Advantages:
The farmer can grow a larger crop because it is easier to fight pests.
In some cases the farmer can use a more environmentally friendly crop spray.
The farmer can also protect the environment by using less crop spray.
Disadvantages:
Genes from the genetically modified rape crop could be transferred to the pests. The pests then become resistant to the crop spray and the crop spraying becomes useless.
Rape plants can pollinate weeds - for example navew which is found in rape fields. When rape plants pollinate the navew their genes are transferred. The navew then acquires pesticide resistance.