Open Marsh Water Management in Delaware: 1979-2007

Open marsh water management (OMWM) is a selective ponding-and-ditching technique for saltmarsh mosquito control that encourages consumption of mosquito larvae by native larvivorous fishes while eliminating or reducing larval rearing habitats. OMWM has been successfully practiced in Delaware since 1979, and after 28 years of use actively continues today. After some refinement of the state’s initial OMWM goals, a target universe of about 9% of Delaware’s tidal wetlands (or about 9,000 acres of moderate to severe larval-production habitats in a total 95,000 acres of coastal wetlands) were identified for OMWM treatment. As of 2007, about 80% of Delaware’s intended statewide OMWM work has been accomplished. Locations where open marsh water management (OMWM) work has been done in Delaware are reviewed with particular emphasis on habitat types and landowner categorizations, and the rate of OMWM installation is also examined. The types and mixtures of OMWM systems used in Delaware (open, sill, and closed systems) are discussed. OMWM’s use in previously drained, parallel-grid-ditched marshes can have notable habitat restoration benefits for waterbirds (waterfowl, wading birds, shorebirds) and aquatic estuarine organisms. PROCEEDINGS NINETY FOURTH ANNUAL MEETING 56 Origin and scope of OMWM work in Delaware Open marsh water management (OMWM) started in Delaware in 1979, driven by a desire of the Delaware Division of Fish and Wildlife to reduce, to the extent practicable, the use of chemical larvicides for salt marsh mosquito control. OMWM must be installed in a manner such that satisfactory mosquito control is achieved (i.e. similar to or greater than the efficacy achieved with larvicide applications) and simultaneously not cause any unacceptable impacts to salt marsh ecological structure and function. The Delaware Mosquito Control Section (DMCS) is an integral part of our state’s fish and wildlife management agency. Whenever more environmentally sound methods of mosquito control can be used, we try to adopt such approaches. Delaware’s OMWM efforts arose from the pioneering development and use of OMWM in New Jersey started in the mid-1960s (Ferrigno and Jobbins, 1968; Ferrigno et al., 1975). OMWM work that was started on Maryland’s Eastern Shore in the mid-1970s (Lesser 1983) then further spurred Delaware to adopt OMWM as the preferred salt marsh mosquito control method. When OMWM started in Delaware, our primary goal was to lower the use of temephos, an organophosphate larvicide. Today with OMWM we want to lower, to the extent practicable, our use of methoprene, a larvicide that works as a juvenile hormone mimic, and our use of Bti, a microbial larvicide. Delaware has a total of about 95,000 acres of tidal wetlands, covering about 8% of the state’s surface area (Tiner 1985), yielding the highest percent surface cover by tidal wetlands of any state. This is due, in large measure, to Delaware’s overall small size. Delaware has only about 1,950 sq. miles and ranks 49th in size among the states, while also containing a disproportionately large amount of coastal wetlands. Given these data, combined with Delaware’s being in the top 10 states for human population density, and with so much of Delaware’s populace within the typical 3-5 mile (sometimes up to 10-mile) flight range of salt marsh mosquitoes, Delaware’s residents and visitors can be confronted with severe mosquito-related problems if salt marsh mosquitoes are not controlled. The saltmarsh mosquito guild of concern in Delaware includes Aedes sollicitans, Ae. cantator, Ae. taeniorhynchus, Culex salinarius, and Anopheles bradleyi. Approximately 11,000 acres of Delaware’s 95,000 acres of tidal wetlands are found in coastal impoundments no longer open to unfettered tidal flow (Meredith and Whitman 1994). Instead, these are now wetlands where marsh water levels and tidal exchanges are managed via water control structures for multiple resource purposes, including some management for mosquito control. Impoundments are created through the construction of dykes or levees to block or eliminate tidal flow across the marsh surface. Marsh water levels are thereby managed within the impoundments via water control structures at either higher or lower levels as opposed to in un-impounded marshes. Coastal impoundments are habitats where OMWM in a more traditional sense is not employed, reflected in the word “open” in the term “open marsh water management.” This indicates that the technique does not involve the use of dykes, levees or other features to block or restrict free flow of tidal water over and across the general marsh surface (i.e. general marsh surface flooding in OMWM-treated marshes continues in the same manner as before OMWM treatment). The Delaware Mosquito Control Section has found OMWM to be a very effective, satisfactory approach for larval mosquito control, yielding >90% reduction in larval populations. The Delaware Division of Fish and Wildlife (of which mosquito control is part) views any possible adverse impacts to marsh plant communities or non-target organisms from OMWM to be quite inconsequential and readily acceptable, especially when viewed in light of greatly reduced larvicide use. How OMWM is regulated in Delaware and the Section’s authority to perform OMWM work are discussed. The future use of OMWM in Delaware is examined. This includes our intention to pursue the remaining 20% of the initially targeted 9,000 acres of OMWM work, our need to maintain about 7,000 acres of completed OMWM work, our possibly making more use of OMWM within coastal impoundments, and a possible expansion of OMWM systems to contend with new larval habitats arising along marsh upland borders because of relative sea-level rise.