Pest control - Moth resists marker plan

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No simple solutions: larvae of the diamondback moth.

By Alec Nicol

Speculation that localised resistance patterns in populations of diamondback moth are an indication of discreet genetic variations has been ruled out by new research.

It puts an end to the idea of developing genetic markers to identify different populations and targeting them with chemicals for which they have no resistance.

Photo: No simple solutions: larvae of the diamondback moth.

Instead, the problem looks far more complex, and may require an integrated pesticide control program across a range of broadacre and horticultural industries.

The diamondback moth is a serious pest for canola, forage Brassica crops and intensive vegetable crops. It has a demonstrated ability to quickly develop pesticide resistance.

Ms Nancy Endersby with colleague, associate professor Steve Mckechnie.Nancy Endersby from the Centre for Environmental Stress and Adaptation Research (CESAR) at Monash University, has just completed a survey of 17 populations collected in Australia and overseas.

While she encountered significant variation between populations in South-East Asia, Africa and Australia, she found no variation between populations within Australia or between Australian and New Zealand populations.

Photo: More data needed: Ms Nancy Endersby with colleague, associate professor Steve Mckechnie.

And she points out that there is a lot of movement going on all the time, even across the Tasman.

She believes localised variations in resistance are the result of concentrated use of a particular chemical in particular areas.

"Very high levels of resistance to pyrethroids developed in the moth in vegetable crops in the 1990s," she says. "These chemicals are no longer used extensively in these areas and the level of resistance is being diluted by an inflow from the major gene pool. That"s why routine spraying for the pest can"t be recommended.

"The more a chemical is used, the higher the background levels of resistance will creep."

Ms Endersby says any new, effective chemical registered for use in high-value horticultural industries could be put at risk by indiscriminate use in broadacre crops.

For this reason, she suspects canola growers may have to tolerate some level of damage.

She says there is a need to determine the threshold that would spark a multi-industry control effort, plus a need for more data on the insect"s movements and life cycle on the different host plants to improve the forecasting model.

"We also need to survey the level of the moth"s natural enemies in canola crops," she says. "It may be that a control strategy will call for the introduction of natural enemies and the provision of a refuge for them between seasons."

Overseas, Brassica crops including canola have been genetically modified to incorporate Bt toxins.

"Brassicas have proved to be relatively simple to modify in this way," says Ms Endersby.

"Bt broccoli varieties have been engineered in the US and there are experimental Bt canola varieties available, and work is progressing to develop the best pyramid of Bt toxins.

"However, use of these varieties in Australia would require careful husbanding to prevent the development of Bt resistance in the insects."

In the meantime, work is continuing on a quick test for the level of resistance in particular moth populations using molecular methods.

For more information:
Nancy.Endersby@sci.monash.edu.au

GRDC Research Code: UMO 0002, program 3