Bioinsecticides - Bt Toxin Overview

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 control pests harmful to crops.
• Microbial pesticides come from naturally occurring or genetically modified microorganisms like bacteria, fungi, algae, and viruses.
• The effect of microbial entomopathogens involves invasion of the insect gut, multiplication of pathogens, insecticidal toxin production, and insect death.

Advantages of Microbial Insecticides

• Highly specific to target insect groups or species, reducing harm to beneficial insects and other organisms.
• Microorganisms used are typically non-toxic and non-pathogenic to humans, animals, and other insects not closely related to the target.
• Suitable for application to crops ready to eat, as residues have no harmful effect on humans or animals.

Disadvantages of Microbial Insecticides

• High specificity can lead to other harmful pests surviving in treated areas.
• Effectiveness is impacted by factors like high temperatures, desiccation, ultraviolet radiation, requiring careful timing and application procedures.
• Some microbial pesticides require specific formulations and storage methods to maintain their efficiency.

Bt Toxin - Details

• The most successful commercial bioinsecticide produced by microorganisms is Bt toxin, also known as 8-endotoxins or Cry protein.
• Bt toxin is a protein produced by Bacillus thuringiensis.
• It's a larvicide toxin, appearing as a parasporal crystal, produced during sporulation, released with spores, and effective against insects with alkaline pH midguts.
• Cry genes are often located on a plasmid in most B. thuringiensis strains.

Bt Toxin Mode of Action

• Cry Endotoxin (inactive toxin) is inactivated until it dissolves after the alkaline pH.
• Gut proteases (trypsin-like proteases) convert the inactive toxin into its active form, Polypeptide toxin.
• The active toxin binds to specific receptors on epithelial cells in the midgut of the target insect.
• Insertion into the cells disrupts ion concentration, causing epithelial cell swelling, and basement membrane destruction.
• Release of gut contents leads to paralysis and larval death.

Bt Toxin Preparation

• Preparation involves using a sterile broth medium, inoculating it with B. thuringiensis, incubating at 30°C for 48 hours, and centrifuging the mixture.
• A supernatant then is separated from the spores and crystals of the Bt toxin, and will be washed to be ready as a suspension (spores and crystals).
• The spore-crystal mix would then go into a suspension in distilled water.

Bioassay - Definition and Types

• A bioassay is a scientific experiment to detect or measure the effects of a substance (toxin) on a living organism.
• Qualitative bioassays estimate the physical effects of a substance.
• Quantitative bioassays determine the concentration of a substance that causes a specific effect, such as killing 50% of a population (LC50).

Bioassay of Bt Toxin - Procedure

• Procedure for a quantitative bioassay of Bt toxin includes 100 ml of water, 10 larvae, starving them, adding food and a defined volume of spore-crystal suspension (toxin), and incubating at room temperature.
• Mortality is calculated using Abbott's formula for correcting control mortality.

Bt Toxin Harmlessness

• Bt toxin is harmless to mammals and humans because of its specificity and the lack of active toxin formation in the low pH of the mammalian intestinal tract.

Bt Toxin in Transgenic Plants

• Bt plants are genetically modified crops with insect tolerance, expressing Cry genes from B. thuringiensis, resulting in delta endotoxin production.

Bt Toxin in Approved Crops

• Potato plants producing Bt toxin were approved in 1995 in the USA.
• Genetically modified maize producing Bt Cry protein was approved in 1996, targeting European corn borer.
• In India, by 2014, using Bt cotton was commonplace in large quantities by farmers.

Microbial Insecticides - Ecological Safety

• Microbial insecticides are ecologically safe because they are highly specific to target insects and do not harm humans or animals.

This quiz explores bioinsecticides, focusing on Bt toxins derived from natural sources. It highlights the advantages and disadvantages of microbial insecticides, including their specificity and safety for non-target organisms. Test your understanding of these environmentally friendly pest control methods.