Arthropod pest resurgence: an overview of potential mechanisms

The phenomenon of insecticide-induced resurgence of arthropod pests has long been known to occur in response to a reduction in natural enemy populations, releasing the pest population from regulation. However, studies of resurgent populations infrequently examine other mechanisms, although numerous alternative mechanisms such as physiological enhancement of pest fecundity, reduction in herbivore-herbivore competition, changes in pest behaviour, altered host-plant nutrition, or increased attractiveness may also cause, or enhance the probability of, resurgence. Additionally, many studies that have identified natural enemy mortality as the primary cause of resurgence do not document a priori regulation by natural enemies and, therefore, are correlative in nature. In this paper, a universal definition of resurgence is proposed and criteria for determining the occurrence of resurgence are listed. Both of these are essential to rigorous evaluation of this important phenomenon. Alternative mechanisms for resurgence and potential areas of future research are identified. It is argued that insecticide resistance is not a mechanism underlying resurgence; rather, it simply enhances the probability that resurgence may occur. The similarity of resurgence to some insect outbreaks is discussed. In some cases natural outbreaks differ only in the initiating factor. The importance of resurgence management to plant protection is that resurgence is totally contradictory to the intended outcome of insecticidal application. This conflict strengthens the need to identify specifically the causal factors for each case of resurgence in order to manage this detrimental phenomenon effectively.