Resistance to pirimicarb insecticide suspected in Western Australia

Growers are warned that pirimicarb may fail to control green peach aphids (GPA), with preliminary tests indicating some Western Australian populations may have developed resistance to the important insecticide.

CSIRO entomologist Owain Edwards, who has conducted previous Grains Research and Development Corporation (GRDC) funded research into insecticide resistance in GPA (Myzus persicae), said molecular tests had provided strong evidence the species had developed resistance to pirimicarb.

As well, several growers had provided anecdotal evidence of control failures with pirimicarb in 2010, indicating resistance in GPA may already be widespread.

Dr Edwards said that if resistance was confirmed through further analysis, the tested populations from Kojonup and Moora would be Australia’s first known cases of pirimicarb-resistant GPA.

"Areas of New South Wales contain GPA with low levels of tolerance to pirimicarb, but the WA populations could have levels of resistance 100-fold higher than unaffected populations,” he said.

Dr Edwards said growers should be particularly mindful of the risk of pirimicarb failure if summer rain was received.

"Summer rain would allow a ‘green bridge’ of volunteer canola or weeds such as wild turnip or wild radish to grow, sustaining GPA populations and increasing the risk of aphids reaching high numbers in 2011,” he said.

"GPA reached high numbers in parts of the WA grainbelt in 2010, with drought-stressed plants being much more susceptible to aphid damage.”

Dr Edwards said confirmation of resistance to pirimicarb in Australian GPA would be a blow to pulse and oilseed growers in particular, as the insects favoured these crops, causing damage by direct feeding and transmitting viruses.

"Pirimicarb, from the carbamate group of insecticides, has until now been seen as a fall-back for aphid populations resistant to other chemical groups,” he said.

"Pirimicarb is also relatively inexpensive and compatible with integrated pest management (IPM) principles, as it is aphid-specific and less harmful to other invertebrates when applied to crops.”

Dr Edwards said molecular tests on the GPA suspected to be resistant had identified a mechanism associated with strong levels of pirimicarb resistance in GPA overseas.

"Follow-up biological tests are required to confirm this resistance, including its severity,” he said.

Dr Edwards said growers could reduce the risk of resistance by:

• Rotating insecticide classes;
• Avoiding the use of ‘insurance’ sprays;
• Applying insecticides only after carefully monitoring and correctly identifying pest species.

He said the risk of pirimicarb-resistant GPA spreading outside WA was potentially higher than for many other aphids because the species was associated with plants in the gardening and horticultural industries.

Information on managing invertebrates including aphids is available on the GRDC website at www.grdc.com.au/pestlinks

The GRDC is funding national research in to the use of integrated pest management (IPM) techniques in Australian cropping systems to help benchmark current pest management practices and increase knowledge of IPM principles.