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A larvicide (alternatively larvacide) is an insecticide that is specifically targeted against the larval life stage of an insect. Larvacides may be contact poisons, stomach poisons, growth regulators, or (increasingly) biological control agents.

The biological control agent Bacillus thuringiensis, also known as Bt, is a bacterial disease specific to Lepidopteran caterpillars. Bacillus thuringiensis israelensis, also known as Bti, and Bacillus sphaericus affect larval mosquitoes and some midges have come into increasing use in recent times.

Bti and B. spaericus are both naturally occurring soil bacterium registered as larvicides under the names Aquabac, Teknar, Vectobac, LarvX, and VectoLex CG. Typically in granular form, pellets are distributed on the surface of stagnant water locations. When the mosquito larvae ingest the bacteria, crystalized toxins are produced which destroys the digestion tract. These larvicides will last only a few weeks in water and pose no danger to humans, non-targeted animal species, or the environment when used according to directions.

Methoprene is another biological agent that interrupts the growth cycle of insect larvae, preventing them from development beyond the pupa stage. Altosid is the trade name for methoprene, which is usually applied to larger bodies of water in the form of time-release briquets which can last from one to five months. Use of this larvicide does not pose an unreasonable health risks to humans or other wildlife and it will not leach into the ground water supply. There is a small acute and chronic risk to some fish and freshwater invertebrate species.

Temephos, marketed as Abate, is an organophosphate which prevents mosquito larvae from developing resistance to bacterial larvicides. Due to the small amount needed and the fast rate that temephos breaks down in water, this type of larvicide doesn't pose an unreasonable health risk to humans. But at large doses it can cause nausea or dizziness. Similarly, there is not a large risk to terrestrial species, but there is a toxic concern for non-targeted aquatic species. Therefore, temephos should be limited only to sites where less hazardous larvicides are ineffective and controlling application intervals.

Larviciding techniques can also include the addition of surface films to standing water to suffocate mosquito wigglers, or the genetic modification of plants so that they naturally produce a larvicide in plant tissues.

• EPA explanations of various laricides

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