Why are single-site pesticides prone to development of resistance among pest populations compared to multi-site pesticides? When a pesticide is used to control a pest, for example an infestation of mites on a landscape shrub, not all of the mites are killed by the pesticide. Mites that are not killed may possess a genetic change or mutation that confers resistance to the pesticide.

The probability of a pest possessing genetically-based resistance to a pesticide is extremely low for pesticides that act on multiple biochemical processes in the pest. For example, imagine a multi-site pesticide that causes a deleterious effect to three different biochemical pathways in a pest. A pest resistant to the pesticide would have to have three genetic mutations conferring resistance to the three target sites of the pesticide. This is unlikely. Conversely, with pesticides that act on a single-site, there is a greater likelihood that some members of the pest population possess a genetic mutation at the corresponding site. Genetically resistant members of the pest population will survive the pesticide application. Even if the number of pests that possess this characteristic and survive are few, their presence can be significant. Since many pests typically have short lifecycles and reproduce rapidly, further pesticides applications will “select” for the genetically resistant portion of the pest population. As these genetically resistant portions of the population reproduce, their offspring inherit the resistant trait and at some point the pesticide-resistant portion of the population may overtake the portion of the pest population that is susceptible to the pesticide. At this point the pesticide will no longer be effective in controlling the pest population. This example is simplistic, because the reality is that pests have many different mechanisms to overcome pesticides, but it illustrates why pesticides with single-site chemistries are prone to the development of pesticide resistance.

Russell, P. 2003. Taking the path of least resistance. Pesticide Outlook 14 (2): 57-61.

E. Bush, last updated 12/19/05

Virginia Cooperative Extension - Virginia Agriculture & Experiment Station