opp (DHRS), Jonas Stewart or Paul Haydt (East Volusia MCD), Carla Wright (Pinellas County MCD), James Robinson (Pasco MCD), Jim Hunt (Brevard MCD), Oscar Fultz (Chatham County MC Commission, Ga.). Impounding Principles The principle of impounding, which has been used extensively along the central east coast, is simple: keep mosquito-producing high marsh covered with a minimum of several inches of water to prevent Aedes from ovipositing. Due to the contour of the marsh surface, the flooding depth can vary considerably. Before the 1970s, mosquito control considerations far outweighed natural resource interests. In the 1950s and '60s, when impoundment construction occurred, little was known about the importance of high marshes to estuarine productivity. Historically, black mangroves, Batis and Salicornia dominated many high marshes that were impounded. These plants cannot sustain continual excessive flooding to the point where the succulent plants or black mangrove pneumatophores are inundated. During those early years of impounding, water levels were maintained at depths that in some locations killed virtually all vegetation. This left some impoundments barren except for areas where red mangroves intruded. Research. The total effects of impounding are not yet clearly understood. However, research on high marshes and impoundments since the 1950s has provided considerable information on which to base management decisions. The vast majority of the research investigating the effects of different impoundment water management programs has been partly funded by the Florida Department of Environmental Regulations' Office of Coastal Management and the Florida Department of Health and Rehabilitative Services' Office of Entomology Services. Estuarine productivity. In many locations low marshes, with their nearly daily tidal flushings, provide more significant, continual nutrient discharges to the estuary than do high marshes. However, in locations where high-marsh acreage far exceeds low marsh, as along the Indian River lagoon, the contribution of high marsh to estuarine productivity may be very important, even though historically these high marshes were essentially dry for nine to 10 months of the year. Provost (1967, p. 169) writes: If it is mainly low marsh that nourishes estuaries, (these) impoundments may not be starving the estuary significantly. On the other hand, the absence of inflowing streams to nourish the estuary may render what nourishment can come from these marshes disproportionately important. High-marsh organisms. Organisms using the high marsh can be classified into two basic groups: Marsh residents: those that can spend their entire life within the marsh, e.g., sheepshead minnow ( Cyprinodon variegatus ), mosquitofish ( Gambusia holbrokii = affinis ) and sailfin molly ( Poecilia latipinna ). Marsh transients: those that use the high marsh during a portion of their life cycle, e.g., ladyfish ( Elops saurus ), snook ( Centropomus undecimalis


Source Reduction 7 ) and tarpon ( Megalops atlanticus ). Marsh transients typically enter the high marsh only during the fall high-tide period, using the marsh for food resources and as a refuge from predators during this several-month, tidal-flooding period. Research has demonstrated that marsh residents can prosper within impoundments and serve as a potential food source for wading birds. However, impoundment dikes can interfere with high-marsh access for transient organisms. Harrington and Harrington (1961, 1982) demonstrated a decrease in the number of fish species using a marsh after impoundment, along with an accompanying change in the diet of those remaining from primarily omnivorous to detritivorous. In some instances, impounding changed heavily vegetated areas, which were previously unattractive to waterfowl, to marshes commonly used by ducks. However, until the early 1980s, little was known of the effects of various impoundment-management methods on the marsh. Rotational Impoundment Management Based on research findings, a rotational impoundment management (RIM) technique is currently the most favorable impoundment management technique. RIM is accomplished by installing culverts with flapgate risers through impoundment dikes to seasonally reconnect the impounded marsh with the estuary. Culverts reconnect the impoundment with the estuary by serving as doorways for nutrient and organism movement. Intensive sampling has shown that fish use these culverts as entrances and exits to the impounded marsh. Culverts are strategically distributed around the dike. Favorable locations are historic, natural tidal creeks or locations where flushing will be optimized. Culvert inverts are generally set at approximately -1.0 feet NGVD (National Geodetic Vertical Datum). Culverts are left open during the fall and winter, then closed in the late spring for the mosquito-breeding season. Spillway height is set by one of several flapgate risers designed at the minimum height necessary for mosquito control, thus allowing water levels exceeding that to spill out into the estuary. During the closure period, approximately early May through September, the impounded marsh is flooded by pumping water as needed from the adjacent estuary using either portable diesel-driven or stationary electric pumps. In early fall pumping ceases, and the culverts are opened to allow the fall high tides to flood the marsh. RIM accomplishments controls saltmarsh mosquito production from the marsh with a minimum of insecticide use; promotes