Step 1: Understanding the Concept:
Instead of external spraying, plants can be genetically modified to produce their own insecticide.
Step 2: Detailed Explanation:
1. The \textit{cry} gene was isolated from the bacterium \textit{Bacillus thuringiensis}.
2. This gene was then incorporated into the corn plant's genome using a vector.
3. As a result, the corn plants in Field B (GM corn) produce the toxic Bt protein internally in all their tissues.
4. When a corn borer attempts to feed on these plants, it ingests the toxin and dies, making the plant naturally resistant to the pest.
Step 3: Final Answer:
The corn plants in field B are transgenic (Bt corn) and produce the insecticidal protein endogenously.