Move over Popillia japonica, there’s a new “Japanese beetle” in town.
As is typical in a given year, Wisconsin sees a few new invasive species in the state each year. In 2016, one of the surprises was the arrival of the “two banded Japanese weevil” (Pseudocneorhinus bifasciatus), often simply called the “Japanese weevil”. Weevils themselves are technically a type of beetle from the hyper-diverse family Curculionidae, which contains a plethora of weevils, curculios, and multitudes of bark beetles. When talking to the public, it’s amazing how often the Japanese beetles feeding on landscape plants during the summer are mixed up with the Asian lady beetles (Harmonia axyridis), which invade homes in the fall. Given the name of this new “Japanese weevil”, I’m expecting this creature to confuse the situation even more.
What is the Japanese Weevil?
The Japanese weevil (P. bifasciatus) is a non-native beetle that feeds on a wide variety of landscape plants, particularly shrubs. Adult Japanese weevils are ~ ¼” long with a gray or brownish, pear-shaped body with black bands across the wing covers (elytra). The pale larvae (grubs) live in the soil and feed on roots of suitable host plants. We haven’t had this pest in Wisconsin long enough to fully understand the local life cycle, but given the pattern in other states, this insect will most likely complete one generation per year, with adult presence and feeding damage occurring during the summer from late June through August.
Where’s the Japanese weevil from?
Very similar to the Japanese beetle (Popillia japonica), the Japanese weevil is native to Japan and first popped up in the US in 1914, via the transport of infested nursery stock. Like the Japanese beetle, the Japanese weevil feeds on a wide range of landscape plants. When it first showed up in Wisconsin in 2016, the Japanese weevil was found on a variety of ornamental plants in Madison, WI. How this species got to Wisconsin remains a mystery, but the movement of infested potted plants is the most likely explanation, as this insect is not capable of flight. While the Japanese beetle is common across many parts of eastern North America, the Japanese weevil has a much more scattered distribution and can be found primarily in the Mid-Atlantic region with scattered cases in the Illinois, Iowa, Wisconsin and Oklahoma.
What does this insect mean for Wisconsin?
Luckily, it seems that the Japanese weevil may not be nearly as big of a threat to landscape plants as the Japanese beetle. When lots of Japanese beetles are present, entire trees can have their leaves nibbled into a lace-like skeleton. In contrast, when the Japanese weevil feeds, it tends to cut notches out of the edges of leaves. This damage can resemble the feeding of many caterpillar species, and healthy plants should be able to tolerate the feeding. Reports from other states suggest that this insect unlikely to cause very severe damage.
Think you’ve found the Japanese weevil?
If you come across any beetles in Wisconsin feeding on landscape plants that resemble the Japanese weevil, send digital images and/or physical specimens to the UW Insect Diagnostic Lab for confirmation, so we can track this new arrival in the state.