revegetation by maintaining sufficiently low water levels during the summer flooding period; and allows marine life to use the previously unavailable impounded high marsh. Numerous ecosystem effects of RIM are currently under investigation. Possibly in the future, management variations such as subcells to minimize stress to impounded organisms, water-level manipulations to benefit waterfowl and endangered species, or aquaculture projects such as marine-stock enhancement can be implemented in impoundments while still maintaining their prime mosquito-control source-reduction purpose. Operating permit When an agency or developer wants to make structural changes to an impoundment, permitting agencies usually require long-term impoundment management improvements frequently in the form of RIM. After these structural improvements are made, the Florida Department of Environmental Regulation requires that the local mosquito control agency assume an operating permit that ensures the impoundment is managed in an environmentally acceptable manner as determined by the SOMM. Careful consideration of water quality is important because many of the transient marine organisms entering impoundments cannot tolerate water-quality extremes. To ensure that impoundment water quality is suitable, DER requires that various water quality parameters (e.g., salinity, dissolved oxygen, turbidity, or muddiness, and temperature) be monitored both in the impoundment and adjacent


Source Reduction 8 estuary. If impoundment water quality is unsatisfactory, appropriate management actions, such as enhanced water exchange by flushing or overpumping, may be necessary. During culvert opening in the fall, care to minimize adverse effects to the estuary is essential. Research and management experience will dictate how these guidelines will change over time. Current limits for the various parameters are: salinity must not exceed 40 parts per thousand (ppt); turbidity must not exceed 29 nephelometric turbidity units (NTU) above ambient; dissolved oxygen should not fall below 2 to 3 parts per million (ppm); and temperature should not exceed 36 C (97 F). Sampling sites are determined by the mosquito control agency and DER. All measurements must be taken in the morning, when dissolved oxygen is generally lowest. Operating permit reports are submitted regularly to DER. Impoundment Structures Environmental permitting. Maintenance of existing impoundment structures to their original design specifications does not require permit approval. However, the construction of a new dike or the installation of new culverts requires approval by several environmental regulatory agencies (e.g., DER, Army Corps of Engineers, water management district or local county). All such work that disturbs estuarine or impoundment sediments can cause turbidity to rise, which may damage fragile marine life. Therefore, such construction requires the use of silt screens to minimize turbidity problems. Dikes. Several major points that were important when impoundment dikes were constructed should also be considered during maintenance or construction of new ones: Size. The dike must be tall enough to prevent normal seasonal high tides from overwashing it. It must be wide enough to allow access passage of maintenance vehicles and equipment and must be sturdily built to minimize wave-action erosion. Construction. Fill must be taken from within the impoundment. When the dike course has been laid out, all vegetation, dead or alive, must be removed; any remaining plant material will weaken the dike. The dike base center consists of a trench, usually one dragline bucket wide and one deep. This core is filled with well-packed, sturdy material. When possible, leave a narrow beach on the estuary side. This can often be accomplished by placing the dike on the natural berm. When vegetated, this beach prevents dike erosion. Riprap placed along the beach can help trap mangrove seedlings, further stabilizing the dike. For the dike top, marl is recommended. Culverts. Culverts are generally installed with a backhoe or dragline. Because of saltwater's high corrosiveness, corrugated aluminum culverts (typically 14 gauge) are preferred. If more than one culvert is necessary in a dike section to ensure proper compaction, each must be installed separately, rather than digging one hole and placing several culverts in it side by side. Culvert diameter depends on the water-volume exchange desired, although the larger the culvert both in diameter and length, the more difficult the installation. Experience has shown that 30 inch diameter culverts provide the necessary biological benefits and are relatively easy to install. Plastic culverts now available on the market may prove acceptable but are limited in size. Flapgates. The following information about the advantages and disadvantages of culvert flapgate designs currently in use was provided by James R. David, Saint Lucie County Mosquito Control District. Standard riser. ( Figure 3 ) Although the standard riser is simple to operate, it is less versatile than flapgated designs. It is quite expensive to construct (approximately $500 per structure), but is commercially available. Simple flapgate. ( Figure 4 )The simple flapgate is easy and inexpensive to build (approximately $50), but its lack of automatic water release is a


