Larvicides — Growth Regulators
These chemicals are typically referred to as insect growth regulators or IGRs. IGRs act on the endocrine or hormone system of insects. These insecticides are specific for insects, have very low mammalian toxicity, are non-persistent in the environment, and cause death slowly. Most of the currently registered IGRs mimic the juvenile hormone produced in the insect brain. Juvenile hormone tells the insect to remain in the immature state. When sufficient growth has occurred, the juvenile hormone production ceases triggering the molt to the adult stage. IGR chemicals, such as methoprene mimic the action of juvenile hormone and keep the insect in the immature state. Insects treated with these chemicals are unable to molt successfully to the adult stage, and cannot reproduce normally.
Larvicides — Bacteria
Bacillus thuringiensis var. israelensis (Bti) is a naturally occurring bacterium that produces a crystalline protein toxin (crystal) and a spore. The larval activity of Bti formulations is due to the presence of the protein toxin. The spore has no larvicidal activity. For mosquito larvae, many factors are necessary to produce the toxic effects of Bti crystals. If the crystals are available in sufficient quantity, to suffer toxicity and die, a larva must : 1) Capture and ingest the crystals, 2) Possess a digestive tract with a highly alkaline pH, 3) Possess the enzymes capable of liberating the toxic proteins, and 4) Possess the gut membrane receptors, compatible with the solubilized toxins. Bti-based products are not insecticides of contact. The active ingredient (crystals) must be ingested to show a toxic activity. This very specific mode of action makes it very safe for non-target organisms present in the same environment.
Bacillus sphaericus (Bs) is also a naturally occurring, spore-forming bacterium found throughout the world. At the time of sporulation, Bs produces crystallin proteins (as in Bti but different) toxic for many species of mosquito larvae upon ingestion.
Very refined oil or surfactants can be used as larvicides/pupicides. These products have the ability to kill both larvae and pupae. The District is not currently using any oils but is using surfactants. The surfactant used is also called monomolecular film (MMF). Using conventional spraying methods, the invisible monomolecular film quickly spreads over standing water habitats. The film reduces the surface tension of the water making it difficult for the mosquito larvae and pupae to attach to the surface which causes them to drown. Emerging mosquitoes are unable to fully emerge and will drown. Mosquito larvicide and pupicide are effective on all species of mosquitoes that breed in standing water and require the air/water interface in their lifecycle.