University of Idaho reports light potato psyllid pressure

Pressure from the tiny, winged insects that spread zebra chip disease in potatoes has been light in Idaho this season, emboldening some potato farmers to scale back on their pesticide programs.

Zebra chip, which is caused by the Liberibacter bacterium and spread by potato psyllids, first arrived in the Pacific Northwest in 2011. The disease creates bands in tuber flesh that darken during frying, rendering spuds unmarketable.

University of Idaho Extension entomologist Erik Wenninger, who runs an extensive field monitoring program to gauge psyllid populations, said the first psyllid of this season was captured in late May — about the same timing as last season.

However, a single psyllid, captured at UI’s Kimberly Research and Extension Center, has tested positive for Liberibacter this season.

Furthermore, Wenninger said, the scouting network, which includes yellow sticky traps surrounding 96 potato fields throughout the state, has captured five to seven psyllids per week, compared with more than 50 psyllids per week last season.

Wenninger said zebra chip was found in a few fields last season, but the incidence of the disease was relatively low, especially given the high psyllid densities.

Erik Wenninger:

“We had a lot more psyllids at this time last year.”

“We also notice lighter pressure from aphids and Colorado potato beetles.”

Potato grower Doug Gross - located in Wilder - estimates he spent $230 per acre spraying insecticides to protect his crops last season. Based on the light pressure from both psyllids and aphids this season, he’s already skipped two insecticidal sprays that he’d made by this time last season.

Doug Gross:

“As a cost-saving measure, we are cutting back our insecticide program.”

“We will still start spraying at some point in time when we reach a threshold, especially if we see any (Liberibacter) positive psyllids.”

Mark Darrington - a grower in Declo - acknowledges psyllid pressure is down, but he’s continued with his usual spraying program, believing it would be too risky to scale back.

Darrington said early season sprays are more affordable because they can be mixed in the same tank as fungicides, he can use chemicals that are prohibited as harvest approaches and he doesn’t have to switch to products that might cost more to avoid chemical resistance.

Mark Darrington:

“Later in the season, if they’re not finding enough hot psyllids.”

“I won’t make additional sprays of the more expensive products.”

Jeff Miller, with Rupert-based Miller Research, recently captured a psyllid on a sticky trap near a research field, but it didn’t test positive for zebra chip. He’s prolonged spraying insecticides on fields in which he used an insecticidal seed treatment at planting.

Jeff Miller, Miller Research:

“I think you would be wise to wait (on applying insecticides), and part of that is driven by the economy.”

“I don’t think it’s time to worry about zebra chip just yet. Keep watching the results (of the monitoring program) and see what we find.”