Splice the genes for a naturally pest-killing microbe into a crop plant so that the corn or wheat or potato plants themselves become lethal to the pests. No more wasteful -- and expensive -- spraying of toxic pesticides. Instead, the bugs taste the leaves and go off to die.Indisputably, it works. The genes of Bacillus thuringiensis, or Bt, have been added to corn, cotton, potatoes, and other crops, with the desired results. Scientists are aware, however, that any effort to manipulate the ecological balance must reckon with the evolutionary consequences. So even before the first Bt crops were planted, scientists, farmers, and regulators began worrying about unintended effects.