While the recent cold snap may have killed some emerald ash borer larvae, arctic outbreaks in Iowa don’t last long enough or come often enough to seriously impede the spread of the tree-killing pests, state forestry officials said Tuesday.
“We will see some mortality, probably less than 10 percent,” said Tivon Feeley, forest health specialist for the Department of Natural Resources.
But even if larva mortality reached 50 percent, “the population would bounce right back within a year,” State Forester Paul Tauke said.
In Minnesota, where arctic outbreaks are colder, more frequent and last longer, they do inhibit the spread of the emerald ash borer, according to Lee Frelich, director of the University of Minnesota Center for Forest Ecology.
In parts of northern Minnesota and North Dakota, frequent, protracted cold spells may actually establish a zone in which ash trees can survive, Frelich said.
“We’re outside that zone in Iowa,” Feeley said.
Citing recent research conducted in Minnesota, Frelich said 5 percent of emerald ash borer larvae die when exposed to a temperature of 0 degrees Fahrenheit. The mortality rate increased to 34 percent at 10 below, to 79 percent at 20 below and 98 percent at 30 below, according to that research.
However, the exposure must last at least several hours to overcome the insulating effect of the bark beneath which the larvae live.
That is not a problem in northern Minnesota, where at International Falls, the temperature dropped below zero at 9 a.m. Saturday and will stay below well into Thursday, according to National Weather Service meteorologist Dean Melde. Lows at the Falls, he said, were 19 below Saturday, 31 below Sunday, 30 below Monday and 26 below Tuesday.
In Iowa, by comparison, “it was cold but not super cold,” said State Climatologist Harry Hillaker. Low temperatures ranged from 11 below at Clarinda in southwest Iowa to 24 below at Cresco in northeast Iowa, both on Monday morning, he said.
At Dubuque, one of the coldest reporting stations, the temperature stayed below zero for about 45 hours and remained cooler than 10 below for about 34 hours, Hillaker said.
Feeley said the green ash and white ash common in Iowa are more cold resistant than the black ash, upon which most of the Minnesota research is predicated. “They have thicker, better insulated bark,” he said.
Because adult emerald ash borer beetles die before winter, extreme cold affects only the larvae, he said.
Tauke predicted last month that all but a minute fraction of Iowa’s 55 million ash trees will be dead within 20 years.
In areas like Fairfield, Burlington, Mechanicsville and Creston, where infestations have been documented last year, all but a few lingering ash trees will be gone within five or six years, he said.Following an initial Iowa confirmation in Allamakee County in May 2010, three years passed before the burst of confirmations in 2013: Des Moines County in July, Jefferson County in August, Cedar County in October and Union County in December.