Biocontrol
[Bacillus thuringiensis subsp. israelensis]
Purpose for use
The bacterium is used worldwide to control mosquitoes and blackflies, vectors of many human infectious diseases. Its larvicidal activity is caused by insecticidal crystal proteins (ICP) that are produced during sporulation. Following ingestion, the crystal dissolves in the alkaline pH prevailing in the larval midgut, releasing protoxin polypeptides which are then activated by proteolytic enzymes and act as a stomach poison. Bacillus thuringiensis subsp. israelensis was originally isolated from a temporary mosquito-breeding site, treatment of larval populations does not result in epizootic outbreaks of disease, in contrast to the situation with classical biocontrol agents. No recycling or amplification of Bacillus thuringiensis subsp. israelensis has been observed under field conditions, and its use as a biological control agent is therefore restricted by its limited efficacy in field. (ref. ID; 6765)
Luria-Bertani [LB] plates from commercial powder (R-153-78, 1,000 IU mg-1; Roger Bellon Laboratories, Neuilly-sur-Seine, Belgium)
Test organisms
Protozoa: Tetrahymena pyriformis (ref. ID; 6765)
[Bacillus thurigiensis]
Some strains, or subspecies of Bacillus thuringiensis commonly known as Bt, are pathogens of insects and some other organisms and are distributed worldwide in many habitats, including soil. Some strains of Bt produce proteins that kill certain insects. These proteins are the active ingredients of commercial Bt insecticidal sprays (e.g., Dipel, Xentari, Javlin, Foray, M-One, VIP). Bt insecticides are classified as biopesticides because some bacteria produce the Cry proteins. The toxins of Bt are highly selective, and different strains of the bacterium kill different insects and only those insects. These are numerous strains of Bt, each with different Cry proteins, and more than 60 Cry proteins have been identified (e.g., Cry1Ac, Cry1Ab, Cry9C, Cry3Bb1, Cry2A, Cry3A, Cry3C, Cry4). (ref. ID; 6712)
Mode of action
Cry proteins produced by the bacterium are usually crystalline (called insecticidal crystal proteins-ICPs) and are protoxins with a molecular mass (Mr) of about 130-140 kDa that require cleavage by proteases to produce the biologically active form (toxins) with a Mr of 60-70 kDa. Therefore, ICPs must be ingested to have an effect and require alakaline conditions, typically in the range of pH 8-11, in the insect midgut, to be solubilized to a form conductive to activation by midgut proteases. Activation appears to require the presence of active indigenous bacteria in the midgut. After ingestion, the protein molecules pass through the peritrophic membrane and bind to specific receptors. Binding is an essential step in the intoxication process and insusceptible insects, the toxicity of a particular Cry protein is correlated with the number of specific binding sites (i.e., receptors) for it on microvilli, as well as its affinity for these sites. Many chewing insects that ingest ICPs do not have the appropriate receptors, even if they have alkaline midguts, and thus, are not sensitive to the toxins. In highly sensitive insect species, the microvilli lose their characteristics structure within minutes of toxin insertion, and the cells become vacuolated and begin to swell. This swelling continues unti the cells lyse, resulting in extensive damage to the midgut wall and in paralysis and eventually death of the insect. (ref. ID; 6712)
Trade name
BT (J Date registered 3/6/1981): Agritrol, Bactospeine, Bakthane, Basilex, Biospor, Biotrol, Cellstart, Dipel, Entobacterin, Selectzin, Thuricide
Purpose for use
Insecticide.
[Bacillus sphaericus]
The mosquitocidal activity of Bacillus sphaericus persists longer in the field, due to several possible factors: protection of the toxic crystal within the exosporium and failure to attach to sedimenting organic particulates in water, reproduction of B. sphaericus within the guts of nontarget arthropods, and recycling (germination, vegetative growth, and sporulation) in the carcasses of toxin-killed larvae. Recycing of ingested spores in the carcasses of mosquito larvae and pupae was also demonstrated for Bacillus thuringiensis subsp. israelensis in the laboratory, but the linkage between its larvicidal activity and proliferation does not necessarily hold in nature. (ref. ID; 6765)
[DCV]
Trade name
DCV (J Date registered 4/27/1974)
Purpose for use
Insecticide.