Insecticide resistance is a reduction in the susceptibility of a pest population to a particular insecticide or insecticide group. The change is heritable, and natural selection can result in a rapid increase in the proportion of the population with resistance, leading to inadequate control of the pest at the insecticide’s registered rate.
Field ‘spray failures’ can also be due to inadequate coverage or other application issues, including water quality or incompatible product mixes, so it is essential for future management decisions that if resistance is suspected, surviving populations are tested to confirm their status.
Read the latest results from resistance surveillance of helicoverpa.
Links to results for other pests, including mirids, mites, cotton aphids and SLW can be found in the Further information section.
The rise of resistance in populations
In insect populations there is always a small proportion of adaptive mutants which carry genes that make them less sensitive to insecticides. Continued use of an insecticide or insecticide group with a common mode of action (MoA), over several generations will select for individuals with resistance (selection pressure). These resistance genes may have initially been at low levels in the population or in some cases can arise as the result of a random (de novo) mutation. Individuals with resistance traits are more likely to survive and breed after exposure, increasing the potential that a high proportion of the following generation will have that trait. Eventually, in the presence of ongoing selection, enough resistant individuals survive that the pesticide application fails to reduce the population to below damaging levels. This is known as practical resistance.
Types of resistance
There are several different mechanisms of resistance. These can require different management approaches.
|Metabolic||Biochemical over-expression: increased production or activity of detoxifying enzymes that degrade/detoxify the pesticide.||The most common form of resistance, often involving multiple gene clusters. Usually inherited as a dominant trait, meaning it is more easily selected for by pesticide exposure.|
|Target site||Point mutation in DNA producing decreased sensitivity at target site: pesticide interaction is blocked or impaired.||Generally conferred by recessive genes and is therefore more suitable for dilution strategies (see management information below).|
|Physical||A physical adaptation that improves protection (e.g. thicker cuticle or faster excretion).||Provides less protection than internal resistance mechanisms, but can improve chance of survival.|
|Behavioural||A behavioural adaptation that avoids the insecticide or helps reduce exposure (e.g. hiding in more sheltered areas).|
Cross-resistance occurs when resistance to one insecticide confers resistance to another insecticide(s). In most cases, this occurs within the same MoA group, although there may be differences in the level of resistance between active ingredients or even different formulations. Occasionally, insecticides that don’t have the same MoA but do have similar molecular structures may be metabolised by a single enzyme. In these circumstances, metabolic resistance may be present for all the products.
Multiple resistance is the presence of two or more resistance mechanisms in the same insect. While less common than cross-resistance, it is much more concerning as it can significantly limit chemical control options.
Tolerance to insecticides is a natural tendency of some species or life stages to be less susceptible for a variety of reasons, including body size, relative waxiness, metabolic processes or behavioural tendencies. In many cases, the reasons for differences in tolerance levels are unknown.
Insecticide resistance management strategies
Management strategies usually aim to minimise the frequency of resistance in a population by:
- Using alternative control methods to kill off resistant individuals with something they do not have resistance to. This includes rotating insecticides from different MoA groups and using cultural or physical control methods within and between crops.
- Diluting the resistant population with susceptible genes to minimise the chance the trait is passed on. This approach targets traits linked to recessive genes where offspring require recessive resistance genes from both parents for the resistance to be expressed. Untreated refuges are used to build up susceptible populations, that mate with resistant individuals, reducing the chances of two resistance genes being inherited by the same individual.
Approaches to pesticide use that minimise the risk of resistance developing:
- Use economic thresholds to minimise insecticide use – do not apply ‘insurance’ sprays
- Delay the use of broad-spectrum insecticides for as long as possible as these products reduce beneficial populations which can lead to flaring of other pests and place further selection pressure on pest populations as a result
- Adhere to industry recommended application windows to limit exposure to any one MoA group
- Avoid the consecutive use of any one MoA group (including following a seed treatment with the same MoA foliar) – rotate between groups where possible and do not exceed the maximum number of sprays stated on the label
- Do not respray an apparent failure with a product from the same MoA group.
Insecticide surveillance of resistance levels in major pests occurs in both the grains and cotton industries and are made available in industry reports and publications. Key pests tested include helicoverpa, mirids, mites, cotton aphids and sliverleaf whitefly (click on the pest to go to the latest surveillance update).
The grains industry has several resistance management strategies for individual pests (along with a description of the science behind them) that can be accessed at the IPM Guidelines for Grains website, including helicoverpa, green peach aphid, red legged earth mite, and diamondback moth.
See also Insecticide resistance in the southern region (2019, GRDC)
Cotton has an industry-wide IRMS that considers several of their major pest species. It is revised annually and printed in the Cotton Pest Management Guide.
While most resistance management strategies are voluntary, it is strongly recommended that they are referred to whenever a spray decision is made. Some strategies are a compulsory part of user agreements for specific crop technologies. For example, growers of Bollgard® cotton must adhere to the technology provider’s Resistance management plan (RMP).