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New research points to lack of insecticide resistance in lucerne flea

Lucerne flea is often at risk of off-target effects from insecticide application. But - to date - researchers have not found the pest has developed resistance.
Photo: cesar

Lucerne flea (Sminthurus viridis) is a pest thatcan be found in most grain crops and pastures in southern Australia.

Due to its widespread distribution and frequent high abundance, it is often at risk from off-target effects by insecticide application.

Research undertaken by cesar and the University of Melbourne has tested for insecticide resistance in populations of lucerne flea and not found evidence of this to date.

This finding was made by screening nine lucerne flea field populations in 2018 for resistance to omethoate and imidacloprid - widely used organophosphate and neonicotinoid insecticides in grain crops.

Populations were collected in the field from cropping locations across Victoria, New South Wales and Western Australia and brought back to the lab for screening.

Researchers performed bioassay experiments to establish the baseline sensitivity of each lucerne flea population to omethoate and imidacloprid.

No resistance found

Fortunately, no resistance to organophosphates or neonicotinoids was found in lucerne flea populations screened.

This is good news, particularly at this time of the year - as April to July is a critical monitoring period for this pest.

Lucerne flea over-summering eggs will have started hatching in March and April.

Crops should be inspected frequently and special attention should be paid to crops immediately after emergence, as plants are most susceptible to damage during this growth stage.

Evidence of lucerne flea feeding damage will manifest as windowing on vegetative parts of the plant, or small pieces of missing tissue.

If spraying for lucerne flea is necessary, target late-stage nymphs and consider spot spraying to reduce numbers - as the pest is often distributed patchily within crops.

Do not use synthetic pyrethroids, however, as lucerne flea has a natural tolerance to this chemical active group.

Selection pressure

Lucerne flea causing damage to a clover leaf. PHOTO Gus Skinner

Lucerne flea causing damage to a clover leaf.
Photo: Gus Skinner

These research results are good news for growers who find lucerne flea in numbers that may warrant a spray treatment.

It is also good news for the story of insecticide resistance in Australia, as there have been increasing concerns about pest species evolving resistance to insecticides - particularly very common species, such as lucerne flea, which may be subjected to regular insecticide selection pressure even though they may not always be the target pest.

A 2019 GRDC report into insecticide resistance in the southern region, produced by cesar in collaboration with the South Australian Research and Development Institute and the Birchip Cropping Group, lists lucerne flea as a pest that has a high risk of evolving insecticide resistance.

Several agricultural pests in Australia are already resistant to many chemical actives, including:

  • redlegged earth mite;
  • green peach aphid;
  • diamondback moth; and
  • cotton bollworm.

As an example of how strong resistance can become in a population, some redlegged earth mite populations have been found to display a resistance factor to synthetic pyrethroids that is about 200,000 times the resistance of susceptible populations.

Instances of insecticide resistance have been getting more common in recent decades, both in Australia and worldwide.

Therefore, it is important that growers minimise insecticide resistance selection pressure placed on pests such as the lucerne flea to preserve the effectiveness of insecticides registered against them.

Preserving insecticides

To minimise the selection pressure placed on the lucerne flea and reduce the risk of resistance evolving, growers are urged to avoid using synthetic pyrethroids - due to lucerne flea's natural tolerance to these insecticides.

Growers are also encouraged to be mindful of the use of organophosphates, neonicotinoids and other insecticides when trying to control pests - as non-target effects can increase selection pressure on commonly found species, such as lucerne flea, and may lead to the evolution of resistance.

If it is necessary to control lucerne flea with insecticides, spray at the correct label rate and make sure to rotate insecticide Mode of Action groups. This will help preserve the effectiveness of insecticides for controlling this pest.

If you suspect a new case of insecticide resistance, contact cesar on 03 9349 4723. Insecticide resistance management strategies are available for several pest species and can be found on the GRDC website.

GRDC Research Code UM00057

NOTE: This work was led by Dr Aston Arthur (cesar), with financial support from GRDC. Western Australian collections of lucerne flea were made by the WA Department of Primary Industries and Regional Development (DPIRD). Project partners are CSIRO, the University of Melbourne and DPIRD.

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