Environmental Management
Indian River County is located adjacent to the Indian River lagoon, a shallow estuarine system. Ocean water input is limited to man-made connections at inlets, therefore lagoonal tides are driven more by seasonal and storm activities than by lunar influences. As shown in the diagram above, the typical lagoonal coastal salt marsh is composed of two distinct zones. These “low” marshes and “high” marshes border both sides of the lagoon.
The “low” marsh is land that is flooded by the daily tides. In Indian River County it is dominated by Red Mangrove (Rhizophora mangle) and coastal grasses such as Smooth Cordgrass (Spartina alterniflora). Low marsh does not breed salt marsh mosquitoes.
The “high” marsh is only flooded by seasonal tides, storm driven tides or rainfall. Vegetation consists of black (Avicennia germinans) and white (Laguncularia racemosa) mangrove and understory succulents such as saltwort (Batis maritima) and glasswort (Salicornia spp.). It is in this irregularly flooded high marsh that mosquitoes breed in voluminous numbers.
Mosquito eggs hatch almost immediately upon flooding, and the mosquito rapidly progresses through four larval stages called “instars”. In later stages, the larvae commonly “ball up” in aggregations (form dense patches on the water surface) that may number 100,000 in a three-foot-wide “ball”. Such “balls” may be found dotted all over every shallow pond and puddle in the marsh, so even a small area of marsh may breed many billions of mosquitoes from just a single rainfall or tide event.
A 1-2 day pupal stage allows the larval form to metamorphize into the adult flying/biting form. The adults emerge in a synchronized flight, mate in the air and then disperse seeking a blood meal to provide protein so they can start the cycle all over again.
Mosquito Control in Coastal Marshes
The best and most economical form of mosquito control in marsh areas is to prevent them from breeding in the first place, called “source reduction”. The IRMCD has exploited a peculiarity of our most common salt marsh mosquitoes, Aedes taeniorhynchus and Aedes sollicitans, by doing what at first glance may seem counterproductive - putting more water into the marsh.
Aedes taeniorhynchus will not lay its eggs directly onto water. Instead, it waits for water levels to fall slightly and lays its eggs (oviposits) in the damp soil or mud bordering puddles and shallow ponds. If the marsh could remain covered with water instead this would prevent this mosquito from ovipositing.
During the 1950s and early 1960s, IRMCD created many so-called “Impoundments” out of our coastal marshes. Low earthen dikes were constructed surrounding the high marshes. Large diesel or electric water pumps are used to add enough river water to the marshes so that all areas are covered a few inches deep. Because of its egg laying peculiarity, Aedes taeniorhynchus will not be able to make use of the marsh and the breeding cycle is stopped.
Impounding is by far the most economical and efficient approach to controlling salt marsh mosquitoes. Once a marsh is flooded, no chemicals need be used except perhaps at the upland margin of the water. Compared to the costs of chemicals and labor, keeping an impoundment flooded through the summer months is quite inexpensive. Most impoundments are opened to the river during the winter to allow fish and other marine life access to the areas. Properly managed impoundments support a vast number of plant and animal species while offering the best means of mosquito control available. The IRMCD has always been in the forefront of scientific marsh management.
When Impounding is Impossible
Some ecologically-insensitive low-lying areas are appropriate candidates for Rotary Ditching. A rotary ditcher is a trenching device which is designed to dig a flat bottomed shallow ditch while flinging the excess dirt completely clear of the area. Normal ditching methods leave a high area on either side of ditch which prevents water from entering the channel until it is high enough to get over the lip. Mosquito breeding may occour in the water which is not high enough to enter the ditch. Rotary ditching eliminates this problem. These machines are usually attached to large tractors which use extra-wide tires to prevent crushing the vegetation or leaving tire ruts which might breed mosquitoes themselves.
