Many homeowners and landscape managers in our area are dealing with their first real onset of damaging populations of the true Japanese Beetle. The moist conditions of 2017 led to a very rapid expansion of the Japanese Beetle populations in Oconto and Marinette Counties, leading to the damage now being observed. We also have multiple native beetles that are closely related to this beetle, most notably the rose chafer and the spring rose beetle. However, there are some distinct differences between the species.
The Japanese Beetle has been in the U.S. since 1916, and has steadily spread from New Jersey, where it was first introduced. This insect can be very problematic because the larvae feed on turfgrass roots and the adults feed on leaves of at least 300 species of ornamental plants, as well as having the potential to cause severe damage to both corn and soybean fields. The adults are usually out in July and August, with each adult surviving most of this two month period. This is a major differerence from our native relatives, as they are feeding on plants in early to mid June and only survive for a couple weeks.
Japanese Beetles feed in a very consistent manner on plant foliage, leaving just the veins of leaves behind. This is termed skeletonization and often looks very similar to lace. Preferred plants will have hundreds of beetles on them and can be heavily damaged or even completely defoliated. They can be found on all types of plants, even some plants in vegetable gardens (especially basil), but prefer ornamentals and fruits such as roses, raspberries, grapes, linden family trees, crabapple and some prunus species, mountainash, and Norway Maple.
The adults are about 1/2 inch long, with a greenish-bronze head and thorax and copper/metallic-colored wing covers. There are white spots easily visible along the sides of the abdomen. They almost always feed in pairs, groups, or clumps. Eggs are laid in managed turf areas, in which the grubs hatch and feed on roots, causing another type of plant damage. The grubs can create significant water and nutrient stress in turf and ornamentals due to severe root pruning. They hibernate in the soil, feed on roots for a few weeks again in the spring, and then pupate, emerging as adults in early July.
Control of the beetles can be accomplished by utilizing insect netting as a protectant over smaller, more valuable plants; hand picking of the beetles off of plants and squishing or placing into a pail with soapy water (note that this is best done in early mornings or evenings when it is cooler and darker as they are strong fliers during warm days); or the use of insecticides. The use of pheromone lure traps may help concentrate the beetles and make them easier to control, but is not usually an effective control method by itself, as there will usually be more attracted than can be caught by the traps.
If choosing to use an insecticide, make certain that you use a product that is labeled for use on the plants you are protecting. Synthetic insecticide active ingredients that are most likely to succeed in managing them include permethrin, bifenthrin, malathion or carbaryl. Lower-impact insecticidal options are not as successful, but neem extracts like Azadractin have been shown to provide short term protection, especially if only small to moderate numbers of Japanese beetles are present.
Grub management options should also be done at this time of the year, as their primary feeding time is from now through the end of September. If we have dry conditions in August and September, the young grubs usually do not survive well, so simply not watering in a drought year can really reduce their numbers. In normal years or in irrigated turf, there are biological or chemical control options. Milky spore disease, parasitic nematodes, and fungal pathogens such as Beauveria and Metarrhizium can be applied to the lawn and may work fairly well in certain conditions. Insecticidal active ingredients that can be applied to the turf for effective grub control include carbaryl, trichlorfon, imidacloprid, halofenozide, and thiamethoxam.
If you want more information on Japanese Beetle management or assistance in identifying them, contact Agriculture & Horticulture Agent Scott Reuss at the Marinette County UW-Extension office: 715-732-7510 or e-mail to firstname.lastname@example.org Multiple publications are available free on the web to assist, as well, and can be found through the UW-Extension publications website or other midwestern universities such as University of Minnesota.