Bti

Bti is a naturally occurring soil bacterium, Bacillus thuringiensis subspecies israelensis, that produces crystal proteins toxic to the larvae of mosquitoes, black flies, and some midges. Unlike chemical insecticides, it works only on dipteran larvae in aquatic or semi-aquatic habitats; it has no effect on other insects, mammals, or fish. The bacterium is applied to water bodies, where larvae ingest the toxin and die within hours or days.

The commercial product comes as a wettable powder, granule, or liquid concentrate containing spores and crystalline inclusions. Formulations are sold under various trade names and typically contain between 1,200 and 3,000 ITU (International Toxin Units) per milligram. Application rates depend on water depth, larval density, and product potency, but are usually in the range of 0.5 to 2 kilograms per hectare of water surface, or 1 to 5 pints per acre in the United States.

Bti breaks down rapidly in sunlight and is biodegradable; it leaves no residue after seven to ten days of normal light exposure. This rapid degradation means repeated applications are necessary during the breeding season if larvae are continuously present. The bacterium must contact the larvae while they are feeding, since the toxin is a stomach poison and does not penetrate the cuticle. It is most effective against early and middle instar larvae rather than pupae.

Practical use and limitations

Bti is standard in mosquito control programs for standing water bodies such as retention ponds, catch basins, ditches, and constructed wetlands. It is also used in wastewater treatment settling tanks and ornamental water features. Its specificity makes it valuable where non-target organisms must be protected. However, it requires water temperature above 15 degrees Celsius to work reliably, and effectiveness drops sharply in cooler conditions or in turbid water that shields larvae from the spores.

The main constraint is cost and labor: repeated applications throughout a season are more expensive than a single broad-spectrum chemical spray, and access to water bodies must be feasible. Resistance has not been documented in field populations, and Bti is approved for organic agriculture in most jurisdictions. It is often combined with other biological controls such as copepods or surface films, or used as a component of integrated mosquito management rather than as the sole control method.