Mention Japanese beetles to your gardening friends or fellow tree lovers and you are sure to get a reaction. Relatively new to the state, Japanese beetles have been in Wisconsin 10-15 years now. Isolated at first, they are becoming more widespread. 2011 has been an especially bad year with high populations in many areas.
Slightly less than ½” long they are shiny and metallic green with coppery-brown wing covers. Adult beetles emerge from the soil in late June through early July. These are from pupated grubs from eggs laid the previous year. The beetles are active and feeding above ground for about 8 weeks, by late August to early September their feeding is done. During that time the foliage and in some cases the flowers of susceptible plants are eaten. The beetles feed on the upper surface of leaves, leaving a lace-like or ‘skeletonized’ appearance. On trees, the feeding usually starts at the top of the crown , which is initially hard to see. As the feeding continues through summer, heavily damaged plants can turn brown and become defoliated.
Japanese beetles cause damage to over 300 species of plants. Some of their favorite hosts in our area include:
· Linden (Basswood) – especially Little Leaf Linden Varieties
· White Barked Birches – River Birch is less susceptible
· Cherries and Plums
· Mountain Ash
· Many other Trees, Shrubs, Herbaceous Flowers and Food Plants!
Light feeding on plants is not that troublesome, however repeated heavy feeding with high populations of beetles is more of a concern. Leaves are plants food producing parts. When large amounts of leaves are consumed, the plants energy reserves are lowered reducing its growth and predisposing it to other problems. Just as important heavily infested plants do not look good either!
What to do:
· Many insecticide sprays are effective against Japanese beetles. Because of the long life cycle of the adult beetle, sprays need to be reapplied over the course of the summer.
· Soil applied systemic insecticides, are effective in controlling the beetles through the course of summer. Applied in spring or the previous fall, systemic insecticides get into the foliage of plants. The beetles still come, but they take a few bites and are killed. You will see some beetle feeding damage, however it will be much less compared to a non treated tree. If beetle populations are high, this alone may not give satisfactory control. Additional crown sprays may be needed. DO NOT use systemic insecticides on food plants – i.e. raspberries, plums, cherries, and grapes.
· On small plants – roses, flowers, food plants, container plants, removing beetles by hand and placing them in soapy water may provide adequate protection when beetle numbers are low. Sprays of Insecticidal soaps with ‘neem’ (a botanical insecticide) are also a good option for small plants and food plants.
· Avoid the use of Japanese beetles trap’s, they attract more beetles to your property resulting in more damage.
Japanese beetles are somewhat cyclical – some years the populations are higher than others. Abundant soil moisture conditions are favorable conditions for the grubs. The beetles are not going away, and they will be back next year, in what numbers and where is unknown. It does appear that this problem is building in numbers and expanding its range. Plan to protect susceptible and valuable plants next year. Call your Wachtel Certified Arborist, to help you to battle the beetle.