Larval Control
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.
Larvicides
The IRMCD currently uses a few different chemical formulations for larvicidal purposes:
- “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. Bt is also used by the agricultural industry 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.