[jillesvangurp]

These are all contributing factors. Mono cultures mean a single problem with pests can rapidly spread. Using pesticides means you wipe out a lot of the local wild life; including any predators that might go after the insects that spread the pests. And if you grow the exact same variety of the same produce, they are all going to be vulnerable to the exact same thing at the exact same time. Using more pesticides just adds to the problem and eventually pests become resistant anyway.

A solution here could be growing a larger variety of produce, using organic farming practices, crop rotation, etc. Pests tend to specialize in specific things and most pests have natural predators. So, if you stop killing those they'll help keep outbreaks in check. And if you rotate crops, you take away the food source for the pests. And if you grown a variety of different things, it won't all get sick at the same time.