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Winter Effects on Japanese Beetles

A voracious pest of gardens and soybean fields, Japanese beetles (Popillia japonica) are a relative newcomer to the state. I had grown up with them my entire life; they seemed to follow me from my childhood home in Ohio to the Midwest. When I was in Iowa, I remember bringing out the soapy buckets of water in the morning to pick and drop them to their doom. The beetles were already clumsy fliers, and when covered in dew they were even easier to grab. Yet, these pests kept coming day after day. The damage these metallic colored insects did to my family’s garden was extensive. Plum trees became skeletons of their former selves, and any legume or rose bush we had was similarly stripped of foliage.

However, during the end of 2013, a harsh winter overcame Iowa after a particularly dry fall. Very little snow fell during that time, and temperatures well below zero were the norm. We were all grateful when spring finally arrived, and when the plants and trees bloomed, I waited, secretly dreading the summer when the Japanese beetles would emerge—but none did. The bucket stayed in the garage, and I wondered where those dastardly creatures went.

Flash forward to New Year’s Eve here in Minnesota, one of the coldest on record. So cold in fact, that the Curiosity Rover currently roaming around on the planet Mars recorded a daily high (-4˚F) which was warmer than Mankato (-7˚F). While both the Japanese beetles in the soil and the humans nestled in their homes hated that cold, some beetles likely owed their lives to the snow that fell that evening. 

People may forget that Japanese beetles spend a great deal of their life cycle underground as larvae and pupae. We see the adults from around July to August, where then they dig a few inches and lay eggs. Similar to their loathsome adult parents, the larval stages of the beetles can also cause damage to plants—feeding on mostly grass roots on large patches of turf.

To keep themselves alive during the winter, the larvae burrow downward below the frost line. The beetles can die when soil temperatures dip below 15˚F. Typically, grubs can go down 2” to 8” to avoid this fate, but if the cold is unrelenting, they can migrate up to a foot below the soil surface. Nevertheless, as long as they stay ahead of the plunging frost line, the beetles survive—probably planning how to eat your roses when they emerge in summer.

So, is there a chance that this year’s cold snap will rid us of these insects? It is important to note that topsoil acts as natural insulator, protecting many creatures from the unrelenting wind and chill above. As you may know, water is also a great insulator in any form, even as snow (think about igloos). When you are in a situation where both deep soil and snowpack exist, the cold air will be unlikely to kill large amounts of grubs.

Yet, finicky weather can fool Japanese beetles. For example, a stretch of warm temperatures—quickly followed by a Mars-like cold streak—tricks the larva into climbing upward to shallower ground, where the sudden cold kills them.
So far, we have gone through several of these thaw/freeze cycles in January, and the odds are looking good the bucket and soap will stay in the garage this summer.

Works Cited:
Cloyd, R. and P. Nixon. 2003. Japanese Beetles: Impact of Winter, University of Illinois Extension.
Rover Environmental Monitoring Station. Centro de Astrobiologia and NASA.