Ecological and genetic factors influencing evolution of pesticide resistance in tetranychid and phytoseiid mites

Among tetranychid spider mites and their phytoseiid predators, the evolution of pesticide resistance is a common event. In most cases, resistance is based on a single dominant or semidominant gene. However, polygenic, less-stable resistance often develops under laboratory selection. More rapid development of pesticide resistance in spider mites and predatory mites than among other arthropods might partly be due to their arrhenotokous reproduction. For both groups of mites, little study has been done on population genetic factors influencing pesticide resistance. A few studies have focussed on ecological factors. An important ecological factor influencing resistance evolution is the level of immigration of susceptible individuals into treated habitats. Spider mites and predatory mites both tend to reside in treated habitats at high levels and to immigrate at only modest levels from untreated habitats. This favors rapid resistance development. Another factor contributing to rapid resistance evolution in both mite groups is their rapid reproductive rate. A food-limitation factor may limit resistance evolution under field conditions more in predatory mites than spider mites. After treatment by a pesticide, spider mites have an unlimited food source, whereas predatory mites have a decimated food source (their prey), which leads to reduced reproduction, starvation, or migration. Because of the common occurrence of resistance among both mite groups, a strategy of resistance management is often feasible for them. Case histories of IPM where the population dynamics and genetics of pesticide resistance of tetranychid and phytoseiid mites have been considered are discussed. The overall conclusion is that greater understanding of the population genetics and ecology of these species will provide for improved systems of resistance management and IPM.