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European Corn Borer and Bacillus thuringiensis

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History of <i>Bacillus thuringiensis</i>

Now let us look at a control method farmers have used to combat the adverse affects of European corn borers. Bacillus thuringiensis (Bt) is a naturally occurring, soil borne organism. Bt was first discovered in a diseased silkworm colony in 1901 by Ishiwata, a Japanese bacteriologist. However, not much research was done until 1911 when a German bacteriologist, Berliner, took interest in this bacterium (Reardon, et al., 1994).

Bacillus thuringiensis can vary in size, ranging from a small bacterium, measuring 0.5 x 1.2 µm (micrometers) to quite large at 2.5 x 10 µm. The Bt cells are rod shaped and can occur as single cells or in chains (Holt, 1984). The bacteria are so small that approximately 8,000 Bt cells can fit on a circle the size of the period at the end of this sentence. (This number was calculated by figuring the total area of a single Bt bacteria (2.5 x 10 µm) and comparing that to the total area of a period on a page.)

Once Bt was discovered and cultured, researchers found that this bacterium produced a toxic crystalline protein lethal to certain insects. The insect must ingest the toxic protein produced by Bt in order for death to occur. The Bt protein will kill Coleoptera (beetles), Lepidoptera (moths, butterflies, and caterpillars), and Diptera (flies and mosquitoes). However, there are many different strains of Bt with Bt proteins specifically lethal to certain insects. For instance, protein from a certain strain of Bt is lethal only to insects in the order Lepidoptera. Another protein is lethal only to the order Coleoptera. (See Table 1, Hoffmann and Frodsham, 1993).
 

Table 1

 

Bacillus thuringiensis strains
Control Characteristics
san diego
Colorado potato beetle and elm leaf beetle adults and larvae
tenebrionis
Colorado potato beetle and elm leaf beetle adults and larvae
kurstaki
caterpillars, such as ECB, cutworms, and cabbageworms
israelensis
mosquitoes, blackflies, and fungus gnat larvae
aizawai
wax moth larvae and various caterpillars, especially diamondback moths

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