Answer: Bt. is short for Bacillus Thuringiensis, a bacterium that acts as a stomach poison when eaten by caterpillars. It has no effect on aphids or their eggs. One of its important attributes is that Bt doesn't affect beneficial insects, but works very well against caterpillars. When you use Bt (Dipel is a trade name), expect that the caterpillars will continue to feed until the bacterium has time to react.
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