What causes insecticide resistance in crop pests?

Insects can adapt to the plant protection products designed to control them. Learn what drives the development of insecticide resistance.

Naturally occurring mutations contribute to genetic diversity in all species. This diversity helps them adapt to their environment.

In the case of crop pests, this includes evolving resistance (partial or complete) to crop protection chemistry.

When an insecticide is first introduced, the pest it targets is usually controlled effectively, with few individuals carrying resistance genes.

However, pests with resistance genes are more likely to survive a spray. Any survivor can pass on the resistance advantage to their offspring (a heritable change).

Through continued application of insecticides with the same mode of action (MoA), selection for resistant individuals continues while susceptible individuals are eliminated.

When this occurs, pest populations become increasingly difficult to control at the label rate.

This often leads to more frequent applications of the insecticide, accelerating the development of resistance.

In some situations, cross-resistance can occur. This is when resistance to one insecticide confers resistance to another, even without exposure to it.

Resistance can develop relatively quickly, with the speed influenced by many factors, including:

• Pest reproduction rate
• Pest population size
• Pest host range
• Pest migration behaviour
• Presence of nearby susceptible populations
• Persistence and specificity of the plant protection product
• Plant protection product rate, timing, number of applications and method of treatment

Following the introduction of synthetic organic insecticides in the 1940s, it did not take long for insecticide resistance to be recorded. By 1947, resistance to DDT was confirmed in houseflies.

With every insecticide introduction, cases of resistance developed in key pest species two to 20 years later.

Resistance increases fastest in situations where pests reproduce quickly, there is little or no immigration of susceptible individuals and when pests are subjected to frequent sprays. This often occurs in protected cropping environments, such as greenhouses.

Additionally, some aphids in protected environments are thought to originate from more genetically diverse, sexually reproducing, populations on imported plant material.

Resistance is present in numerous crop pests, including aphids, whiteflies, thrips, beetles and spider mites.