During the year, we often get asked questions regarding the impact of winter on pests such as Japanese beetle. Just as a weatherperson has difficulty predicting the weather, it is not always possible to determine the extent of winterkill. However, it is important to note that winter conditions affect natural enemies (parasitoids, predators, and pathogens) as well as pests. Additionally, just because we experience extremely cold air temperatures doesn’t mean that the insects are affected. The effect of cold depends on whether there is a snow cover.
Snow acts as insulation (just like insulation in a house) to buffer the soil from extremes in air temperatures. In fact, the soil may be as much as 20 to 30 degrees warmer than the ambient air temperature, depending on depth of snow cover. Snow cover reduces insect mortality and allows more insects to survive the winter. For example, even air temperature of –35°F may have minimal effect on insect survival if snow cover is deep. In contrast, barren soil (with no snow cover) will be colder deeper down within the soil profile. This has the greatest impact on insects in the soil and leads to higher mortality.
This year, portions of Illinois, such as the Chicago area, experienced winter conditions with little or no snow cover; and cold temperatures occurred over an extended period. This allowed soil temperatures to get cold enough to increase the likelihood of killing insects in the soil. Winterkill may be substantial, especially when unusually cold weather occurs without an insulating layer of snow. Soil temperatures below 15°F likely result in near 100% mortality.
Japanese beetles overwinter in the soil as immature grubs (larvae). At first frost, grubs move deeper in the soil. Most of them typically overwinter at 2 to 8 inches, although they can migrate down 11 to 12 inches below the soil surface. The depth at which grubs are located may depend on soil structure. Other grub species, probably true white grubs, have been found 6 feet deep in the soil during extended subzero conditions in northern Illinois with little to no snow cover. Although grubs migrate down to avoid freezing temperatures, some mortality may still occur over the winter if rapidly changing conditions prevent the grubs from moving far enough to escape lethal environmental conditions. For example, sudden cold spells with little snow cover often cause high mortality.
A heavy snow and thick sod cover usually result in grubs’ being closer to the surface and surviving with only low mortality. They tend to move deeper in relatively barren soil. Lack of substantial snow cover on barren soil typically results in extremely high grub mortality. Japanese beetle grubs are killed at soil temperatures near 15°F and die when soil temperatures are consistently around 32°F for 2 months.
In Illinois last winter, consistently cold temperatures with little or no snow cover resulted in the soil’s being frozen longer and deeper than has been common in recent years. Reports of the soil’s being frozen 15 inches deep in central Illinois and 30 inches deep in northern Illinois for about 2 months were common. Emergence of Japanese beetle adults has been slow and late this year, probably due to the prolonged cool spring. Much lower numbers are being reported in many areas of central and northern Illinois, although there are reports of spots with very high populations. It is possible that prolonged cold soils caused increased wintering grub mortality. Areas with high beetle numbers may be those where deeper snow insulated the soil.