Bioinsecticides PDF

Bioinsecticides Bt toxin  Introduction Insect management in agriculture is important to safeguard crop yields and productivity. A large number of chemical insecticides that effectively control pests have been proven to be harmful to human health and environment. There is a ne...

Bioinsecticides Bt toxin  Introduction Insect management in agriculture is important to safeguard crop yields and productivity. A large number of chemical insecticides that effectively control pests have been proven to be harmful to human health and environment. There is a need to reduce the dependence on pesticides by using safer alternatives to manage pests. Many insecticidal proteins and molecules are available in nature, which are effective against agriculturally important pests but innocuous to mammals, beneficial insects and other organisms. Biopesticides are biochemical pesticides derived from natural sources as animals, plants and microorganisms. They are living organisms or their products (phytochemicals, microbial products) which can be used for the management of pests that are injurious to crop plants. Microbial pesticides come from naturally occurring or genetically modified microbial entomopathogens including bacteria, fungi, algae and viruses. The effect by microbial entomopathogens occurs by Invasion of the insect gut Death of the Multiplication insect of the pathogen Insecticidal toxin production  Advantages of microbial insecticides: 1. The toxic effect of microbial insecticides is highly specific: ✓They are specific to a single group or species of insects, and this specificity means that most microbial insecticides do not directly affect beneficial insects (they facilitate the survival of beneficial insects) in treated crops. ✓The microorganisms used in microbial insecticides are essentially nontoxic and nonpathogenic to the animals, humans, and other insects not closely related to the target insect.  Advantages of microbial insecticides: 2. The microbial insecticides could be applied in the crops which are ready to eat (fresh vegetables and fruits) because their residues have no harmful effect to humans or other animals.  Disadvantages of microbial insecticides: Naturally, there are some limitations for using the microbial bioinsecticides, but do not prevent the successful use of them. These limitations just provide users to select the effective microbial products and effective application method in the field achieve successful results.  Disadvantages of microbial insecticides: 1. Because of the high specificity of the microbial insecticides to only a specific species or group of insects in the treated area, the other harmful pests will survive and may continue to cause damage. 2. The efficiency of some microbial insecticides are affected by several environmental factors as high temperatures, desiccation, or exposure to ultraviolet radiation. So, suitable timing and application procedures are especially important for some products. 3. Special formulation and storage procedures are necessary for some microbial pesticides to maintain their efficiency. (Store all pesticides according to label directions.) The most successful commercial bioinsecticide produced by the microorganisms is a Bt toxin which: is called δ-endotoxins, crystal protein or Cry protein. is a protein produced by Bacillus thuringiensis. is a larvicide toxin. appears as a parasporal crystal. is produced during the sporulation. is released along with the spores. is effective against insects that have an alkaline pH in their midgut. In most strains of B. thuringiensis, the cry genes are located on a plasmid.  Mode of Bt action Cry Endotoxin (Inactive toxin) Alkaline pH Dissolving the crystal and formation of Protoxin Gut proteases (trypsin-like proteases) Polypeptide toxin (active toxin) Binding the active toxin to specific receptors located on the epithelial cells of the midgut  Insertion of the toxin into the cells Imbalance in ion concentration - Swelling the epithelial cells - Destroying the basement membrane Releasing the gut contents into the body of larva Paralysis and death of the larva Preparation of Bt toxin suspension Sterile broth medium Inoculating the medium with B. thuringiensis Incubating the inoculated medium at 30 °c/48 h Centrifugation  Sediment Supernatant (spores + crystals) Washing Centrifugation Sediment Supernatant (spores + crystals) Preparing a spores & crystals suspension in distilled water  Bioassay Bioassay Scientific experiment designed to detect or measure the effects of certain substances (toxins) on a living organism Quantitative bioassay Qualitative bioassay Estimating the Estimating the physical concentration of a effects of a substance substance must be used  Quantitative bioassay LD50 LC50 The lethal dose of a The lethal concentration substance to kill 50% of a substance that kills of the population in a 50% of the population specific time within a specific time - The unit of LD50 is g - The unit of LC50 is mg/l or mg/kg of the body of the toxin. weight.  Bioassay of Bt toxin 100 ml of water + 10 larvae  Starving the larvae  Addition of: Food larvae Definite volume of the spore-crystal suspention  Incubation at the room temperature   Calculating the mortality using Abbott's formula which is a mathematical equation to correct the control mortality in bioassays Mortality = [(Mortality (%) in treated population – Mortality (%) in control population) / (100 – Mortality (%) in control population)] X 100 The Bt toxin is harmless to mammals and humans because: 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) BT plants are transgenic plants or genetically modified crops with insect tolerance by expressing cry genes from B. thuringiensis. The resulting crops contain delta endotoxin.  In 1995, potato plants producing Bt toxin were approved safe by the Environmental Protection Agency, making it the first human-modified pesticide-producing crop to be approved in the USA. In 1996, genetically modified maize producing Bt Cry protein was approved, which killed the European corn borer and related species. In India, by 2014, more than seven million cotton farmers, occupying twenty-six million acres, had adopted Bt cotton. ❖The microbial insecticides are ecologically safe, Why? ❖The Bt toxin is applied in a form of a spore- crystal suspension, why?