Can a Cold Snap Kill Invasive Species?

Feb 22, 2021


Did the recent cold snap help us in our fight against invasive species? The Masked Biologist says “yes and no” in this week’s Curious North-inspired edition of Wildlife Matters.

Recently, listener Bill Boldon went onto Curious North at WXPR and asked the question: Is the extended cold period long enough to help kill invasive species?

Well, it depends. Like my dad used to say, “yes with an if or no with a but.” Invasive species are extremely adaptable by nature. They can move into an unoccupied niche and exploit it, and they lack the natural suppressive measures that would keep them in check in their home environment.

Here in the Northwoods, our environment is fairly hostile toward otherwise aggressive invaders. That being said, we definitely have aquatic invaders that have pulled it off, like Chinese mystery snails, eurasion water milfoil, and zebra mussels—presumably by hitching a ride with unwitting boat owners and water recreation enthusiasts. Once they get here, there are some wins and losses, and winter weather might set it back temporarily or set the stage for success. Eurasian water milfoil may senesce due to low light conditions and thick ice in shallow parts of a lake, for example. That’s a win, temporarily. However, the native plants are set back by the same conditions, so then it will be a race to see if desirable native plants will pioneer into the open areas or if the aggressive invader will get there first.

Invasive land plants really won’t see an impact. Spotted knapweed, glossy buckthorn, garlic mustard, these all snooze the winter away in dormancy or aggressively reproduce by seed. They are not impacted by anything winter has to throw at them.

When it comes to insects, sadly, it is a similar story, one step forward, one step back. A brief cold snap may kill some individual larvae, setting back local populations. However, the conditions would have to be extremely harsh for a long time to do enough damage to result in natural control. the larvae that survive will be the heartiest and most adaptable individuals, and they will emerge in spring and get back to work. Local population numbers may be smaller, so the damage and rate of expansion may be reduced for a year, but with the number of eggs an insect can lay, it will not take long to build numbers back up. When it comes to invasive insects, there is even more bad news. The climate has been moderating for a long time; our Wisconsin winters are weeks shorter now than they were when my mom was child. Overnight cold temperatures are less, and lake ice forms later and leaves earlier generally speaking than it did the middle of the last century. There is some indication that the resulting extended plant growing season may favor invasive pests (that only reproduce once a year because of our short growing season) allowing two breeding cycles in a single growing season! This would dramatically increase the number of individual larvae each winter, more than offsetting some winter mortality due to a cold snap.

Wildlife species are probably the most vulnerable. For example, I highly doubt we will ever see the problem Florida has with iguanas freezing and falling from trees. Our weather is too cold for too long. We won’t see invasive snakes, either. Really any cold-blooded reptiles are out of luck unless they can find a creative way to spend the winter somewhere warm or deep below the frost line. I have written about some potential parakeet invasions in far southern Wisconsin, but they would need the urban heat bubble in Milwaukee or Madison to help them survive cold snaps. Furbearing animals would be the most likely to survive; we occasionally get a report of a feral hog up here, but overall our cold weather and deep snow minimize invasions of that type. However, since we are seeing more frostbit opossums up here, I do think as our climate moderates according to the most reliable models over the next half century we could see some small burrowing animals like armadillos venture north.

So will our recent cold snap help reduce invasive species? It depends. Yes, if it got cold enough and stayed cold enough some local populations could see a temporary setback. Also, I’d say no, but cold snaps are important to help keep local wildlife populations healthy and balanced by reducing the numbers of sick and weak individuals. Cold snaps also help reduce impacts to wildlife of certain pests like ticks and mange, which helps their populations in the long run.

Thanks for the question Bill. I’d encourage all listeners to submit your questions to Curious North, they make great ideas for new recordings like this one.