How Bt Kills European Corn Borer
The Bt toxin that kills the ECB is also known as a crystal protein. This crystal protein is made in the bacteria cell and as the cell goes through its lifecycle, the crystal protein is exuded. The bacteria then develop into spores and the crystal protein is attached to the spore (Fig. 8). In the case of genetically engineered plants, the genetic code for the crystal protein is put into the plant. Therefore, when the plant tissue is ingested by the insect, only the crystal protein is eaten. On the other hand, when Bt is used as a spray applied insecticide, the entire spore and crystal protein is ingested. This is the same way ingestion occurs in nature (Madigan, et al., 2000). Ingestion of the crystal protein, by the ECB, must occur in order for death to occur.
Fig. 8: Spore and crystal protein (Madigan, et al., 2000)
The crystal protein is chemically activated within the larval mid-gut and becomes a toxin. The crystal protein can only be activated in a mid-gut environment with a pH of 9.5 (Deacon, 2001).which is characteristic of the ECB. 'The toxins bind to cells within the gut and cause leakage of cell fluid and then lysis to occur' (Madigan et al, 2000). These crystalline toxins paralyze the digestive tract of the ECB larvae and cause them to stop eating. The crystalline toxins are labeled “CRY” with a number representing order of discovery (i.e.: CRY 1 was the first CRY protein to be characterized). Affected larva become inactive, stop feeding, their body becomes flaccid and the head may appear overly large for the body of the larva (Hoffmann and Frodsham, 1993). The larvae stop eating and die from 12 hours to 5 days later. However, this depends upon the size of the larvae and how much of the Bt toxin was eaten (Willie, 2001).To see how Bt causes death in the ECB larva, click the animation below.
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