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We use larvicides to control mosquitoes when they are in their larval or pupal stages. Controlling mosquitoes when they are in their larval stages is an effective approach in our mosquito control program. If we can stop mosquitoes before they reach their adult stages then this reduces our need to spray adulticides. It’s better to stop the source. We use several different products. Each one is different and can be used for particular situations. Some of our products when applied will treat several generations of larva. This cuts back on re-treatment.
Our Larvicides:
Bacillus thuringiensis, subsp.israelensis (Bti)
Bti is a natural occurring, target specific larvicide that we use to control mosquito larva. Mosquito larvae ingest a lethal dose of Bti crystals and spores. The toxins destroy the walls of the mosquito’s stomach, paralyzing and damaging the mosquito’s digestive system. Larvae become sluggish and die within 24 hours. 
(S) Methoprene
Methoprene is an insect growth regulator that interferes with normal mosquito development. Methoprene is absorbed through the skin of the larvae and results in their inability to complete their metamorphosis. We currently use three different forms of methoprene. Pellets, briquettes  and liquid.
Pellets- Releases the insect regulator as they erode. Pellets can be placed when no water is present. This allows us to place pellets ahead of time in problem areas.  They release effective lengths of insect growth regulator for up to 30 days under typical environmental conditions.
 Liquid- Diluted mixture of methoprene. This mixture is usually mixed with Bti to form Duplex. This liquid version of methoprene is only good if dispersed in water and only maintains it efficiency for a smaller period and only of water is still available.   
 Briquets- Larger than pellets. They are also a sustained release insect regulator. They release the insect regulator as they erode. Since they are larger we use smaller amounts. They are often used in treating storm drains.
Monomolecular Film
Monomolecular film spreads quickly over the surface of the water. It reduces the surface tension of the water. This makes is difficult for larvae and pupae to attach to the surface of the water. The larvae cannot attach to the surface to breath, they eventually drown. 
A mixture of liquid methoprene and Bti. This allows for a larger range of control.