Bacillus thuringiensis israelensis (Bti)

The discovery of Bacillus thuringiensis israelensis (Bti) in 1976 revolutionized mosquito control by providing an environmentally safe and efficient biological larvicide. There are at least 86 named varieties of Bacillus thuringiensis bacteria, capable of producing protein crystals effective against different groups of insects. The protein crystals produced by Bti are effective against larvae of mosquitoes (Culicidae) and blackflies (Simuliidae), and even against non-biting midges (Chironomidae) and dark-winged fungus gnats (Sciaridae) at higher dosages.
The mechanism of Bti-induced mortality is complex, and the bacteria itself is not harmful for mosquitoes. The active ingredient is the protein crystals that are produced by the bacteria while sporulating. These crystals must be ingested and end up in a stomach environment with high pH where the crystals can disintegrate and become free proteins (protoxins). These protoxins can then be cleaved into four cell-specific toxins by enzymes in the midgut and attach to and destroy specific gut cells, creating holes in the stomach which will induce the death of the larvae. The long and complicated chain of events from ingestion of protein crystals over protoxins to toxins, and the requirement for reacting with specific gut cells, ensures the selective effect of Bti against mosquitoes and a few other nematoceran insect families. Mosquitoes have plenty of the specific receptor gut cells which makes them very sensitive to Bti, while non-biting midges have few and are less sensitive.
Bacillus thuringiensis israelensis is named after Israel, where it was discovered in 1976 by Joel Margalit, Professor at the Ben Guiron University. The discovery of a mosquito-selective variety of Bacillus thuringiensis opened new possibilities for environmental-friendly mosquito control. The first field test of Bti against mosquitoes were performed 1978 against the floodwater mosquito species Aedes vexans in the River Rhine Valley of Germany. After almost 40 years of use by the German Mosquito Control Association, ”Kommunale Aktionsgemeinschaft zur Bekämpfung der Schnakenplage e.V.” (KABS), Bti is still highly efficient against mosquitoes and no resistance problems have been observed. Nowadays, Bti is used in many countries, on all continents, and is generally efficient against all mosquito species if the larval sites can be found for the treatment. However, the important vector species Aedes aegypti and Aedes albopictus, breed in small and often hidden containers or puddles, and thus are difficult to control with Bti.

The Bti bacteria has a worldwide natural distribution, and have been documented in humid soil environments over the entire world except for mountain tops and polar areas. In the Nordmyra wet meadow, near the village of Tärnsjö in the Nedre Dalälven region, researchers from Stockholm University documented natural Bti during the 1990s, several years before the first VectoBac G treatments.