Bacillus sphaericus (Bs) Strain 2362, the active ingredient in VectoLex, is a naturally occurring, spore-forming bacterium found throughout the world in soil and aquatic environments. At the time of sporulation, Bs produces a delta-endotoxin, which is toxic to many species of mosquito larvae when ingested.


When VectoLexis applied to a larval habitat, larvae ingest the delta-endotoxins which are attached to the bacterial spores and encased in a parasporal body. The toxins are activated by enzymes and the high pH that is found in the larval midgut. The toxins then exert their effect upon cells of the midgut epithelium, causing pore enlargement and cell destruction. Fluids from the larval haemolymph then mix with midgut fluids and the pH of the midgut is neutralised which leads to larval mortality which is usually seen within 48 hours of ingestion. Early instars are highly susceptible to the toxins, however, non-feeding, late fourth instar larvae, and pupae will remain un-affected.


Extended residual activity in the environment is an outstanding property of VectoLex WG. This persistence is thought to be the result of the stability and slow settling rate of the Bs toxin. Added to this, many of the larvae that die accumulate on the water surface. The Bs spores then germinate inside the larval cadavers, grow, and produce toxin which is then ``recycled'' back into the larval habitat. It is often seen that the greater the larval population upon initial treatment, the greater residual effect of Bs. It should be noted that spot treatments with VectoLex WG tend to deliver less residual activity than treatment of the whole water body and that VectoLex WG also gives good efficacy and residual effect in highly polluted habitats.
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