Microbial Insecticides Quiz

Bioinsecticides - Bt toxin

• Bioinsecticides are biochemical pesticides derived from natural sources like animals, plants and microorganisms.
• They are living organisms or their products (phytochemicals, microbial products) used to manage pests harmful to crops.
• Microbial pesticides come from naturally occurring or genetically modified microbial entomopathogens, including bacteria, fungi, algae and viruses.
• The effect of microbial entomopathogens involves the invasion of the insect gut, multiplication of the pathogen, insecticidal toxin production and eventually the death of the insect.

Advantages of Microbial Insecticides

• Highly specific to a single pest species or group of insects
• Do not directly affect beneficial insects
• Microorganisms used in microbial insecticides are essentially non-toxic and non-pathogenic to animals, humans and other insects not closely related to the target pest.
• Residues have no harmful effect to humans or other animals; can be used in crops ready to consume (fresh vegetables and fruits).

Disadvantages of Microbial Insecticides

• High specificity to only a specific insect species in a treated area; other harmful pests will survive.
• Effectiveness affected by environmental factors (high temperatures, desiccation, UV radiation)
• Special formulation and storage procedures are necessary for some microbial pesticides to maintain their efficiency

Bt Toxin

• The most successful commercial bioinsecticide produced by microorganisms is Bt toxin.
• It's called 8-endotoxins, crystal protein or Cry protein.
• It's a protein produced by Bacillus thuringiensis.
• It's a larvicide toxin.
• It appears as a parasporal crystal.
• It's produced during sporulation.
• Released along with the spores.
• Effective against insects with an alkaline pH in their midgut.
• In most strains of B. thuringiensis, the cry genes are located on a plasmid.

Mode of Bt Toxin Action

• Cry Endotoxin (inactive toxin) in the alkaline pH condition.
• Dissolving the crystal and formation of Protoxin.
• Gut proteases (trypsin-like proteases) convert Protoxin into Polypeptide toxin (active toxin).
• Active toxin binds to specific receptors located on the epithelial cells of the midgut.
• Toxin inserts into the cells, causing an imbalance in ion concentrations.
• This leads to swelling of epithelial cells, destruction of the basement membrane, release of gut contents into the larva body, and eventually paralysis and death.

Preparation of Bt Toxin Suspension

• Sterile broth medium
• Inoculating the medium with B. thuringiensis
• Incubating the inoculated medium at 30°C for 48 hours.
• Centrifugation of the medium
• Washing of the sediment
• Preparation of spores and crystals suspension in distilled water

Bioassay

• Scientific experiment to detect or measure the effects of certain substances (toxins) on living organisms.
• Types of bioassays:
  • Qualitative: estimating the physical effects of a substance.
  • Quantitative: estimating the concentration of a substance.
• In quantitative bioassays:
  • LD50: lethal dose of a substance to kill 50% of a population in a specified time (unit: g or mg/kg body weight)
  • LC50: lethal concentration of a substance to kill 50% of a population in a specified time (unit: mg/l of the toxin).

Bioassay of Bt Toxin

• 100 ml of water + 10 larvae
• Starve the larvae
• Add food larvae and definite volume of spore-crystal suspension
• Incubate at room temperature
• Calculate mortality using Abbott's formula.

Bt Toxin Harmless to Mammals and Humans

• The toxin is very specific in its action.
• The active toxin is not formed in the acidic pH found in the intestinal tract of mammals.

Bt Toxin in Transgenic Plants (BT plants)

• Genetically modified plants with insect tolerance.
• Expressing cry genes from B. thuringiensis.
• Resulting crops contain delta-endotoxin.

Bt Toxin in Commercial Applications

• Potato plants producing Bt toxin were approved in 1995 by the Environmental Protection Agency (first human-modified pesticide-producing crop in the USA).
• Genetically modified maize producing Bt Cry protein, approved in 1996, killed the European corn borer and related species.
• In India, by 2014, more than seven million cotton farmers adopted Bt cotton, occupying over 26 million acres.