mc_marsh

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.

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.

There are many hundreds of acres of coastal marshland and low-lying woodlands which the IRMCD cannot make into impoundments. Some property owners did not wish their land to be impounded while other areas have peculiar topography which would have made impounding too expensive. In these areas, the IRMCD must maintain an active mosquito control program involving larval inspectors and the use of mosquito larvicides applied from the ground or air.

The IRMCD employs about half a dozen field inspectors who routinely visit hundreds of known mosquito breeding areas and check the water for larvae. If the affected area is small, these inspectors will usually apply an appropriate larvicide immediately to kill the mosquitoes. If the area is large or inaccessible, the IRMCD applies larvidical chemicals using an airplane.

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.

* "Altosid" (methoprene) : is a synthetic insect growth hormone. By applying it at the appropriate time in the mosquito life cycle, they cannot make the transition from larvae to adult. Methoprene is an extremely safe larvicide, also approved for usein homes and kitchens. It works on only a few kinds of insects, and at the dosages used by the IRMCD only mosquitoes are affected. Methoprene is approved for use in even the most protected environmentally-sensitive marshlands.

* "BTI" (bacillus thuringiensis israelensis) : is an naturally
derived preparation which incorporates an extract from a salt-pond bacteria.Bti affects only insects which have an alkaline digestive chemistry (and are inclined to eat it). It is harmless to humans and almost every other species. Bti is also used by the agricultural industry under the trade name "Dipel" to protect food crops from various species of weevils and worms and the Bt gene has even been spliced into experimental food crops so the plants will produce Bt by themselves. For mosquito control purposes, the Bt toxin is coated onto corncob grit or mixed with water for air or ground application.While somewhat expensive, it is extremely effective and ecologically friendly. Like methoprene, Bti is also approved for use in the most environmentally-sensitive marshes.

* "Abate" (temephos) : is a conventional organophosphate insecticide to which mosquito larvae show extraordinary sensitivity. The dosages which kill mosquito larvae are far below those required to affect most other insect species. By taking advantage of this quirk of mosquito biology, the IRMCD can make good use of this effective larvicide without impacting non-target species. Abate may be coated onto sand or gravel and applied by air or sprayed as a liquid from the ground.

* "Agnique" - is what's called a "monomolecular film" product. It is sprayed onto the surface of the water and immediately spreads out into a layer one molecule thick - suffocating mosquito larvae and pupae. Long ago, diesel fuel was used for a similar purpose but the additives made it rather toxic to the environment. Agnique is a heavy alcohol ester with extremely low toxicity. It is normally applied as a liquid concentrate although there have been rumors about a pellitized product that can be dispensed by an airplane.

Agnique is that it's the only product currently availible that will kill mosquito pupae - which are nearly invulnerable to any reasonable dose of conventional insecticides.

None of these larvicides are persistent. They kill the mosquito larvae present at the moment and then rapidly decompose. If a heavy rain occurs a few days after treating with these larvicides there is no alternative but to treat once again. The manufacturers of all three larvicides also sell time-release "pellets" or "briquettes" which can be used in small areas that are difficult to access. Expense, ecological concerns and the possibility of the insects developing resistance to these larvicides preclude the use of time-release pellets on a larger scale.

(Peter O'Bryan & Jim Mason)