Source Reduction 9 drawback. It can be used in combination with other flapgate types to enhance circulation. Figure 3 Figure 4 Flapgate riser. ( Figure 5 ) The flapgate riser is highly sensitive to water level variations and is easily adjustable by manipulating the riser boards. Although relatively expensive (approximately $800 to $1,000), it is commercially available. Figure 5 Gator Gate. ( Figure 6 ) An all-aluminum structure fabricated by Gator Culvert is easy to open and close and fits into typical risers. If lagoon water levels exceed impoundment flooding level, water will enter impoundment while high impoundment water levels can spill out over top aluminum plate. Approximate cost: $925. Figure 6 Modified tidegate weir. ( Figure 7 ) The modified tidegate weir (MTGW) is inexpensive (approximately $130), versatile (allows excess water to release automatically) and extremely sensitive to water-pressure gradients. Extra weights are necessary to reduce buoyancy during the closed period. This gate can be constructed of 3/16-inch T6 aluminum for roughly double the wooden structure's cost. St. Lucie County Mosquito Control District inverts this structure, allowing continuous drawdown of impoundment bottom water. Figure 7 Weir. A weir is a wide, low-level dam typically constructed so the height of the weir opening can be regulated by placing riser boards in it. Weirs rather than culverts are used in Louisiana and elsewhere and have been suggested for use in Florida impoundments. None have been tried to date. Proponents suggest that because of their large surface area, weirs allow far greater water flow (hence nutrient and organism transport) than culverts during periods of high water or when totally open. However, there are serious disadvantages to weirs that have been documented. These include: Vehicle transport. After construction, weirs obstruct vehicle transport unless expensive bridges are built across them.


Source Reduction 10 Erosion. Weirs frequently leak, resulting in erosion that is difficult and expensive to correct. Water exchange during high tides. Because of their large surface area, weirs allow large water volumes to flow across their riser boards during extreme tides in closed periods. However, an advantage of flapgated culverts is they can open if estuarine water levels exceed those in the impoundment. Also, multiple culverts can provide the horizontal distance that weirs provide. Expense. Weirs are more expensive to install than culverts. Summary Since the early 1970s, we have become aware of the environmental importance of estuaries and their role in high marshes. These ecosystems support many forms of living organisms. They have their own special flora and fauna, they are breeding and nursery grounds for many kinds of marine organisms, and they can be prolific producers of saltmarsh mosquitoes. The balance of nature is always delicate, but perhaps nowhere more so than in estuaries, where the nutrient-rich brackish waters support large food chains, from microorganisms to fish, birds and mammals. Man's intrusion into estuaries has created disturbances in many ways, all of which to some extent negatively affect the estuarine balance. Mosquito-control source-reduction activities are but one of many such disturbances. Over the past decade, we have learned through research and experience that improved salt marsh management for natural resource enhancement is possible while still maintaining the benefits of mosquito-control source reduction. RIM and OMWM using rotary ditches are valuable tools in managing Florida's sensitive salt marsh wetlands in environmentally acceptable ways while still controlling saltmarsh mosquito populations with a minimum of pesticide use. Further fine tuning may be necessary, but we have made major accomplishments in the multipurpose management of our saltmarsh resources. Awards for Excellence Over the past several years, two mosquito-control source-reduction programs have received high distinction from agencies responsible for wetlands resources. U.S. Fish and Wildlife Service Conservation Service Award to Jack Salmela. Under the directorship of Leon Jack Salmela, Brevard County undertook the state's largest source reduction program. Jack's care, perseverance and success in managing these marshes for both mosquito control and wildlife resources was highlighted in 1986, when he received the U.S. Department of Interior, Fish and Wildlife Service's Conservation Service Award. As described by the Fish and Wildlife Service: This is the highest honor bestowed by the secretary to private citizens and groups for direct contributions to the mission and goals of the Department. It was presented to Mr. Salmela for his endless contributions to wildlife conservation through effective mosquito-control techniques and his personal dedication to effective management of wildlife resources. Jack was further honored for his conservation efforts in 1987 when the Florida Mosquito Control Association presented him with the first Maurice W. Provost Award, an award for developing management and mosquito-control methods that reduce pesticides application while minimizing habitat alterations. DER Secretary's Environmental Award to the St. Lucie County Mosquito Control District. In January 1990, the St. Lucie County Mosquito Control District (Frank Evans, director, Jim David, assistant director) received the Florida Department of Environmental Regulations Secretary's Environmental Award for its innovative impoundment management program. This DER award is for wetland enhancement and management which has significantly contributed to protection, conservation or restoration of the air, water or natural resources of the state.


