Monoculture plantings are more susceptible to pest pressure than mixed stands. Not only does a pest find it more difficult to locate its preferred host in a mixture, but the pest s natural enemies are often more abundant or effective in mixed plantings. Conversely, large fields of single crops create an ideal environment for pest attack. When crops are genetically uniform, as most modern varieties are, the opportunity for pest damage is greater still. Plants under stress from drought, a lack of nutrients or other factors are more vulnerable to pests.
Some plants are actually better food for their pests when under stress.
Aphids, in particular, take advantage of the nutrients available in stressed plants. Recent research has shown that when fed on by certain insects, some plants emit chemical signals that call natural enemies to their aid.