There’s bad news for Northwest pear growers: Pear psylla, just like spider mites, are showing alarming levels of resistance to the industry’s wide array of insecticides.
“Our production systems are on the brink of field failure,” Washington State University entomologist Elizabeth Beers wrote in her final report for the Northwest Pear Research Review in February in Hood River, Oregon.
The update didn’t exactly shock Beers or growers, who have been warned about increasing chemical resistance of weeds, insects and fungi for decades.
Psylla specifically have been showing resistance since 1965 when researchers determined the chemical Morestan had become less effective against the pests.
“Psylla have had a long and proud history of becoming resistant to everything we throw at them,” Beers said at the meeting.
However, Beers’ presentation — on behalf of four other researchers involved in the project — painted a pretty grim picture of results from a $48,700 resistance study in 2014 that showed the six pesticides currently used to combat pear psylla are only partially working, if at all.
“All of the insecticides tested produced low or moderate mortality on the average in winterform adults,” Beers said.
The researchers tested six insecticides at various rates, including the maximum label rates, for winterform psylla resistance. Beers suggested caution interpreting the results.
Only the two pyrethroids — Pounce (permethrin) and Warrior (lambda-cyhalothrin) — target the standard test stage of psylla, winterform adult.
Piperonyl butoxide, or PBO, an adjuvant normally used with pyrethroids to help overcome resistance, was not used in the bioassays, which could have altered the outcome.
They came up with the following results:
—Three of the six insecticides, those with historical data for comparison, showed increased resistance over time. They were AgriMek (abamectin), Pyramite/Nexter (pyridaben) and Provado/Admire (imidacloprid).
—The remaining three insecticides — Delegate (spinetoram), Pounce and Warrior — caused only moderate mortality of the winterform adults, which may be a sign of resistance.
—Delegate and Nexter caused the highest rate of psylla mortality, but still only killed about half. In addition, one population of tested psylla, taken from a Washington orchard, had very low mortality from both Delegate and Nexter, possibly a sign of cross resistance.
Transcriptome analysis, studying genetic markers of psylla RNA, of the populations may provide more insight into the mechanisms of resistance.
What’s more, while psylla are showing resistance to insecticides, many of their natural predators are not, meaning the chemicals may be doing more harm than good, Beers said.
Beers, an entomologist at the Tree Fruit Research Center in Wenatchee, Washington, was a collaborator on the project.
The principal investigators are Tom Unruh, an entomologist who retired from the U.S. Department of Agriculture lab in Wapato in October last year; Peter Shearer, an entomologist who resigned in 2016 from Oregon State University’s Mid-Columbia Researcher and Extension Center in Hood River; Richard Hilton, an entomologist with Oregon State University’s South Oregon Research and Extension Center in Central Point; and Joanna Chiu, an entomologist at the University of California, Davis.
The work involved psylla collected from 20 orchards in Washington and Oregon, Chiu’s lab in California analyzed the transcriptomes, studying genetic markers of psylla RNA, that may provide more insight into the mechanisms of resistance in the future.
The project started in 2014 but the researchers had asked for no-cost extensions, Beers said.
At the research review, Beers followed her pesticide resistance presentation with a similar report on miticide resistance. “Otherwise known as more bad news,” she said to a chorus of chuckles.
Unfortunately, Beers and her colleagues don’t have any simple advice for how growers should react to this bad news.
She and others have more projects underway, but she suspects the industry will have to adopt softer integrated pest management programs.
“We have repeated anecdotal evidence that organic programs work, but they are not easy to manage,” she said. “My colleague John Dunley coined the term ‘organic-ish.’ It is still an apt description of where we need to head.” •
– by Ross Courtney