COLUMBIA, Mo. — Although we may wish recent sub-zero temperatures and snow would have a positive effect by killing insect pests — such as emerald ash borer (EAB) larvae — it does not appear to be the case in Missouri. Most EAB, the tiny invasive green insect that is threatening the Show-Me-State’s ash trees, probably survived the extreme cold.
“It’s hard to estimate how many were possibly affected by the severe temperatures,” Missouri Department of Conservation (MDC) Forest Entomologist Rob Lawrence said. “A few EAB larvae probably were victims of the cold, but most will likely survive. And unless the weather has more serious impacts on larvae later this winter, we can expect EAB populations will continue growing.”
According to Lawrence, when temperatures fall below minus 10 degrees, some EAB larvae can be killed by the cold. And the colder it gets below that, the more larvae that are killed. However unlike humans, wintering larvae are not affected by wind chill. Air temperature is what matters.
EAB over-winters as larvae under ash tree bark and are naturally protected against the cold. Like most temperate climate insects, EAB produce a type of “anti-freeze” that helps them withstand very cold temperatures and ash tree bark provides some insulation, raising temperatures even a bit further for larvae underneath. The air temperature in most Missouri locations did not go below minus 15 in the recent cold spell.
The emerald ash borer, which originated in Asia, was first detected in Missouri in 2008 in Wayne County. It has now been found in 8 other counties; Bollinger, Butler, Madison, Perry, Pulaski and Reynolds counties in southern Missouri and in Jackson and Platte Counties in the Kansas City metro area.
Just like chestnut blight and Dutch elm disease before it, EAB is capable of eliminating its host tree, native North American ash trees, from forests and cities. This makes it one of the most serious environmental threats now facing North American forests.
Lawrence says it is too early to determine if the cold temperatures will affect other pest populations such as ticks, chiggers or mosquitoes. Those pests tend to overwinter in or near the ground, which can be warmer than winter air temperatures, especially when covered with an insulating blanket of snow.
According to the Sanborn Field weather station, which is located in Columbia, at 9:20 a.m. on January 6 air temperature was minus 6.7 degrees with wind chill putting the temperature we humans feel at minus 23.1 degrees. In comparison, the soil temperature two inches down was 31.8 degrees and four inches down, 32 degrees.
Lawrence said there could be more of an impact on insect’s survival rate if it gets warm for a couple of days and then the temperature drops severely.
“When we get a few consecutive warm days in January or February, around 50 or 60 degrees, some insects may lose their ability to withstand cold temperatures,” said Lawrence. “If then we get a hard freeze, that freeze could kill those insects.”
It’s a guessing game when trying to predict spring pest populations. What the weather does during the rest of winter will have a big impact. We will just have to wait until spring to see what develops.
For more information about the EAB in Missouri, visit mdc.mo.gov/node/5326.