Pest resurgence is the rapid reappearance of a pest population in injurious numbers, usually brought about after the application of a broad-spectrum pesticide has killed the natural enemies which normally keep a pest in check.
A well-known example in rice cultivation is the resurgence of brown plant hopper (BPH). If no pesticides are used, BPH is kept under control by its natural enemies (mirid bugs, ladybird beetles, spiders and various pathogens). Pesticides kill the beneficials and create a situation where populations of BPH can multiply rapidly and thus become a man-made pest.
Resurgence can be easily avoided by not spraying pesticides. But for many farmers it is difficult to recognize that resurgence has occurred in their field. They spray regularly because they see pests in their fields, without realizing that it is actually the spraying which is causing the pest problem.