Source Reduction 11 References Balling, S.S., T. Stoehr and V.H. Resh. 1980. The effects of mosquito control recirculation ditches on the fish community of a San Francisco salt marsh. California Fish and Game 66:25-34. Balling, S.S. and V.H. Resh. 1982. Arthropod community response to mosquito control recirculation ditches in San Francisco Bay salt marshes. Environmental Entomology 11:801-808. Balling, S.S. and V.H. Resh. 1983. The influence of mosquito control recirculation ditches on plant biomass, production and composition in two San Francisco Bay salt marshes. Estuarine, Coastal and Shelf Science 16:151-161. Balling, S.S. and V.H. Resh. 1983. Mosquito control and salt marsh management: factors influencing the presence of Aedes larvae. Mosquito News 43:212-218. Bidlingmayer, W.L. and E.D. McCoy. 1978. An inventory of the salt marsh mosquito control impoundments in Florida. Unpublished report to Fish and Wildlife Service, U.S. Department of Interior. 103 pp. Bidlingmayer, W.L. 1982. Survey of saltmarsh mosquito control impoundments. Journal of the Florida Anti-Mosquito Association 53:4-7. Carlson, D.B. 1983. The use of salt marsh mosquito control impoundments as wastewater retention areas. Mosquito News 43:1-6. Carlson, D.B. and J.D. Carroli Jr. 1985. Developing and implementing impoundment management methods benefiting mosquito control, fish and wildlife: a two year progress report about the Technical Subcommittee on Mosquito Impoundments. Journal of the Florida Anti-Mosquito Association 56:24-32. Carlson, D.B., R.G. Gilmore and J.R. Rey. 1985. Salt marsh impoundment management on Florida's central east coast: reintegrating isolated high marshes to the estuary. In: Proceedings of the 12th Annual Conference on Wetlands Restoration and Creation, pp. 47-63. Carlson, D.B. and R.R. Vigliano. 1985. The effects of two different water management regimes on flooding and mosquito production in a salt marsh impoundment. Journal of the American Mosquito Control Association 1:203-211. Carlson, D.B. 1987. Salt marsh impoundment management along Florida's Indian River Lagoon: historical perspectives and current implementation trends. Proceedings of a Symposium on Waterfowl and Wetlands Management in the Coastal Zone of the Atlantic Flyway, August 1986, pp. 358-371. Carlson, D.B. and R.L. Knight. 1987. Mosquito production and hydrological capacity of southeast Florida impoundments used for wastewater retention. Journal of the American Mosquito Control Association 3:74-83. Carlson, D.B. and P.D. O'Bryan. 1988. Mosquito production in a rotationally managed impoundment compared to other management techniques. Journal of the American Mosquito Control Association 4:146-151. Carlson, D.B., P.D. O'Bryan and J.R. Key. 1991. A review of current salt marsh management issues in Florida. J. American Mosquito Control Association 7:83-88. Carlson, P.R., O.A. Jarbro, C.F. Zimmerman & J. R. Montgomery. 1983. Pore water chemistry of an overwash mangrove island. Florida Scientist 46:239-249. Clements, B.W. and A.J. Rogers. 1964. Studies of impounding for the control of salt marsh mosquitoes in Florida, 19581963. Mosquito News 24:265-276. Chapman, H.C. and F. Ferrigno 1956. A three-year study of mosquito breeding in natural and impounded salt marsh area in New Jersey. Proceedings of the New Jersey Mosquito Extermination Association 43:48-64. Ferrigno, F. and D.M. Jobbins. 1968. Open marsh water management. Proceedings of the New Jersey Mosquito Extermination Association 55:104-115. Ferrigno, F., D.M. Jobbins and E. Ruben. 1964. Practical aspects of mosquito-wildlife investigations


