Why are the apple farmers seeing these new, stronger bugs?

The strongest bugs survive and if they reproduce they can pass on the genes of being stronger to their offspring

Explanation:

Not sure if this is for bio or not, but if it is then that should be the right answer, if not it's probably still correct.

The bugs are a stronger and more immune to the bug killer that the farmers put out to kill the bugs last time. The more the bugs are affected by the bug killer the bugs body builds stronger to fight off the bug killer and the bug killer has no affect on the bug.

Explanation:

answer; planning ahead;