The impact of a deep freeze on the emerald ash borer


It’s bitter cold in the Ann Arbor area and throughout the country; temperatures have been below zero most of the last two days.

One of the things we’re looking for a silver lining in super cold weather is its impact on bugs, especially invasive species like the emerald ash borer that are not used to unusual weather. A story in the The Gazette (Cedar Rapids Iowa) has a description of the effects of extreme cold on these destructive beetles.

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….

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.

More details on Minnesota Public Radio News, again from Frelich:

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.

The recent study is Venette and Abrahamson, Cold Hardiness Of Emerald Ash Borer, AGRILUS PLANIPENNIS: A new perspective. The abstract:

This study was designed to assess the cold hardiness of emerald ash borer larvae, the
overwintering stage of the insect. We began by measuring larval supercooling points, the
temperatures at which larvae freeze. We found that larvae collected from naturally infested trees in St. Paul, MN between late October and early December had an average supercooling point of -25°C (-13°F). Research elsewhere indicates that when these insects freeze, they die. Our laboratory assessments of cold hardiness were confirmed during field tests. Naturally infested logs were held outdoors in St. Paul, MN (low winter air temp=-28°C) and near Grand Rapids, MN (-34°C) for ca. 5.5 weeks. Approximately 40% of larvae from logs in St. Paul were inactive or brown, both evidence of death; approximately 90% of larvae from logs near Grand Rapids were inactive or brown, compared with the approximately 10% that showed evidence of death prior to exposure or after being held under cool, non-lethal conditions. Overwintering mortality
may help to minimize the damage caused by emerald ash borer in areas with extremely cold
winter climates.

So there you go; the colder the weather, the more of the bugs die.

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