—by Ross Courtney
As the staple chemical thinner carbaryl comes under increasing scrutiny, alternatives are stepping in for the Northwest tree fruit industry.
Carbaryl has been a primary ingredient for fruitlet thinning for decades. It’s still registered, though public concern about pollinator health has growers and retailers looking for other solutions.
“Some folks would like to have more options,” said Lauren Gonzalez of G.S. Long Co., who helps the grower services company manage new product development.
Honey bees, and pollinators in general, have captured public sympathy as beekeepers wrestle with colony loss year to year. Carbaryl is not the biggest cause, but it is an insecticide. In January, the U.S. Environmental Protection Agency issued a draft opinion that discusses that concern.
Searching for alternatives now helps the industry brace itself, Gonzalez said.
“At least we’re prepared, because it takes a while to understand these new thinners with our increasingly extreme springs,” she said.
Concern over carbaryl
When carbaryl is applied at the label timing of 80-percent bloom or later, beehives should have been removed, said Tory Schmidt, a program manager for the Washington Tree Fruit Research Commission. But that doesn’t mean there aren’t a few blossoms that could still attract insects from a neighboring block or nearby habitat, Schmidt said.
A research paper co-authored by Washington State University entomologist Brandon Hopkins listed carbaryl as one reason tree fruit pollination poses the highest risk point in the yearly cycle of migratory honey beehives.
The project concluded that pollinating apple and cherry orchards in the Northwest in April and May exposes bees to more insecticides and insect growth regulators than when they pollinate almonds and seed crops, or when they produce honey in Dakota clover fields.
Most of the blame went to buprofezin, a growth-regulator-based insecticide used to control mealybugs, leafhoppers and pear psylla, but carbaryl was one of the insecticide residues detected during colony inspections. The researchers also linked brood mortality to carbaryl exposure but found no evidence of acute toxicity to adult bees during pollination.
Still, the results led Hopkins to suggest giving honey bees more of a time buffer before using carbaryl as a thinner, just to avoid one more stressor on colonies.
“Those bees have to survive for them to come back the next year,” said Hopkins, apiary and laboratory manager of the WSU bee program.
Alternatives emerge
Meanwhile, growers who rely on pollination have been showing more interest in alternatives. Schmidt has been running trials on apples.
Among the alternatives, metamitron holds the highest promise, he said.
Metamitron, originally developed as a herbicide for sugar beets, has delivered the most consistent, statistically significant reductions in fruit set, larger fruit and improved return bloom over 12 years of trials, Schmidt said.
“Because (the registration process) has taken so long, we’ve been able to work with it a long time,” he said.
Metamitron, which will be marketed by ADAMA US as Brevis, briefly stops photosynthesis. That starves developing fruitlets, the laggards of which the tree will then abort.
The product causes only minor phytotoxicity when used at rates the label likely will call for, Schmidt said.
Metamitron is already registered in more than 30 countries where carbaryl is not allowed, said Diane Reynolds, head of marketing for ADAMA US.
“While it is new to us here in this country and in this room, it is not new to ag as a global community,” Reynolds told growers in January during a session about carbaryl alternatives at the annual G.S. Long grower meeting in Yakima, Washington.
ADAMA expected registration sometime in February and its first commercial sales in April in Washington, Reynolds said, possibly in time for use this year. (Editor’s note: After this story was printed in the March 15, 2025, issue, the U.S. Environmental Protection Agency granted ADAMA US the registration for Brevis, which uses the active ingredient metamitron.)
Recommended application timing resembles that of carbaryl, Reynolds said, but the product offers flexible timing. It can be applied from petal fall to 20-millimeter fruitlet size and in temperatures ranging from 50 to 84 degrees Fahrenheit.
With the new product, the company also is launching BreviSmart, a model to help growers adjust rates and timing depending on weather and carbohydrate development.
Other carbaryl alternative candidates include ethylene promoter 1-aminocyclopropanecarboxylic acid — usually called ACC — and a new formulation of the plant growth regulator benzyladenine.
ACC, sold as Accede from Valent BioSciences, uses a natural, plant-derived metabolic precursor to ethylene. It is registered by the EPA and labeled for tree fruit.
Growers have thinned with ethylene products such as ethephon, but they can gas off before doing much good, Schmidt said. ACC is absorbed and metabolized, helping the plant produce its own ethylene, which promotes thinning and return bloom.
Schmidt only has a few years of trials with ACC and has yet to produce a successful result of his own, but trials elsewhere have him convinced it will work in Washington tree fruit.
Luke McKee of Valent told growers at the G.S. Long meeting that ACC is best known for its late thinning window, with a timing of 15–20 millimeters in fruit.
“This is a good thing to have in your back pocket,” he said.
However, Valent researchers are trying to find use patterns to make it work for earlier timing as well, McKee said.
Drew Hubbard of Fine Americas told growers about a yet-unnamed, improved formulation for the plant growth regulator Exilis 9.5 SC (6-benzyladenine), a plant growth regulator intended to work more consistently and in cooler weather.
“We found with our reformulation package that this product is more consistent, which means we’re having more consistency in cool weather,” Hubbard said. The company also expects registration this spring.
Benzyladenine, a plant growth regulator, works best when paired with other products, including carbaryl, Schmidt said. He reports success in thinning apples and pears with this new formulation from Fine Americas. •
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