One of the most successful forms of genetically modified crops are the species that have been engineered to express bacterial proteins that are lethal to insects that ingest them. These crops have picked up the name “Bt,” for Bacillus thuringiensis, the bacteria that originally made the toxins. There are Bt versions of food crops such as corn and soy beans, as well as the commercial crop cotton.
The danger with these crops is that they’ll do what every other insecticide has done throughout history: select for the evolution of resistance. In the US, government regulations require that Bt crops be planted along with some fields sown with their non-Bt versions, called refuges. This ensures that any rare resistant individuals will likely mate with non-resistant animals that fed on the insecticide free crops, diluting out the resistance genes.
Interesting read on the arms race between insect resistant crops and insects who evolve to tolerate what was engineered to be toxic to them. And because of the short lifespan of insects, we can see the evolution unfold before us.