Another hidden problem with pesticide use, and reason for needing more and more pesticide for a given level of control, is that organisms that weren't formerly major pests can become so when the system is altered by pesticide use. Such pests are called "secondary pests." The secondary pest may be one that wasn't sensitive to the particular pesticide used, or it may have acquired resistance more rapidly.
Secondary pests are given a competitive advantage by being tolerant to the pesticide, whereas before they were kept minor by other organisms that competed with them or preyed upon them but which were pesticide-sensitive. This emergence of secondary pests as a result of pesticide use has happened time and again.
Spider mites, certain scale insects, and aphids are all examples of these secondary pests. In fact, many of our most serious pests are secondary pests. We'll see an example in the Indonesian rice case study
Click ">>" at the tbottom of this page for some case studies involving pesticides and their impact on agroecosystems. Many of the effects described in these case studies would have been difficult to predict in advance; we don't understand very well the interactions of all the organisms that make up the agroecosystem. However, some of them should have been predictable had people used ecological reasoning and foresight. A better marriage between ecology and agriculture is called for!
("Navigate " for reminders on how to do that.)