Psyllids decline, but overwintering concerns arise
POCATELLO, Idaho — Researchers say the number of potato psyllids found harboring the Liberibacter bacteria that causes the crop disease zebra chip dropped significantly in the Pacific Northwest during 2014.
A new study out of Washington, however, has heightened concerns about resident psyllid populations overwintering in the Pacific Northwest.
Zebra chip, which causes bands in potato flesh that darken when fried, first arrived in the Pacific Northwest in 2011. By 2012, infections remained minimal as a percentage of the overall Idaho crop but ran as high as 15 percent in certain spud fields, said University of Idaho Extension entomologist Erik Wenninger.
In 2013, 33 of 1,093 psyllids captured through a UI monitoring program tested positive for Liberibacter.
Just 170 psyllids were captured in 2014, with four testing positive for Liberibacter. No infected spud plants were found.
Almost all of Idaho’s 2014 psyllids were of the Northwest haplotype, compared with 2013 when many Western psyllids were also found, suggesting to Wenninger that there may have been more “homegrown” psyllids this year, rather than insects blowing in from other regions.
Oregon State University tested 15,000 psyllids in 2014 — half of the 2013 total — finding 0.5 percent were positive for Liberibacter and “few plants in the field were symptomatic.”
Washington’s monitoring program confirmed only two Liberibacter-positive psyllids of 1,000 tested, said Joe Munyaneza, a research entomologist with USDA’s Agricultural Research Service in Yakima, Wash. Munyaneza led an experiment showing psyllids that feed on bittersweet nightshade — a common plant in the Northwest known to support the bugs between potato crops — seem to have greater cold hardiness than insects that feed on potatoes alone.
“We believe the bittersweet nightshade is conferring some cold-hardy property to the psyllids,” Munyaneza said.
Munyaneza’s team has also found a second psyllid host plant, called goji berry, or Lycium. His team continues to find psyllids surviving at five Washington goji berry sites.
Wenninger said Idaho’s monitoring program thus far shows psyllids arrive in Idaho during the second week of June, with numbers gradually increasing as the season progresses and higher populations found further west in the state, where temperatures are higher.
During the past two seasons, the network has included intensive weekly monitoring of 13 fields with vacuum samples and 10 sticky traps, plus light weekly monitoring of 75-94 fields using four sticky traps. UI has base funding through a federal grant to continue some level of monitoring, but has applied again for an Idaho Potato Commission grant to continue the exhaustive network.
“We’ve asked at several stakeholder meetings if people want us to keep sampling, and we’ve found pretty overwhelming support for it,” Wenninger said.
Other new psyllid research by Wenninger shows more than 90 percent infection in plants exposed to high densities of Liberibacter-positive psyllids at least seven weeks before harvest, compared with minimal infection by plants exposed closer to harvest. The study suggests to Wenninger growers may not be at especially high risk when psyllid numbers spike late in the season.
Other new research shows zebra chip infection increases in storage, but cooler temperatures can curb the rate of increase.