ONTARIO, Ore. — An Oregon State University scouting program has confirmed onion thrips — tiny, winged insects that spread iris yellow spot virus in onion fields — are reproducing in Western Idaho and Malheur County, Ore., nearly a month early.
OSU Extension onion specialist Stuart Reitz fears thrip pressure will be especially high this season, increasing rates of iris yellow spot in growers’ fields.
Reitz began finding immature thrips on plants in mid-March — something that doesn’t usually happen in the region until mid-April. Reitz’s testing of volunteer onions hasn’t confirmed any virus yet this season, but he’s certain the disease, which causes yield losses and smaller onion size, will surface all too soon.
“This is the earliest I have seen reproducing thrips out in the field,” Reitz said. “We had a really mild winter increase their survivorship, and we had enough warm weather back in early February that the thrips were able to start reproducing again.”
The average February temperature throughout the area served by the National Weather Service’s Boise office was a record 43.9 degrees, 7.4 degrees above normal. The previous February record high was set in 1934, according to David Decker, program leader with the NWS Boise office.
“We’re also on pace to have the mildest winter in terms of average temperatures,” Decker said. “This is going to be the mildest February and winter for a lot of different sites around Idaho.”
Reitz monitors onion thrip populations with yellow stocky cards placed around four fields in Malheur County and two fields in Western Idaho.
Reitz said growers can limit their risk of iris yellow spot by controlling volunteer onions and weeds that help the insects and virus overwinter. He said growers may also plant varieties that better resist the virus, though they’re all susceptible to some degree, and implement thrip insecticide programs.
In OSU’s 2014 research trials, Reitz said the most successful pesticide programs reduced thrip pressure by 65 percent and increased yields by 30-35 percent over untreated check plots. Treated plots also had larger-sized bulbs.
Reitz said onions infected with iris yellow spot also tend to rot at higher rates in storage.
Paul Skeen, president of Malheur County Onion Growers, anticipates spraying a bit earlier than normal.
“(Growers) really need to watch the onions along the roadsides and the onions that are volunteers coming in the middle of the field,” Skeen said.
Skeen, who had already planted half of his onion crop as of March 19, said everything seems to be happening about two weeks early this season.
Nyssa, Ore., farmer Reid Saito has placed an emphasis on planting less susceptible varieties, noting dark-leaved varieties tend to attract more thrips.
“If the thrips come in early, then we will have to start treating a little earlier,” Saito said. “That could translate to more cost if we have to use an extra spray.”
Idaho and Oregon require growers to treat or dispose of cull onion piles by March 15 to control populations of onion root maggot. Saito said that policy has also helped to curb thrip numbers.