Quote from Chapter 2 The Obligation to Endure

Since DDT was released for civilian use, a process of escalation has been going on in which ever more toxic materials must be found.This has happened because insects, in a triumphant vindication of Darwin's principle of the survival of the fittest,have evolved super races immune to the particular insecticide used, hence a deadlier one has always to be developed-and then a deadlier one than that. It has happened also because ,for reasons to be described later, destructive insects often undergo a flareback, or resurgence, after spraying , in numbers greater than before, Thus the chemical war is nver won, and all life is caught in its violent crossfire.

(my word)This is why DDT in use escalated for such reasons.

Under primitive agricultural conditions the farmer had a few insect problems.These arose with the intensification of agriculture-the devotion of inmense acreages to a single crop.Such a system set the stage for explosive increases in specific insect population.Single crop farming does not take advantage of the principles by which nature works;it is agriculture as an engineer might conceive it to be.Nature has introduced great variety into the landscape the man has displayed a passion for simplifying it.

(my word) if you are messing around with the nature, things could go terriblly wrong.