An attempt to make potato plants resistant to sap-sucking insects has highlighted the unpredictability of genetic engineering. The modified plants unexpectedly turned out to be vulnerable to other kinds of insect pests, demonstrating how important it is to assess each transgenic crop individually. Crops such as maize and cotton have already been made resistant to chewing insects by adding a gene for the bacterial toxin Bt. But Bt doesn't deter sap-suckers like aphids, so genetic engineers are looking at other natural substances to keep insects at bay, such as the lectin proteins found in many plants and seeds.
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