Final answer:
The USDA's Animal and Plant Health Inspection Service regulates genetically engineered plants to ensure their safety and prevent ecological risks.
Step-by-step explanation:
The branch of the USDA that is mainly responsible for regulating genetically engineered plants, particularly those that are potentially invasive or could carry pests, is the Animal and Plant Health Inspection Service (APHIS). APHIS closely monitors transgenic plants and other GMOs to ensure they are safe for human consumption and do not pose any risks to the environment or other species. This involves extensive testing to maintain ecological stability and prevent the spread of foreign genes to natural species.