The chance discovery of diamondback moth in canola-growing areas with resistance to several new insecticides is causing concern among growers and advisers; researchers are seeking to determine if it was just an anomaly or whether something more alarming has occurred
Very high levels of synthetic pyrethroid resistance and significant Affirm® and Group 28 resistance were found in two unrelated DBM samples provided to SARDI entomologist Greg Baker.
PHOTO: David Cappaert, Michigan University
When South Australian Research and Development Institute (SARDI) entomology leader Greg Baker received two diamondback moth (DBM) samples for resistance testing in September 2013, he did not expect to find substantial insecticide resistance to any of the new DBM insecticides.
The samples were from Nundroo, near Ceduna on the far western Eyre Peninsula, SA, and from York, Western Australia.
“Canola is not a big part of the crop rotation in Nundroo. The agronomist who supplied this sample wanted to know whether synthetic pyrethroids (SPs) would work against the pest,” he recalls.
The York sample came from a trial being run by Agrisearch. The trial coordinators took the opportunity to screen for insecticide resistance in DBM.
SARDI tested the DBM sample with the SP alpha-cypermethrin, as well as Affirm® (emamectin benzoate) and Success Neo® (spinetoram), which were recently registered for use in canola, and a Group 28 diamide, which is registered for the control of DBM only in vegetable crops.
To his surprise, Mr Baker found high levels of SP resistance and significant Affirm® and Group 28 resistance in the two unrelated samples.
“We did not expect that,” he says. “We were working on the assumption that SP resistance in the Nundroo sample would be low, and that there would be no significant resistance in either the SA or WA samples to the three newer chemistries because of their very limited or nil use in these districts.
“We had to discount the possibility that we had made a mistake in the testing, which meant it was telling us something very interesting.”
Early in 2014, Mr Baker and his team repeated the tests with the same field samples reared on in the lab, and found the results were the same. The findings mean there has already been a significant shift in resistance to both Affirm® and the Group 28 chemistry.
“These results are concerning,” Mr Baker says. “It doesn’t mean canola growers will immediately experience spray control failures with Affirm®, but it does tell us a resistance shift is underway, which was unexpected at this early stage in the uptake of Affirm®.
“Affirm® has been viewed as a saviour because up until now it has provided good levels of control,” he says. “And there are no other new chemistries in the immediate pipeline.”
The cause of the resistance shift is a mystery to the investigating team. Mr Baker thinks there are two possible explanations: either resistant population movements across large distances from vegetable production areas, or a cross-resistance to emamectin benzoate and Group 28 diamides being conferred by existing SP resistance.
Fortunately there was no evidence of resistance to Success Neo® or Bacillus thuringiensis, which are the other two DBM insecticides available to canola growers.
DBM is a sporadic pest. In most seasons it is of no great consequence. However, in drier winters accompanied by higher daily maximum temperatures, which may occur once every three to five years, significant damaging outbreaks can occur.
Although DBM is not a target pest for grain crops, some brassica weeds and volunteer canola may harbour the pest in low numbers. When this occurs, DBM can be unintentionally and repeatedly exposed to generalised insecticide sprays on non-canola crops.
Mr Baker believes that a similar situation may be arising in DBM as has occurred with another off-target pest, green peach aphid, which has now developed resistance to more insecticides than any other pest species.
“Green peach aphids are not deliberately sprayed in grain crops but they are often unwittingly present and therefore exposed,” he says. “Simultaneously they are getting direct selection pressure from horticultural spray programs.”
Mr Baker stresses that much more work is needed to understand the extent of the issue.
“The weak link in the investigation at the moment is the very limited number of initial samples,” he says. “During 2014, we are keen to hear from agronomists and growers who have DBM developing on their property so we can collect more samples and conduct further testing.”
The situation that has arisen with DBM provides a timely reminder to growers to avoid the use of prophylactic ‘insurance’ sprays as a pest management strategy. Exploring integrated pest management options and spraying only when necessary extends the useful life of available chemistries.
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