This can have the unintended effect of actually causing an upswing in the pest population. Second, many insects can very quickly develop populations resistant to a given chemical. And third, when one pest is eliminated the door is opened to others that were less apparent before their competition was killed off. At first the solution to these problems seemed to be to just use more, or different, chemicals, but eventually it became obvious this was no more than a good way to line the pockets of chemical manufacturers. The first tenet of IPM is to try to avoid the problem in the first place.
Plants have natural defenses against pests.
Problems are more likely to occur when a plant is stressed by its general condition. A plant that receives inadequate moisture or too much sun is less able to put up a resistance to the onslaught of insects. And those that are planted in too shady a spot, are planted too closely together or get watered in late evening are more likely to suffer from fungi. To minimize problems with fungi, make sure the plants are mulched, have plenty of air circulation and are watered in morning. Try to keep them outside the drip lines of nearby trees and remove the severely damaged twigs and foliage.