Bacillus thuringiensis

Bacillus thuringiensis is a widely distributed bacterium that during sporulation produces a crystal inclusion which is insecticidal when ingested by the larvae of a number of insect orders. Susceptible orders include Lepidoptera, Diptera, and Colcop-tera. The action of B. thuringiensis was first observed in 1901 as the cause of a disease of silkworms. Several strains of the bacterium have been identified with activity against a range of insects including cabbage looper, tobacco budworm, mosquito, black fly, and more recently nematodes, ants and fruit flies. While the bacterium appears an ideal insecticide (having a toxicity 300 times greater than synthetic pyrethroids), it requires careful use. It is most effective against neonates and early larval instars so that spraying must be timed for egg hatch. It also has no contact activity and must be ingested so the plant must be well covered to ensure the insect receives a lethal dose. Furthermore it has a half-life in the field as short as 4 h, so careful timing is essential for it to be effective. Despite these limitations, it has been shown to be an important component of crop management programes.

One way of overcoming the problems of application of B. thuringiensis is to incorporate the gene responsible for expression of the protein into the crop plant. This has been achieved with maize (Zea mays) to protect against the European corn borer, with cotton (Gossypium hirsutum) to protect against a range of budworms and bollworms, and with potato (Solanum tuberosum) against Colorado beetle. (Cotton may seem irrelevant in a text on food but cottonseed oil is used extensively in cooking oils, margarines, and industrial fats.) This genetic modification has great benefits but care has to be taken that the food product has not changed in some unpredicted way. All genetically modified foods have to be extensively tested and cleared by regulatory agencies before release.

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