Here's one resident who may welcome the extreme cold wave headed for Minnesota. Your local ash tree.

The extreme temperatures moving in with Sunday's arctic blast may kill off a significant percentage of emerald ash borer larvae, according to one of the premier forestry experts in Minnesota.

Fig. 1. Location ofUnitedStatesandCanadaweatherstationsusedinthisstudy,andrangesofgreen,white,andblackashinNorthAmericaaccordingto Little (1971). R.D. DeSantisetal./AgriculturalandForestMeteorology178–179(2013)120–128

As I gauge the incoming cold wave event, my latest read is that the Twin Cities area will see close to 84 consecutive sub-zero hours starting Saturday night, and lasting into Wednesday morning.

Model forecasts vary, but the deepest cold should peak somewhere between -20 and -26 Monday and Tuesday mornings. Here's the latest look at the Global Forecast System model's temperature output for early next week.

Image: Weatherspark

In northern Minnesota the sub-zero cold will be even deeper, and slightly longer in duration.

Image: David Cappaert, Michigan State University

Emerald ash borer and extreme cold?

Given the magnitude of the incoming cold wave, I wondered about the potential positive benefits of the extended sub-zero streak.

Lee Frelich

I've spoken many times about the mortality of the pine bark beetle at -40 in northern Minnesota based on discussions with University of Minnesota forestry expert Lee Frelich, director of the Center for Forest Ecology.

Lee is widely regarded as a premier expert on all things Minnesota forest, including insect mortality in extreme cold.

Astute MPR News listener and Updraft reader Krista Loke got me thinking about how the emerald ash borer might fare in the upcoming cold.

Hey Paul, was just reading the updraft. Even though I never miss you in the morning with Kathy. Though I find this latest temperature dip a tough one I'm glad to see it/feel it. Didn't we used to get cold like this for weeks at a time? And they never cancelled school! Kids have it so easy these days. I digress.... I am wondering how cold and for what duration do we need to kill off the emerald ash bore? I thought it was something along the lines of -20 for 2 weeks straight. Focus on the positive! I just received my first wave of seed catalogs. Time to hunker down and start planning the garden. Take care, stay warm. Krista Loke

Great question Krista. I had my suspicions on how the extreme cold might affect the little buggers, but I went straight to the source for this one. Here's the excellent and timely response Lee sent my way.

Paul: I think the forecast temperatures that we will experience in the next several days will cause a lot of mortality for emerald ash borer in MN. Details below--probably more than you need. I looked up the most recent research this morning, because I figured I will get a lot of questions about this over the next week. Winter mortality for emerald ash borer is definitely temperature dependent. The larvae can supercool to a certain point, but they die if they freeze, and there is variability in tolerance among individual insects. A recent study from the Forest Service (Venette and Abrahamson--see attached) in Minnesota showed that 5% of the insects die at 0F, 34% at -10F, 79% at -20F and 98% at -30F.

Table 1. Coldest temperatures recorded in largest and smallest diameter logs used in larval COLD HARDINESS OF EMERALD ASH BORER, AGRILUS PLANIPENNIS: A NEW PERSPECTIVE Robert C. Venette1 & Mark Abrahamson2

However, there is the question of what temperatures the insects actually experience, since they spend winter under the bark of trees, and some of them close to the ground, where they may be insulated by the bark itself and possibly by the snow.

This insulation effect can have a substantial effect if overnight minimum temperatures take a brief plunge and recover quickly. In such cases minimum temperatures under the bark can be 2-7F warmer than air temperature.

However, with prolonged cold lasting all night or, as we may experience in the coming days, prolonged cold well below zero all day and all night, then the insulating effect of bark becomes minimal.

A recently published study (DeSantis et al--see attached) integrates these factors to show where emerald ash borers are likely to have substantial mortality--see color maps in figures 2 and 3 in the DeSantis paper. There is a large area in northern MN that has a lot of ash trees that experiences temperatures cold enough to greatly limit the survival and reproduction of the insect.

That is good news for our 900+ million ash trees, many of which grow in the swamps of northern MN. However, with warmer winters likely to occur in the future, this could change. Also, the few insects that do survive the upcoming cold spell might be more resistant to cold than an average insect, and give rise to a new generation of more cold-tolerant insects, although we don't know much about this type of selection in emerald ash borer.

In addition, as you know, there is huge variability in minimum temperature across the landscape, on south or north facing slopes, hill tops, valley bottoms.

We will know more about that over the next year, as we acquire data from our network of Hobos measuring hourly temperatures at 150 forested locations within forests in the BWCAW and Voyageurs National Park, and 150 on Isle Royale. There is also the urban heat island in the Twin Cities, which will help the insects a lot.

Lee

Lee E. Frelich

Director, The University of Minnesota Center for Forest Ecology

So for Minnesota's ash trees, the magnitude and duration of the upcoming cold wave may be a very good thing! No doubt Lee and his colleagues will be checking in to see how the ash borer larvae fared during this extreme cold wave.