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Delaying Resistance  |
| Bt is a protein produced in nature by a common soil bacterium called Bacillus thuringiensis (commonly abbreviated as Bt. B. thuringiensis) is found throughout the world. It was first found in diseased silkworm larvae in Japan and then later in diseased meal moth larvae in Germany. The name thuringiensis comes from the name of the province of Thuringia in Germany where it was found in the meal moths. It was later recognized as a useful biological control agent and has been sold commercially since the 1920s. When chemical pesticides were developed they were more effective at controlling pests and after that Bt was largely ignored. In 1970, a new strain of Bt was discovered that was much more potent and competitive with chemical insecticides as far as effectiveness and cost. This was produced by Abbot Laboratories under the trade name Dipel® in the early 1970s. As more efficient methods for growing the bacteria and more effective formulations were developed, it became more commonly used. Over time more strains of Bt were discovered and technology made the use of Bt proteins more effective. A major technological advancement was the incorporation of the gene for the Bt proteins into plant genomes. This allowed plants to produce the proteins within its cells and provided better control of pests than did foliar and soil chemical control methods. It also provided a more effective way to treat the roots and other plant tissues where it is very difficult if even possible to treat chemically. Bt plants also allow for a more targeted approach by killing only the insects feeding on the plants and not harming the beneficial organisms. |
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| Cry Proteins for Corn Rootworm As of the beginning of 2016, four different strains of Bt have been identified and released commercially for control of corn rootworm. These strains of Bt produce unique crystals that give them their insecticidal properties. These crystals are referred to as Cry proteins, Cry being short for crystal. They are the Cry3Bb1 (Monsanto), Cry34/35Ab1 (Dow/DupontPioneer), mCry3Aa (Syngenta) and eCry3.1Ab (Syngenta). Cry proteins are classified based on their insecticidal properties and their molecular relationships as well as amino acid identity. These differences are based on evolutionary divergence and provide valuable insight into evolutionary relationships. Not all cry proteins are insecticidal, with each one having a narrow spectrum of activity. Cry proteins that have the same number after Cry (e.g., Cry3Bb1 and mCry3Aa) are in the same hierarchical class. This is significant because they would have similar modes of action (mode of action is the way in which the pesticide works or kills the pest). This would imply the possibility for cross-resistance. This would indicate that if a population develops resistance to one Cry3 protein then it highly possible that it would also show resistance for other Cry3 proteins. |
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| Bt Mode of Action/How Does it Work? A major advantage of using Bt is that it is highly specific in what types of organisms it will attack. This specificity is due in large part to how it works, or its mode of action. The mode of action of Bt is a long process that will happen only under certain conditions. This makes it benign to most organisms, but deadly to the targeted pests. A number of steps and conditions must be met for the insecticidal properties of the protein to be activated. There are 4 steps in the activation of the Bt proteins. They are:
Each of these steps must happen and in the correct order to get the desired effect. This allows Bt to be very specific in the types of insects it might target. The first step is that Bt has to be eaten by insects and enter the digestive system. This by itself provides a major advantage by limiting the impact to herbivorous (plant eating) insects. The second step is the dissolving (solubilization) of the protein. This is when the protein dissolves in the stomach, but the toxin is still not active. Solubilization is very specific to particular insects since it must fall within a certain pH range to dissolve. For example, the Bt strains used for European Corn Borer do not work with Western Corn Rootworm since the Corn Borers generally have a strongly alkaline midgut pH of 9.5 or higher while the pH of corn rootworms is generally slightly acidic (around pH 5.75). For reference, the human stomach generally has a pH of about 2. Insect guts must have the right midgut pH to dissolve the proteins. Proteins only become toxic if they become dissolved. Once the protein has been dissolved it still needs to be activated. The third step is the activation of the protein by digestive enzymes. The insect gut releases digestive enzymes, which will digest the protein. These digestive enzymes will break the protein down into the toxin. The enzymes that break down each Bt Cry protein are very specific to a group of insects. Without specific digestive enzymes the Bt protein will not be broken down to become toxic. This specificity (usually specific to a group of organisms) adds another layer of specificity. Bt Cry proteins are harmless without specific enzymes. At this point, after the digestive enzymes start to break down the protein it becomes toxic. It’s toxicity is due to the fact that the broken down proteins will bind to receptor sites in the cell membrane and disrupt the normal flow of materials in and out of the cell. This causes the cells to burst and severely damages the midgut of the insect; ultimately leading to its death. |
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| Summary In order for the Bt protein to have insecticidal properties and potentially kill the insect, all four steps must take place and happen in this order: ingestion, solubilization or dissolved, activation and then binding to the receptor site. These steps make Bt Cry proteins used in crop protection very targeted to the organisms intended. Reducing risk to other non-target organisms and reducing the need for less targeted chemical control methods. |
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