High pest population could pose problem if potato leafroll virus becomes established

By DAVE WILKINS

Capital Press

Green peach aphids, a vector for potato leafroll virus, are at it again in Idaho.

The aphids have been observed at the University of Idaho Research and Extension center at Kimberly and in the Rupert-Burley area, according to recent postings on the Pacific Northwest and Treasure Valley Pest Alert Network.

Green peach aphids are a concern to potato growers because of their ability to transmit potato leafroll virus, a disease that can cause net necrosis, a serious internal defect in tubers.

Growers in south-central Idaho were "hammered" by the disease about 15 years ago, said Phil Nolte, a seed potato specialist with the University of Idaho.

Outbreaks since then have not been as serious, mainly because of the use of systemic insecticides.

The insecticides have helped to reduce aphid populations, which in turn has reduced the presence of the virus.

"They have really made a difference," Nolte said. "We have been in this downward trend since about the mid-1990s since these systemic insecticides came into widespread use."

Researchers aren't sure whether aphid populations this year are any worse than normal. Even large populations don't pose a problem unless the insects are carrying the virus.

Yet, growers shouldn't let their guard down, Nolte said.

If the systemic insecticides for some reason begin to break down or the aphids began to build up resistance, the virus could re-emerge as a major threat to potato growers.

Producers such as organic farmers who don't use systemic insecticides, already "could be at some level of risk," he said.

The drawback to systemic insecticides is they are nonselective. They kill target species as well as the beneficial insects that feed on them.

Unnecessary insecticide applications should be avoided, said Erik Wenninger, an entomologist at the UI Kimberly center.

"While scouting for aphids, it's important to take note if you have these other beneficial insects. If you wait a little while, they may take care of the problem for you," he said.

Ladybugs, damsel bugs and parasitic wasps are a few of the beneficial species that can aid farmers in their battle against aphids.

Green peach aphids aren't the only aphids that have been spotted in Southern Idaho fields this summer.

Wenninger has found black bean aphids in sugar beet fields at the Kimberly center.

If natural enemies are absent, growers should consider an insecticide application if bean aphids are on most beet leaves and colonies cover 20 to 40 percent of leaf surfaces, he said.

Black bean aphids usually start to be noticeable in mid to late July. Their populations will continue to build up through August, so it's still difficult to know how much of a problem they will be this year, Wenninger said.

Online

Treasure Valley Pest Alert Network: www.tvpestalert.net