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What does a wimpy winter mean for insects?

Marissa Schuh, IPM Extension Educator. Reviewed by Anthony Hanson, Field Crop IPM Extension Educator.

The National Weather Service's Seasonal Temperature outlook for March, April, and May suggests our dud of a winter has wrapped.  Many people are wondering how this will impact insects, and as always, it depends! Here are some things to think about.

National Weather Service

Each species of insect, both beneficial and pest, has a different strategy for riding out the winter.  While this winter was overall pretty mild, we still had a cold snap in January, and we still got below-zero temperatures many nights.  We had minimal snow cover, but we still had plenty of freeze thaw cycles.  Each one of these factors will impact each insect  that spends the winter in Minnesota differently.

We likely had enough cold snaps to make sure insects that don’t typically spend the winter in Minnesota weren’t able to spend it here. Insects like corn earworm, potato leafhopper, and  black cutworm were most likely killed during the cold snaps we did have.

When it comes to insects that do spend the winter here, there was enough weirdness this winter where this winter wasn’t likely stress-free for every pest.  What will each pest’s population do this year? Hard to say.

Let’s take the example of Japanese Beetle. The insect spends the winter as a grub under the soil surface.  Research suggests they can survive in soil temperatures down to 27F. Research in Minnesota looked at winter soil temperatures and Japanese Beetle grub survival.  The temperatures in the study and the survival rates were:

  • 2018-2019 averaged 30.87°F - 36% survival

  • 2019-2020 averaged 33.12°F -  77% survival

  • 2020-2021 averaged 35.04°F - 100% survival

Soil temperature data is hard to track down. A UMN Weather station in Morris, Minnesota recorded an average soil temp of 31.9F for 4-inch soil depth Dec 1 - Feb 29 and 27F for a low soil temp. So there was likely some Japanese beetle mortality in Western Minnesota, but the metro area was warmer than Western Minnesota in the winter months, meaning we likely had less mortality in the Twin Cities region.

Will pests emerge earlier? Probably, and we can see this reflected in degree day models. University of Wisconsin’s Vegetable Disease and Insect Forecast Network and University of Minnesota both have degree day maps for common fruit and vegetable insects. If you are trying to time the beginning of scouting or when row covers should be placed, these tools can be a good piece of data to use. 

Photo: VegEdge

Looking at an example insect, let’s see what the models are saying about cabbage maggot.  Cabbage maggots are one of our first pests to come out each growing season, as they can develop and become active anytime temperatures are above 43 degrees, and adults will emerge and start laying eggs at 300 degree days. As of 3/13, Cabbage maggot are at approximately 110 degree days in southeast Minnesota,  90 degree days in the metro, and 24 degree days in Duluth. Most of the state has some cooler weather on tap, which will slow down these degree day accumulations. Depending on what the weather does and when your brassicas get planted, part of the cabbage maggot flight period may have passed by the time our cole crops are planted.

What do we do about all this during the growing season?

The best tool is your own observation skills – scouting will be as important as ever, as the rhythms and patterns of insects you’ve noticed in the past may have shifted or not hold true this year.  Make time to scout weekly.

As always, myself and other extension educators will report here about what we are seeing on farms and hearing from farmers.

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