Why extreme cold is good for Wisconsin ecosystems

Cold air might not be enjoyable but it's necessary and desperately needed for our local ecosystems. On Sunday morning, Feb. 7 Wisconsin will experience some of the coldest air in over two years and a stretch of below-average temperatures will continue for the following week.

Multiple morning lows could reach -10°F or colder with wind chills even colder than that. But why is this a good thing? 

There are many invasive species in Wisconsin but some of the most sensitive to extreme cold are insects. Most of the annoying bugs trying to crawl into your home in the fall are in fact from northern Asia such as Asian Lady Beetles and Brown Marmorated Stink Bugs but since being introduced these invasive insects have ravaged local habitats. The Emerald Ash Borer has likely killed off most of your neighborhood's Ash trees and the Hemlock Wooly Adelgid is killing off massive amounts of Hemlock trees in the eastern US, fortunately not yet in Wisconsin. 

While most of these invasive insects come from a similar climate to the United States they can't handle the extremes quite as well as native insects and plants. Our native species have evolved to withstand arctic air outbreaks and can handle dangerous cold no problem, and survival rates are even higher with adequate snow cover like we currently have. Cold weather that kills invasive bugs allows local ecosystems to rebound with their absence. 

Generally, to kill off significant populations of invasive Asian insects you need prolonged stretches of three days or more of extreme cold. For Wisconsin's infestation, we need at least -20°F or colder of morning low temperatures to half these hibernating populations, and the more days in a row the better odds their numbers are reduced. The problem is these bugs often find locations insulated enough like houses or underground to not be dramatically impacted. This is why targeted bug control from an exterminator can be very helpful but rampant pesticide use can do more harm than good to native insects. 

To the delight of many, our low temperatures won't reach -20°F most likely in the next two weeks but we will get close. Anything below -5°F can kill a small percentage of less insulated invasives. 

What we commonly call a stink bug is more accurately called a Brown Marmorated Stink Bug, these insects destroy millions of dollars worth of orchard crops annually and their numbers have rapidly increased since the year 2000 when they were first accidentally introduced. Lack of predators which is a common theme with invasive species is one of the many reasons this bug has become more and more widespread. 

The Emerald Ash Borer is a bug you typically won't see until it's already too late. Their larvae bore through the cambium layer of a tree and girdle the Ash tree, and as a result, nutrients and water from the roots can't reach the crown and almost always kills the tree. Native woodpeckers will eat them but because of their population explosion, there's no way for woodpeckers to keep up. 

One invasive bug that we generally call a ladybug isn't actually native but a close Asian relative known as an Asian Lady Beetle. The biggest distinction is a white "M" shape on their heads compared to no "M" on native ladybugs. Sadly, these have taken over ecosystems and outnumber native ladybugs to a point now that most ladybug looking beetles are likely the invasive Asian lady beetle. A native ladybug usually won't go in your home to avoid the cold, they can handle the cold just fine. The invasive Asian Lady Beetle is what swarms your house in the fall. 

While there are many other invasive bugs the final one we'll mention is the Hemlock Wooly Adelgid. This insect has already drastically impacted Candian Hemlock trees in the Appalachians and fortunately hasn't made it to Wisconsin yet. Before the late 1800s, the Canadian Hemlock was one of Wisconsin's most common forest trees. Their bark had lots of tannin for making leather and Milwaukee at one point was one of the biggest leather producers in the world. Sadly overlogging decimated our hemlock population. Only a small fraction of its original range still exists and this bug, if it reaches us, could decimate what we have left up North. One positive is the Wooly Adelgid cold-weather tolerance is much worse than the other bugs mentioned in this article. But a warming climate might not mean it stays that way forever.  

References for this article include the U.S. Forest Service, Minnesota DNR, and the Mequon Nature Preserve.