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May 21, 2021 - Japanese Beetles in the Landscape

Japanese BeetleOne of the more common questions directed to the Horticulture Help Desk is how to deal with the dreaded Japanese beetle. Even though they have been a significant pest in the general Green Bay area for years, it always seems they are popping up in new places, causing great concern for residents.
These pests are fairly easy to identify with their metallic-green and copper colors and their ladybug size. Though relatively small, they gather in huge numbers in our neighborhoods and leave behind a legacy of “skeletonized” leaves on the more than 350 species of plants they consume! They have their favorites though.  Most complaints revolve around roses, birch and linden trees, grapevines and raspberries.
To help understand how to manage these insects, it’s important to know their life cycle. The adults emerge from the ground in late June to early July and begin to feed and mate. Soon after they lay their eggs in surrounding turf areas and this continues until adults die-off in September. The grubs (white, up to 1” long) that develop from the eggs feed on grass roots and, if in high enough numbers, can severely damage lawns. Extensive feeding actually results in the turf being separated from the earth that can be rolled back like a carpet! This is a classic sign of grub damage and coupled with holes in the lawn from grub-foraging birds, skunks and raccoons, there is little doubt that grubs are afoot (or underfoot in this case!). As temperatures drop in fall, the grubs descend about 6-8” down and dream about roses and linden leaves all winter long. When soil temperatures warm in spring they begin feeding activity once again and pupate in early June emerging as adults around the beginning of July, completing their one-year life cycle.
Controlling Japanese beetle adults can be a real challenge, but there are options for you. To begin, do not use the pheromone traps/bags! They will capture beetles, but they attract more to your yard then they catch so you definitely do not come out ahead. And if you have beetle activity on established trees/shrubs, no control is recommended since these can handle some defoliation without a problem and will re-leaf in spring. For low numbers of beetles, you can simply go around the yard and flick the beetles into a container of soapy water to end their chewing days. If populations are high, there are many insecticides labeled for control of Japanese beetle adults, but you really need to consider how these products affect our beneficial insects like pollinators! One bee-friendly option contains the active ingredient chlorantraniliprole (found in Acelepryn but note that this product is only labeled for use on ornamental plants, trees and shrubs). Another is Btg or Bacillus thuringiensis subsp. Galleriae (found in beetleGONE!), which is a natural bacterium that also reduces the risk to non-target insects. For vegetables and berry crops, consider using one of the many organic products that contain pyrethrins, but apply in late evening after pollinators have ceased feeding for the day. Whatever chemical option you choose to employ, make sure it is labeled for use on the plant species you plan to spray and follow label directions carefully!
Grub control is only required in lawns where numbers are high enough to cause turf damage. Proper chemical product and timing is critical to be effective. Refer to University of Wisconsin Garden Facts:  White Grub Control in Turfgrass (XHT 1018) for complete information.  Note that grub control only solves turf issues and should not be considered a solution to next years adult beetle problem. Even if you successfully eliminate all the grubs in your yard, the adults can fly for miles and can quickly inundate your landscape!
If you have further questions on control of Japanese beetle adults and grubs, or any other home landscape issue, please contact the Horticulture Help Desk at Extension Brown County or visit us on-line at and we would be happy to help you!
Source: UW-Madison Division of Extension ( Publications: Japanese Beetle (XHT 1062) and White Grub Control in Turfgrass (XHT 1018)
Written by:  Doug Hartman, Extension Brown County Horticulture Assistant