By Dan Marzu
Lincoln & Langlade County UW-Extenion Agricultural Educator
No, I’m not talking about the rock group from the ‘60s. Unfortunately, I’m talking about the more destructive kind that have found their way into several ornamental, fruit and vegetables gardens. The two main beetles that I have been receiving complaints about are the Japanese beetle and the Lily Leaf Beetle.
The Japanese beetles are about a half inch long, have coppery brown wing covers and are metallic green. There are also patches of white hairs along the sides and back under the edges of the wings. The larvae are grubs that live in the soil. The Japanese beetle will eat over 350 species of plants. This includes ornamental flowers and bushes, field and forage crops, and vegetable and fruit crops. They are primarily active during the afternoon. The adults will feed in the upper canopy of most plants, eating the leaf tissue between the veins. The grubs will eat roots of grass and ornamental plants. In severe cases the grubs may eat enough roots that turf will not be anchored to the soil and can be rolled back like carpet.
The adults emerge around mid to late June to mate and lay eggs. The female will lay the eggs in moist areas to ensure survival of the next generation. The eggs will hatch in two weeks. In fall, as soil temperatures start to decrease and the first frost occurs the grubs start moving deeper into the soil. They will overwinter to about two to six inches but have been known to go as deep as 20 inches. Once soil temperatures fall below 50° F the grubs become inactive. In the spring, they move up the soil profile and continue to feed for about three to five weeks.
There are several products on the market used to control the Japanese beetle adults and grubs. If chemical controls are not a preferred method then handpicking and placing into a bucket of soapy water may provide some control. This however may only work if numbers are low. The beetles are extremely mobile and if one female is in the area she will attract more beetles to the plants. To control grubs, try to keep the infested soil area dry. Remember the grubs need moisture to survive. There are also a couple biological control products for grubs. These products have inconsistent control, but are a good alternative if chemicals will not be used. These include milk spore disease, insect-parasitic nematodes, and fungal pathogens such as Beauveria bassiana and Metarrhiizium.
The next beetle is the Lily Leaf Beetle. This beetle was introduced to Wisconsin first in Marathon County in 2014 and has since followed the Wisconsin River north as far as Tomahawk. True to its name, the lily leaf beetle lays eggs and feeds on true lilies (genus Lilium) and fritillaries (genus Fritillaria). True lilies include Asiatic, Oriental, Easter, Turk’s cap, and tiger lilies, as well as native lilies such as the wood lily. The lily leaf beetle will not feed on canna lilies, calla lilies or daylilies. They have been known to also feed on Solomon’s seal, bittersweet nightshade, potatoes, Hollyhocks and Hostas. However, it appears they do not lay their eggs on these plants.
The adult lily leaf beetle has a red body with a black head, antennae, legs and underside and is ¼ to ½ inch long. Females will lay up to 450 eggs in irregular strings on the underside of the leaves, sometimes over two growing seasons. The eggs hatch in one to two weeks. These orange to light green larvae will cover themselves with their excrement to repel predators. The larvae will feed for 16 to 24 days before burrowing into the soil to become a fluorescent orange pupa. After 16 to 22 days they emerge as adults. The adults will feed until fall and then overwinter in plant debris in protected areas that are shaded, cool and moist. When the adults emerge in the spring they will mate and lay eggs. There is only one generation per year.
Both adults and the larvae feed on the leaves, stems, flower buds and flowers. Most of the damage is caused by the larvae. There are no natural enemies for the lily leaf beetle. Some Oriental varieties appear to be resistant to the beetle, as where many if not all the Asiatic lily hybrids are susceptible. To control the lily leaf beetle non-chemically, look at the plants early in the morning and brush off the adults and larvae into a container of soapy water. Look on the undersides of the leaves for eggs and remove the leaf or crush the eggs. Insecticides that contain active ingredients such as permethrin, cyhalothrin, deltamethrin, pyrethrin control beetles that are present. Neem oil and insecticidal soaps also have some control on larvae. Several applications may need to take place as the beetles move from garden to garden.
If using any insecticide use caution to ensure non-target insects, including pollinators, are not present when the application is made. Always read and follow labeled directions when making any pesticide application. Feel free to contact the UW-Extension office at 715-539-1072 if you have any questions regarding these pests or other horticulture and agriculture questions you may have.