Microbial insecticides are toxins derived from various bacteria and fungi. The most highly developed group of compounds are derived from the common bacterium, Bacillus thruingiensis, or "Bt" for short. The many different strains of this bacterium produce a variety of crystalline protein-like toxins which commonly have toxic activity against certain insect groups.
A second group of compounds have been derived from the soil fungal actinomycete, Streptomyces avermitilis. This fungus produces a variety of toxins which are called avermectins. Commercial pesticides derived from this group include ivermectin and abamectin.
Bt
Bacillus thruingiensis (Bt) is a common bacterium found in soils around the world. Scientists have known for a long time that strains of this bacterium produced crystalline protein toxins which had insecticidal activity. However, it wasn't until the late 1960's that fermentation technology was developed which allowed for the large scale rearing of this bacterium and extraction of the toxin.
Though several strains of the bacterium were known to be toxic to insects, the most widely developed materials were derived from B.t. variety ‘kurstaki’. The toxins derived from this variety are toxic only to the larvae (caterpillars) of butterflies and moths. Products with ornamental labels are: Bactospene™, Biobit™, Caterpillar Attack™, Dipel™, Larvo-Bt™, Thuricide™, Victory™ and others.
In the early 1980's, another strain, B.t. var. ‘israelensis’ was developed which has activity against the larvae of certain flies, especially mosquitoes. These products are not effective against the dipterous leafminers of ornamental.
In the late 1980's, the third strain, B.t. var. ‘tenebrionis’ (=B.t. var. ‘san diego’) was developed which has activity against certain beetle larvae. The elm leaf beetle has been the most common ornamental pest target. Products with ornamental labels are: M-One™ and Trident II™.
Apparently, Bt crystalline toxins attack the cell membranes of the gut lining. This causes the insect to stop feeding as soon as it ingests the Bt product and death often occurs several days after gut bacteria have invaded the insect body cavity. In order to get maximum efficacy out of Bt products, it should be targeted towards the younger larvae which have less well developed gut linings.
Mammalian Toxicity: Bt toxins are considered relatively non-toxic to mammals and other animals. Some formulations may have carriers which can cause eye irritation, but this is not caused by the Bt toxin.
Uses: Bts are useful alternatives to standard pesticides where caterpillars and beetle larvae are a problem. Unfortunately, most of the Bt products are not very effective once the larvae have exceeded half of their growth. Therefore, continuous monitoring of caterpillar or elm leaf beetle populations must be performed in order to target applications correctly. Where reinfestations regularly occur, as in the elm leaf beetle, several applications may be necessary to achieve satisfactory control.
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