Source Reduction 12 on Cumberland County marshes. Proceedings of the New Jersey Mosquito Extermination Association 51:74-84. Ferrigno, F., L.G. MacNamara & D.M. Jobbins. 1969. Ecological approach for improved management of coastal meadowlands. Proceedings of the New Jersey Mosquito Extermination Association 56:188-202. Ferrigno, F., F. Slavin and D.M. Jobbins. 1974. Salt marsh water management for mosquito control. Proceedings of the New Jersey Mosquito Extermination Association 62:30-38. Frye, O.E., Jr. 1970. Fish and wildlife benefits in mosquito impoundments. Proceedings of the Southeast Conference on Mosquito Suppression and Wildlife Management. pp. 16-18. Gilmore, R.G., D.W. Cooke and C.J. Donohoe. 1982. A comparison of the fish populations and habitat in open and closed salt marsh impoundments in east-central Florida. Northeast Gulf Science 5:25-37. Gilmore, R.G. 1987. Fish, macrocrustacean, and avian population dynamics and cohabitation in tidally influenced impounded subtropical wetlands. Proceedings of a Symposium on Waterfowl and Wetlands Management in the Coastal Zone of the Atlantic Flyway, August 1986. pp. 372-394. Gosselink, J.D., E.P. Odum and R.M. Pope. 1974. The value of the tidal marsh. Publ. No. LSU-SG-74-03. Center for Wetland Resources, LSU. Haeger, J.S. 1960. Behavior preceding migration in the saltmarsh mosquito Aedes taeniorhynchus (Wiedemann). Mosquito News 20:136-147. Harrington, R.W., Jr. and E.S. Harrington. 1961. Food selection among fishes invading a high subtropical salt marsh: From onset of flooding through the progress of a mosquito brood. Ecology 42:646-666. Harrington, R.W., Jr. and E.S. Harrington. 1982. Effects on fishes and their forage organisms of impounding a Florida salt marsh to prevent breeding by saltmarsh mosquitoes. Bulletin of Marine Science 32:523-531. Heitzman, B. 1978. Management of salt marsh impoundment for waterfowl in North Carolina. N.C. Wildlife Research Commission, 1978, 47 pp. Herke, W.H. 1968. Weirs, potholes and fishery management. In: J.D. Newsom, ed., Proceedings of the Marsh and Estuary Management Symposium. Thos. J. Moran and Sons, Inc., Baton Rouge, LA, pp. 193211. Herke, W.H. 1969. An unusual inland collection of larval ladyfish, Elops saurus in Louisiana. Proceedings of the Louisiana Academy of Sciences 32:29-30. Herke, W.H., E.E. Knudsen, B.D. Rogers, and P.A. Knudsen. 1987. Effects of semi-impoundment on estuarine-dependent fisheries. Proceedings of a Symposium on Waterfowl and Wetlands Management in the Coastal Zone of the Atlantic Flyway, August 1986, pp. 403423. Kahl, M.P., Jr. 1964. Food ecology of the wood stork ( Mycteria americana ) in Florida. Ecological Monographs 34: 97-117. Lake, R.W. and F.T. Murphy. 1970. Highand low-level impoundments for mosquito control in Delaware. Proceedings of the Southeast Conference on Mosquito Suppression and Wildlife Management pp. 10-13. Leenhouts, W.P. and J.L. Baker. 1982. Vegetation dynamics in dusky seaside sparrow habitat on Merritt Island National Wildlife Refuge. Wildlife Society Bulletin 10:127-132. Leenhouts, S.P. 1985. Soil and vegetation dynamics in a rotary-ditched mosquito control impoundment on the Merritt Island National Wildife Refuge. In: Proceedings of the 12th Annual Conference on Wetlands Restoration and Creation, pp. 181-192. Lesser, C.R. and D. Saveikis. 1979. A study of the impacts of a mosquito control integrated pest management program on selected parameters of the ecology of Chesapeake Bay high marsh communities


