Biotechnology has allowed us to genetically modify corn with a gene that acts as a pesticide. what could be a possible environmental impact of this development?

Since a gene that acts as a pesticide is inserted into a corn, that corn is now safe from insects. This will happen because the corn will probably synthesise protein according to that inserted DNA sequence and that protein will serve as a repellent to the insects. However, the Red Queen's hypothesis in evolution explains that organisms must constantly adapt and evolve in order to survive. This refers to insects. If they could not eat genetically modified corn, they would extinct. So, they must adapt or evolve in order to survive. So, it is expected that insects that are resistant to this specific pesticide would develop through the time.

You should never redesign your experiment. try and see what in your experiment you made a mistake on or try and test your experimemt with somebody else. see if they get the right results.