# Resurgence (pest)

Pesticide induced resurgence, often shortened to resurgence in pest management contexts, can be described as a constraint of pesticide use, by which they fail to control pests such as insects and spider mites: instead ‘flaring up’ populations that may have been of minor importance. Although there are more than one mechanisms by which this takes place, mortality of natural enemies following the use of broad-spectrum insecticides and acaricides is often implicated. This is sometimes called the “pesticide treadmill”: a term coined by Robert van den Bosch to describe a self-reinforcing over-dependence on agrochemicals and inimical to natural biological controls. Notable examples include the flare-up of rice brown planthopper populations, following over-use of broad-spectrum inscticides.

## Causes and mitigation

With an increasing population, world food production has been subject to intensification and since the 1950s, the use of chemical pesticides. Examples are included below, but because of its importance to rice, outbreaks of the brown planthopper were intensively studied. The differential mortality of the pests and their natural enemies (spiders, parasitoids, predatory bugs, etc.) was a major factor causing outbreaks. However, there were other resurgence factors such as increased pest fecundity due to increased protein content of the rice phloem, on which the insects feed.
Sustainable intensification encompasses Integrated pest management (IPM) practices, which are widely recognised as an appropriate strategy for, not only reducing financial costs to farmers and growers, but also managing technical constraints associated with pesticide resistance, resurgence and the risks of high crop residues. An obvious solution is to reduce or eliminate pesticide use, but where pest pressure is high (e.g. with many tropical crops), this is not always feasible: but various measures can be taken, including better timing, application and the use of less deleterious products such as biopesticides.

## Examples
### Cottony cushion scale Icerya purchasi

Management of Icerya purchasi populations is a well-known early success for classical biological control, after introduction of the vedalia ladybird in the late 19th century. However, crop spraying with DDT and organophosphate insecticides in the 1950s, resulted in high vedalia mortality and cottony cushion scale resurgence; this was especially caused by drift of malathion or DDT applied by airplane during the early spring months.

### Spider mites

First generation insecticides such as DDT were often replaced by synthetic pyrethroids for agricultural and horticultural use by the end of the 1970s, due to their relatively low persistence and mammalian toxicity. However, their broad spectrum of activity varies for both spider mite control, and their ability to invoke resurgences of different spider mite species on various plants. Pesticides may be both lethal and repellent to Phytoseiids and other spider mite predators. After synthetic pyrethroids applications, spider mite development is shortened, the sex ratio can becomes more biased towards females and onset of winter diapause is possibly delayed.