The green peach aphid (GPA; Myzus persicae) is a widespread pest of horticultural and broadacre crops. Management of this aphid is an ongoing challenge which is exacerbated by the widespread occurrence of insecticide resistance. Globally, GPA has been confirmed resistant to at least 80 insecticides from a variety of chemical groups.

In Australia, it has evolved high-level resistance to synthetic pyrethroids and carbamates, and low-level resistance to organophosphates and neonicotinoids. Quite recently, low-level resistance to sulfoxaflor has also evolved in some Australian field populations.

Australian research has also now found populations of GPA with resistance to spirotetramat, the active ingredient in Movento® 240 SC Insecticide, as well as several generic products containing the same active ingredient. Spirotetramat, a Group 23 insecticide, is an important tool in GPA management in vegetable crops in Australia.

Resistance has been confirmed in a small number of GPA populations collected from Queensland vegetable crops and, importantly, this resistance has been shown to persist after multiple generations of culturing in the laboratory, demonstrating there is an underlying genetic basis.

This discovery was made through collaborative research between Cesar Australia and the University of Exeter (UK), with investment from Bayer Crop Science and GRDC.

Spirotetramat resistance in GPA has not been found in any other country, despite this product being widely used to manage GPA in a range of crops around the world.

Spirotetramat is not registered for use in Australian grain crops and so this new resistance is not of immediate concern to grain growers. However, it serves as an important reminder of the ease with which some species, such as GPA, are able to evolve genetic modifications which limit the effectiveness of chemical controls.

The evolving challenge of GPA

Management of GPA is an evolving challenge as increasing resistance means growers are left with fewer control measures to minimise crop damage.

In the grains industry, canola is the crop most at risk from GPA. When conditions are favourable, high aphid population levels can build up and inflict serious damage, particularly when plants are moisture-stressed. However, the greatest risk comes from GPA being the major vector of turnip yellows virus (TuYV), which can inflict serious yield losses when infections occur before the rosette stage in canola.

When using insecticides to control GPA, it is important spray booms are set up to optimise coverage and deliver the chemical to where the aphids are located. Growers should also pay attention to dust and honeydew on the leaf as these factors can all reduce product performance. High water volumes, correct nozzle selection and appropriate ground speeds will help to ensure that the product provides the expected efficacy. It is also critical that the full label rates are used, as lower rates will often be insufficient and only serve to increase selection pressure for further resistance.

Understanding the activity of the chemical product is also important. For example, most insecticides exhibit only acropetal movement in the plant (they move upwards in the xylem or stem) and do not translocate downwards. When using these products, applications should be made before crop canopy closure in order to control GPA populations colonising older canola leaves.

Non-chemical strategies

In order to limit the spread and restrict further evolution of resistance, it is important to manage GPA within the framework of an Insecticide Resistance Management Strategy.

In regions with high TuYV risk, the following strategies should be considered:

• Eliminate the green bridge, particularly volunteer canola, wild radish, wild turnip and marshmallow, a minimum of 14 days before sowing;
• Do not sow crops early in the season; sow on usual dates, to avoid early aphid flights; and
• Where feasible, sow into standing stubble and use agronomic practices to minimise bare ground at crop establishment to reduce aphid landings.

Promoting beneficial insects, such ladybird beetles, lacewings, hoverflies and parasitic wasps, can also effectively suppress GPA populations and negate the need to spray at all. When chemical intervention is required, use products that are less harmful to non-target species. The grains industry is fortunate to have several new aphicides that are less toxic to beneficial insects compared with conventional broad-spectrum chemicals.

Useful resources

Growers are encouraged to follow recommendations outlined in the GPA IRMS developed by the National Insecticide Resistance Management working group. Although updates to this strategy are needed, it remains the best source of information for growers.

GRDC has produced the Green Peach Aphid – Best Management Practice Guide.

A new tool is also available that enables growers and advisers to understand the toxicity of insecticide sprays on beneficial insects which play an important role in pest control.

Further details can be found using Cesar Australia’s beneficial chemical toxicity table.

If growers have chemical control difficulties or suspect they have resistant GPA, they can contact Cesar Australia (03 9349 4723) for advice and/or to request a free insecticide resistance test.

Acknowledgements: Bayer Crop Science. Lisa Kirkland, Anthony van Rooyen, Evatt Chirgwin, Aston Arthur, Samantha Ward, Karyn Moore, Marielle Babineau, Chris Bass, Adam Pym, Jo Mackisack, Andrew Mathews.