Ask any gardener or landscaper in the Midwest what their least favorite insect is, and the Japanese beetle will probably be near the top of the list. Think of the plants that this insect feeds on: ornamental trees and shrubs like lindens, birches, crabapples, and roses, fruit crops like apples, pears, plums, raspberries, and strawberries, and garden and vegetable crops like beans and corn, as well as hundreds of other plants. It’s no wonder gardeners have to be on alert for this insect.
So why the resurgence of Japanese beetle activity in 2016? Every winter, a common topic I’m asked about is my “insect forecast” for the coming year. Before winter had even ended, there was good reason to believe that the Japanese beetle would pop back up the state in 2016–and Mother Nature may be the cause. Japanese beetle larvae (white grubs) spend the winter in the soil below ground. In July, August, and September, these grubs can be found in the upper portion of the soil where they feed on the roots of turfgrass. As winter approaches, the grubs tunnel deeper into the soil where they avoid being killed by a hard freeze. In most parts of Wisconsin, we had a reprieve from Japanese beetles the past two years. I suspect this may be due to the brutal winter of 2013-14, which had some extended periods of sub-zero temperatures. It’s quite feasible that this deep frost killed many grubs and led to lower adult populations the following summer (2014). Given enough time, the Japanese beetle populations were destined to rebound at some point, and the mild (el-Niño) conditions this past winter might have been just what they needed to bolster their numbers. Unless we face another brutal winter in the next few years, I suspect that Japanese beetle numbers will be up for the foreseeable future in the state.
Ironically, there’s an important milestone to recognize for this invasive pest this year–the Japanese beetle was detected for the first time in New Jersey 100 years ago, in 1916. Slowly, but surely, this insect spread through many parts of the eastern US, and has been spotted on occasion in isolated spots in the western states. We also have an interesting history of Japanese beetles in Wisconsin. Technically, our first detections occurred in the southeast part of the state in the 1960’s, although these populations struggled to take hold. At the time, this seemed to be a comforting sign–perhaps, our famed “frozen tundra” was simply too cold for them. However, by the 1990’s, Japanese beetles had gotten a solid foothold in the state and they’ve been around much to the chagrin of gardeners ever since.
Interestingly there’s also a new “Japanese beetle” that showed up in Wisconsin this year: the invasive Japanese weevil (sometimes called the “two-banded Japanese weevil”). Stay tuned more information about our newest invasive species in Wisconsin.