—story by Kate Prengaman
—photo by TJ Mullinax
—graphic by Jared Johnson
Flooding apple orchards with sterile codling moths can overwhelm the local population and reduce mating success, lowering fruit damage rates.
The practice provides the foundation for pest control in British Columbia, but as Washington growers have tried the technique over the past six or seven years, to supplement mating disruption and insecticide programs in high-pressure blocks, they encountered mixed results.
“We need to know when it’s a useful tool and when it’s a waste of money, but that answer is elusive,” said Jeff Allen, director of technical services at G.S. Long Co. “We don’t know enough to recommend it like we can recommend other things, but it’s too valuable of a tool not to offer.”
Deploying the sterile insect technique (SIT) effectively is a “numbers game,” according to Teah Smith, pest consultant for Zirkle Fruit Co. She has used sterile moths in the company’s organic acreage since 2018. “It’s really density-dependent for how long it’s going to take to clean up a problem,” she said. “It comes down to the density of the wild population and staying on your sprays.”
She points to Canadian research showing that a ratio of 40-to-1 — 40 sterile moths released for every one wild moth — will sufficiently overwhelm the wild population. Her own data shows that in blocks with lower wild moth populations, she sees greater reduction in damage when using SIT, while blocks with higher pressure take longer to improve.
“Codling moth damage is decreasing where the numbers are conducive to SIT,” she said. “The costs are high, but transitioning to conventional to clean up a problem is a loss, too.”
The challenge, according to Allen, is that most growers who would fall below the recommended 40-to-1 ratio already have decent codling moth control and aren’t looking to spend upward of $400 an acre.
“It’s mostly the people who are already spending a ton of money” and don’t have other options to achieve sufficient control organically, Allen said. “The existing standard only applies to customers who don’t need it, and customers willing to take the risk are gambling. Somewhere in the middle lies a threshold where you get value, but we need more research to know where that is.”
Different experiences across the border
When M3 Consulting Group began importing sterile moths from the production facility in the Okanagan Valley in 2019 and using drones to deliver them to orchards around Washington, customers asked for thresholds to guide deployment.
“All along, everyone has been asking for more feedback on a ‘label rate’ for how this works best,” said Dustin Krompetz, chief operating officer for M3. “We started with the Canadian base rate of 800 insects per acre and basically said, ‘Here you go, let’s see what it can do for you.’”
But the Washington use case is entirely different.
Conventional Canadian growers cleaned up orchards aggressively in the 1990s, before introducing SIT as an areawide control method for codling moth, originally intended as eradication. In Washington, organic growers, who are already struggling to control the pest with mating disruption and pesticide sprays, want to use it like a fire extinguisher.
The experts at the Okanagan-Kootenay Sterile Insect Release Program, which produces the moths, originally thought M3’s approach for Washington was likely to fail, Krompetz said, because research shows that SIT works best when the wild populations are low. Instead, “it got placed in the fires, and it passed.”
Not every time, he acknowledges, but growers found enough success that the acreage M3 supplies has gone up from 1,200 acres in 2019 to 4,000 acres in 2024. The company hopes to serve 8,000 acres next season — and potentially up to 20,000 — based on OKSIR excess capacity. About 80 percent of the acreage re-enrolls, he said, with some dropping off because they have cleaned up the problem and others because it didn’t work well enough for them.
Because Washington growers are using SIT to supplement mating disruption and spray programs, Krompetz believes that the 40-to-1 ratio is too conservative. Allen agrees, saying that G.S. Long’s customers succeeding with SIT rarely hit that ratio and, moreover, it’s a hard metric to assess.
“The overflooding ratios are based on trap counts, and trap counts are hard to get right,” Allen said, especially with mating disruption in the mix in Washington.
But for other customers who ended up frustrated after investing in the program and not seeing it move the needle enough, it can be hard to pinpoint why.
“We have learned a lot about the process but not enough to answer all of our questions,” Allen said.
Krompetz believes more guidance is hidden in the data of M3’s customers over the past six years, and that those records — in the hands of the right scientist — could shed light on what the appropriate application rates should be for supplemental use in organic apple blocks.
Smith agrees. “I have six years of data that somebody could analyze,” she said.
Guided by findings from such an analysis, Krompetz envisions M3 offering multiple rates for customers, depending on the pressure they face — although those rates would still have to be locked in ahead of the season, due to the fixed costs of sterile moth production.
OKSIR takes this approach for its growers, using double rates up to twice a week to combat hot spots, said Michelle Cook, general manager of the program.
Doing the moth math
In British Columbia, what they consider a high-density population might catch 40 moths a year, Smith said, while she’s had orchards catch over 400 a year. To understand how well SIT is working, she uses monitoring data and damage rates to evaluate its effectiveness in each block — or to determine whether the block is even a fit for SIT at all.
“It’s about the sweet spot where it’s needed, not where the odds are against it,” she said. In those blocks facing steep odds, she’ll recommend switching to conventional management to get it cleaned up.
In higher-pressure blocks, using SIT over one season reduced trap catch by 25 percent, but she still saw damage. In blocks trapping 100 wild codling moths or fewer in a year, SIT delivered significant catch reduction and almost no damage.
The price per acre is high, but Smith said that after several years of SIT, she has been able to significantly reduce virus sprays (the biological insecticide common in organic use), more than enough to cover the cost of the sterile moths. For context, in a bad block, she might recommend 20 virus sprays, but SIT could bring that down by half or more.
That said, she waits for the SIT to start working before she pulls back on sprays, and she keeps mating disruption in place.
“Yes, you have to put more money in at the beginning, but hopefully in three to five years, SIT is paying for itself, and you are using less virus and have less of a chance to build up resistance,” she said.
She’s also testing a “maintenance rate” by applying fewer moths in areas with less pest pressure. It’s showing potential.
However, reducing application rates doesn’t cut the price as much as you might expect. Krompetz said the moth production accounts for about half of the price; the other half is in the logistics involved in weekly import, transportation and application. As M3 scales up its services in Washington, he expects the logistics cost to go down.
“The biggest problem is cost; we see that,” he said. “I think we can shave 20 percent off the price as we get scale.”
Growers considering SIT for codling moth control need to be prepared for a long haul and to continue an aggressive approach with the rest of their codling moth program.
“It works, but it takes a couple years,” said Dain Craver, an organic grower. “I’ve cleaned up two spots using sterile release and a lot of Cyd-X and picking the fruit off, so it doesn’t go into the second generation.” •
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