Examining winter survivability of bugs

My humble reader, old-timers wish for a hard, freezing winter because it will kill off most of the insects that plague the crops of farmers in the spring and summer. Even though much of the bugs you see in the warmer days out in the countryside mostly vacate the area for warmer climates, others will stay put and settlein for the winter. But can the cold winter months, indeed, be severe enough to kill off the majority of the insects? Variables in the environment can affect the presence/survivability of insects during the winter, but vary in impact.

If you have never met an entomologist, or at least someone with extensive education on the subject matter, you are missing out on the most fascinating conversations. Chatting with Dane Elmquist, Ph.D of UW-Extension, he serves as the Conservation Cropping Outreach Specialist but is involved in research looking at the effects of insects in cropping systems. When I asked him my question on winter survivability of insects his mind raced like a kid in a candy store and provided good insight during our discussion.

Bugs do not produce their own body heat, and assume the same body temperature as the environment they are in. So, when insects stay in Wisconsin over winter, they can seek shelter from the cold under the ground or under residue and can even produce a substance very similar to the antifreeze that we put in our cars. This elevates their tolerance to the cold but it will vary depending on the insect and where they are hiding from the cold. For example, northern corn root worm eggs can handle down to -16 F deep in the soil, but yet soybean aphid eggs can take it down to -30 F inside the leaf buds of buckthorn (the buds can withstand windchill temperatures). A bean leaf beetle will duck for cover under residue/leaf litter on the soil surface and can only survive temperatures in the 20s F. Second generation, those hatched in summer, corn borer larvae can handle sustained sub-zero temperatures for months. It is easy to point at these temperature ranges and wonder how bugs even make it to spring. But when thinking about how winter affects insects, we are talking about populations. Elmquist said it best, “an individual insect is either alive or dead. But a population (a group of insects) can have different rates of survival.” Let’s say it is a mild winter and only 10% of the overwintering insects die, that leaves 90% of them to live into the spring. On the other hand, in a “hard” winter, perhaps 80% of the population will die leaving only 20% of the population to survive into the spring. So, it is not so much a matter of if insects can survive, but how many insects will survive.

Ground cover can provide a variable for survivability. Snow and ice although cold, are insulators. On a -10 F day, things that are covered with snow and ice will remain much above zero, even into the 30s F. The opposite is also true. When there is little to no-snow cover the insulating element is eliminated, and colder temps can penetrate into the soil profile, decreasing chances of survival for insects.

While severe freezing can be sublethal to bugs, it is the freezing/thawing processes that usually cause insect death. Coming out of a ‘chill coma’ puts a lot of strain on insects, and they need a lot of energy to emerge and find food in the spring. Until they find food, they are at the mercy of their own body reserves. A mild spring can encourage premature emergence before enough, or any, food is available resulting in starvation. Fluctuating temperatures in spring can also deal a final death blow, with warm temperatures followed by cold-snaps and drops in temperatures catches the insects unprepared to handle the change in degrees, decreasing their survival.

Time will tell if this winter deals a blow to the pests lying in wait below the snow and frost, or what kind of spring it will bring. So much must synchronize between the weather, the environment, and the bug’s life cycle to give them a chance to even become a pest. But they will wait-it-out until spring, because they were never given the option in life to quit. And maybe that resiliency is what makes us fascinated bugs, for better or worse.

I would like to give credit to Dane Elmquist of UW-Ext. as a resource, and publications from Iowa State University Extension “Survival Effects of Fluctuating Temperatures on Insects” (April 22, 2020), and from Michigan State University Extension “How insects survive cold: The potential effect of a mild winter” March 14, 2023

THE SOIL

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MATT OEHMICHEN AGRONOMIST