It was our coldest January in eight years. Did the Japanese beetles die?

UNIVERSITY OF MINNESOTA EXTENSION - www.extension.umn.edu

While people may choose to start their year out with all kinds of new year’s resolutions, insects only have one resolution: live long enough to reproduce. Winter survival is one hurdle in a bug reaching its goal.

Many people assume, after months of dangerous wind chills and arctic air masses, that insects have been killed off, but it is rarely that simple.

Let’s check in on one of our least favorite garden pests, the Japanese beetle, and see how they likely fared after a cold start to 2022.

The Japanese beetle life cycle

Where are Japanese beetles right now?

Japanese beetles spend the winter as immature grubs. This is the white grub life stage that is responsible for damage to lawns.

As temperatures cool off in the fall, grubs burrow into the soil. They can dig down to depths of 2 to 6 inches. They will tunnel even deeper if the soil gets too cold during the winter.

So, did the Japanese beetle grubs die?

Air temperature vs soil temperature at two soil depths in St. Paul, Minnesota, at the end of January 2022.

Unfortunately, the answer is “probably not.”

The grubs are a few inches down in the soil, which itself is underneath a few inches of snow. All of this acts as layers of insulation. We know from studies that Japanese beetle grubs can survive down to around 9°F. While Minnesota has been much colder than that, especially when considering wind chill, it is a little bit of a different story a few inches underground.

What is it like temperature-wise down there? The Minnesota Department of Agriculture has a series of stations that collect soil temperature data. Across Minnesota, they tell a consistent story. Soil temperatures hover anywhere from 20 to 30 degrees, while the air temperature is much colder and fluctuates wildly.

Based on what we know about Japanese beetles and soil temperatures, many grubs likely got through January just fine, and many will hang on long enough to emerge as adults next summer.