Factors
Propensity of pest populations to develop resistance is probably caused by a number of factors. First, pest species are usually capable of producing large number of offspring. This increases the probability of random mutations and ensures the rapid build-up in numbers of resistant mutants once such mutations have occurred. Secondly, pest species have been exposed to natural toxins for a long time before the onset of human civilization. For example, many plants produce phytotoxins to protect them from herbivores. As a result, coevolution of herbivores and their host plants required development of the physiological capability to detoxify or tolerate poisons. Thirdly, humans often rely almost exclusively on insecticides for pest control. This increases selection pressure towards resistance. Pesticides that fail to break down quickly and remain in the area contribute to selection for resistant organisms even after they are no longer being applied.
In response to pesticide resistance, pest managers may resort to increased use of pesticides, exacerbating the problem. In addition, when pesticides are toxic toward species that feed on or compete with pests, the pest population will likely expand further, requiring more pesticides. This is sometimes referred to as pesticide trap, or a pesticide treadmill, since farmers are continually paying more for less benefit.
Insect preys and parasites which live on other insects generally have smaller populations and are therefore much less likely to develop resistance than are the primary targets of the pesticides, such as mosquitoes and those that feed on plants. This can compound the pest problem because these species normally keep pest populations in check. But resistant predators of pest species can be bred in laboratories, which can help keep pest populations down.
The fewer sources of food a pest has the more likely it is to develop resistance, because it is exposed to higher concentrations of pesticides and has less opportunity to breed with populations that have not been exposed. Other factors in the speed with which a species develops resistance are generation time and fecundity (shorter generations and more offspring lead to resistance more quickly).
Read more about this topic: Pesticide Resistance
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