The pest population is developing genetic resistance to the environmental changes.
Pests are evolving resistance to our pesticides at a fast rate.
Insects eat our crops so we spray with pesticides.
The pesticides kill the majority of pests, but a few survive.
These breed with each other and so propagate a version of pest that is resistant to that particular pesticide.
We then change our pesticide and the cycle continues.
The end result is a "superbug" that is resistant to all currently used pesticides