Populations of onion thrips are on the rise in southeastern Oregon and southwestern Idaho.

“With later planting this year, things started a little bit slower,” Oregon State University Malheur Experiment Station Director Stuart Reitz said in an interview. But by the end of June, populations were near year-ago levels.

In a June 30 update for the Pacific Northwest Pest Alert Network, he reported strong pressure from thrips, including increases in immature and adult insects in most parts of the region. Numbers varied among fields. Monitoring is set up near seven towns.

Thrips transmit Iris Yellow Spot Virus, which Reitz reported was found in fields near Ontario and Nyssa, Ore., and Fruitland, Idaho, as of June 28. The highest levels were around 10% in a couple of fields.

Infected plants have been more abundant closer to field edges, suggesting most virus transmission at the time was from adults dispersing into fields. He said that would change later in the season if immature thrips survive on infected plants and then transmit the virus to other plants within a field. He recommended growers spray at sufficient volumes and pressures to get insecticides down into the necks of plants.

The virus stresses the onion plant. When thrips feed in greater numbers or carry more of the virus, the plant’s photosynthetic ability drops and the onion is smaller than it could have been otherwise, Reitz said.

Overall, though, “the onions look pretty good,” said Malheur County Onion Growers Association President Paul Skeen, who farms near Nyssa. Summer’s cool start minimized plant stress.

As for thrips, “they’re always hard to kill. You’ve got to stay with your program,” he said July 2, when he was preparing to start a fourth spraying of the season.

Onions planted later than normal aren’t as big, and thrips usually go to bigger plants, Skeen said.

Reitz said thrips historically build through June, and then peak as July unfolds and onion plants start to mature. Pressure usually drops around the the end of July or the start of August as plant tops fall and leaves become less suited to thrips.

“We still have a few more weeks of dealing with thrips in the crop overall,” he said July 1.

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