Source Reduction 13 in Maryland. Unpublished EPA grant report (X00314701), 193 p. Lesser, F. and J.K. Shisler. 1979. Historical development of OMWM in New Jersey. Equipment and Technique. Proceedings of the Utah Mosquito Abatement Association 25:40-44. Linley, J.R., F.D.S. Evans and H.T. Evans. 1970. Seasonal emergence of Culicoides furens (Diptera: Ceratopogonidae) at Vero Beach, Florida. Annals of the Entomological Society of America 63:1332-1339. Linley, J.R. and J.B. Davies. 1971. Sandflies and tourism in Florida and the Bahamas and the Caribbean area. Journal of Economic Entomology 64:264-278. McCoy, E.D. 1979. Ecological control of mosquitoes on Florida's east coast: an overview. Proceedings of the Florida Anti-mosquito Association 50:20-23. Meredith, W.J., D.E. Saveikis and C. J. Stachecki. 1983. Studies on the environmental effects of Open Marsh Water Management, a saltmarsh mosquito control technique. Estuaries 6:270. Meredith, W.J., D.E. Saveikis and C.J. Stachecki. 1985. Guidelines for Open Marsh Water Management in Delaware's salt marshes objectives, system designs, and installation procedures. Wetlands 5:119-133. Meredith, W.J. and D.E. Saveikis. 1987. Effects of Open Marsh Water Management (OMWM) on bird populations of a Delaware tidal marsh, and OMWM's use in water bird habitat restoration and enhancement. Proceedings of a Symposium on Waterfowl and Wetlands Management in the Coastal Zone of the Atlantic Flyway, August 1986, pp. 298-321. Murdoch, C. 1986. Mosquito control project in Upper Tampa Bay Park. In Proceedings of the 13th Annual Conference on Wetlands Restoration and Creation. Nielsen, E.T. and A.T. Nielsen. 1953. Field observations on the habits of Aedes taeniorhynchus Ecology 34: 141-156. O'Bryan, P.D., D.B. Carlson and R.G. Gilmore. 1990. Salt marsh mitigation: an example of the process of balancing mosquito control, natural resource and development interests. Florida Scientist 3:189-203. Odum, W.E., C.C. McIvor and T.J. Smith III. 1982. The ecology of the mangroves of south Florida: a community profile. U.S. Fish and Wildlife Service, Office of Biological Services, Washington, D.C. FWS/OBS-81-24, 144 p. Provost, M.W. 1952. The dispersal of Aedes taeniorhynchus I. Preliminary studies. Mosquito News 12:174-179. Provost, M.W. 1958. Facts about the salt marsh and its mosquitoes. Unpublished Fla. State Board of Health handout. 12 pp. Provost, M.W. 1959. Impounding salt marshes for mosquito control and its effects on bird life. Florida Naturalist. 34 (4). Provost, M.W. 1967. Managing impounded salt marsh for mosquito control and estuarine resource conservation. LSU Marsh and Estuary Symposium. pp. 163-171. Provost, M.W. 1969. Ecological control of saltmarsh mosquitoes with side benefits to birds. Proceedings of the Tall Timbers Conference on Ecological Animal Control by Habitat Management (1973), pp. 193-206. Provost, M.W. 1972. Environmental hazards in the control of disease vectors. Environmental Entomology 1:333-339. Provost, M.W. 1973. Mean high water mark and use of tidelands in Florida. Florida Scientist 36:50-66. Provost, M.W. 1974. Salt marsh management in Florida. Proceedings of the Tall Timbers Conference on Ecological Animal Control by Habitat Management (1973). pp. 5-17. Provost, M.W. 1976. Tidal datum planes circumscribing salt marshes. Bulletin of Marine Science 26:558-563.


