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Larval Control
Larviciding is a technique mosquito control agencies use to control the larval stages of mosquitoes. Districts use larvicides to manage juvenile mosquito populations in their aquatic habitat. Larvicides are natural agents or chemical products that are specifically designed to kill insect larvae. The Collier Mosquito Control District (CMCD) uses a form of Bacillus thuringiensis (Bt), a beneficial bacteria that is commonly used as an agricultural microbial pesticide, for its primary larviciding compound. It acts as a stomach toxin and must be ingested to be effective. However, its mode of action is highly specific, and therefore creates little negative impact on non-target species in the aquatic environment.

The CMCD uses Bacillus thuringiensis israelensis (Bti) as a larvicide. This subspecies of Bt was discovered in 1976 by Dr. Joel Margalit and Mr. Leonard Goldberg. It was isolated from a stagnant riverbed pool in Israel and named Bt variety israelensis. It was noted to have excellent mosquito larvicide activity.

Bti is a stomach toxin. Each Bti organism can produce five different microscopic protein pro-toxins that are packaged into a protein container or a crystal. The crystal form of the toxin is referred to as delta-endotoxin. If the endotoxin is ingested, the five proteins are released into the alkaline environment inside the insect's gut. The toxins work alone or together to break down the gut wall. This leads to paralysis and eventually death for the mosquito larva.

The application of Bti for larval control is very time-specific. Mosquito larvae go through four life stages called instars. They do not feed between each instar, nor do they feed much during the fourth instar, as they prepare to become pupae. Therefore, it is crucial that the Bti is not applied during these non-feeding periods. Otherwise, the application is wasted and control is not achieved. It is best to treat larval habitats with Bti when the larvae are in their second or third instar. Larval kills can be observed within one hour of ingestion of the Bti. However, higher larval mortality is noted within twenty-four hours of application.

Bti is an excellent product to use for Integrated Mosquito Management (IMM) because it does not disturb most beneficial insects and predators. It leaves no residues and is quickly biodegraded. The mode of action for the stomach toxin is very specific. Therefore, if non-target organisms were to ingest the endotoxin without the proper alkaline stomach environment, the toxin will not work. Also, because five different endotoxins are produced from the bacteria, the probability of larval resistance is very poor. Therefore, Bti will probably continue to be a useful larvicide for the future.

The CMCD applies Bti in liquid and granular form. The liquid Bti is used for ground application. Inspectors apply the Bti using a power sprayer with a long hose attached and mounted on a truck. The granular form of Bti is used for aerial applications made by a specially equipped helicopter. This is necessary for areas where the liquid Bti cannot penetrate the canopy or the inspectors cannot reach, or adequately treat because of size, with their ground equipment.

The District also uses Altosid, an insect growth regulator product for larval control. Altosid works by disrupting the development of the juvenile mosquito, preventing it from maturing and emerging as an adult. Altosid is applied as a small briquette that looks like a bottle cork made of charcoal. This product is used in small, permanent or semi-permanent breeding sites such as culverts, catch basins and ditches. The product provides a residual effect in the breeding site and can be effective for 30 days or more. This makes it an ideal choice for treating these types of breeding sites.