One of the unexpected consequences ("side-effects") of industrial agriculture's widespread and continuous use of synthetic pesticides during the past 68 years has been the emergence of more and more pesticide-resistant pests.
Pesticides are a "selection agent" in evolution. In rapidly reproducing organisms, which most pests are, pesticides kill the vulnerable genotypes and favor the resistant ones. In this way, pesticides drive evolution toward populations of survivors.
Just as indiscriminate continuous use of our most effective antibiotic drugs in medicine and animal agriculture has resulted in antibiotic-resistant bacterial pathogens, so has pesticide use resulted in more than 500 species of arthropod "pests" (mostly insects and mites) and plant "weeds" developing resistance to one or more pesticides.
Industrial agriculture is based on extensive monocultures. Monocultures necessarily require pesticides. Pesticides select for better pests.
Rachel Carson, in Silent Spring, stated that 'every pesticide selects for its own failure.'
While this sounds like it would be a caution sign for pesticide use, it actually turns out to be an incentive for agrochemical companies to continue to develop and market new formulations. This is sometimes called the "pesticide treadmill." One pesticide leads to another and another, etc.
Small-scale, diversified farming involving more human care and fewer chemical toxins promotes greater landscape diversity and thereby benefits from a large array of natural "pest" predators and controls.
This illustration is from my 1992 FoxSense book of environmental editorial graphics. Not much has changed, except we now have a few more generations of pesticide and improved pests.
"Pesticide Resistance" from FoxSense. © Fred Montague