Source Reduction 14 Provost, M.W. 1977. Source reduction in saltmarsh mosquito control: Past and future. Mosquito News 37:689-698. Resh, V.H. and S.S. Balling. 1983. Tidal circulation alteration for saltmarsh mosquito control. Environmental Management 7:79-84. Rey, J.R. and R.A. Crossman. 1987. Physical and biological studies of impounded marshes along the Indian River Lagoon in east-central Florida. Proceedings of a Symposium on Waterfowl and Wetlands Management in the Coastal Zone of the Atlantic Flyway, August 1986, pp. 395-402. Rey, J.R., R.A. Crossman, T.R. Kain and D.S. Taylor. 1985. An overview of impounded mangrove forests along a subtropical lagoon in east-central Florida, U.S.A. Proceedings of the National Symposium of Biological Utilization and Conservation of Mangroves, pp. 341-350. Rey, J.R., R.A. Crossman, T.R. Kain, F.E. Vose and M.S. Peterson. 1987. Sampling zooplankton in shallow marsh and estuarine habitats: gear description and field tests. Estuaries 10:61-67. Rey, J.R. and T.R. Kain. 1989. A guide to the salt marsh impoundments of Florida. 447 pp. Rey, J.R., J. Shaffer, D. Tremain, R. Crossman and T. Kain. 1990. Effects of reestablishing tidal connections in two impounded subtropical marshes on fishes and physical conditions. Wetlands 10:2745. Savage, T. 1972. Florida mangroves as shoreline stabilizers. Florida Dept. of Natural Resources Professional Paper 19, 46 pages. Schooley, J.K. 1980. The structure and function of warm temperate estuarine fish communities. Ph.D. dissertation, University of Florida. Shisler, J.K, 1979. The effect of water management on coastal productivity in New Jersey. Proceedings of the Florida Anti-mosquito Association 50:37-39. Shisler, J.K. 1979. Early succession in a low-level salt marsh impoundment. Proceedings of the Florida Anti-Mosquito Association 50:4043. Shisler, J.K., F.H. Lesser and T. Candeletti. 1979. An approach to the evaluation of temporary vs. permanent measures in saltmarsh mosquito control operations. Mosquito News 39:776-780. Smith, N. 1980. A comparison of tidal harmonic constants computed at and near an inlet. Estuarine and Coastal Marine Sciences 10:383-391. Subcommittee on Managed Marshes. 1989. Information for permit applicants entering the impoundment management plan submittal process. 13 pp. Subcommittee on Managed Marshes. 1989. Guidelines for salt marsh ditching management plan evaluation. 9 pp. Todd, R.G. and M.E.C. Gigliolo. 1983. The failure of Gambusia puncticulata and other minnows to control Aedes taeniorhynchus in a mangrove swamp on Grand Cayman, W.I. Mosquito News 43:419425. Wade, R.A. 1962. The biology of the tarpon, Megalops atlanticus and the ox-eye, Megalops cyprinoides with emphasis on larval development. Bulletin of Marine Sciences of the Gulf and Caribbean 12:545622. Woodburn, K.D. and R.A. Routa. 1970. Mosquito control and submerged land management in Florida. Proceedings of the Southeast Conference on Mosquito Suppression and Wildlife Management, pp. 211-222. Workshop on saltmarsh management and research. 1988. Bulletin of the Florida Anti-Mosquito Association. Number 1, 19 PP Zimmerman R.H. and E.C. Turner Jr. 1982. Mosquito distribution and abundance in an inland salt marsh, Saltville, Virginia. Mosquito News 42: 105-109.