That article contains the simplest and clearest one-paragraph explanation I've seen on why spraying pests is a bad idea, even if your spray of choice is mild, "organic", or comprised of trusted household ingredients:
Kaiser was in Costa Rica pursuing his graduate studies when a colleague, who was studying two citrus orchards, noticed something unusual. The first orchard, planted on the edge of a forest that was dense with trees, bushes and wild vines, had more than 90 percent fewer pests than the second orchard, which was in an open plain almost a mile away.
Kaiser was stunned. “You can’t get 90 percent reduction with chemical sprays,” he says. “And sprays kill everything—the pests and the beneficials.” (Beneficials are insects that don’t eat crops but instead help them grow. Bees, for example, help pollinate; others, such as ladybugs and praying mantises, eat the insects that eat the crops.) Every farmer wants beneficials; after spraying, however, the pests always come back faster and stronger than beneficials do. (Biologists explain that this is because the pests breed faster and more prolifically, and have been toughened by centuries of adversity). More sprays follow, and the death spiral widens. Paradoxically, this process of destruction occurs whether the sprays are synthetic or organic.
In Costa Rica, Kaiser and his colleagues realized that the pest-free orchard escaped this fate for a simple reason: the beneficials were hanging out in the foliage bordering the orchard, which gave these insects an easy commute to manage the crops.