This summer we are receiving some questions about landscape plants showing leaves that have been fed on by Japanese beetles. A typical sign of an attack by Japanese beetles is leaves that have been partially or completely eaten.
On some varieties all the leaf tissue is consumed in a given area of the leaf, and for plants with a different vein structure, the beetles prefer to eat only the parts of the leaf between veins, leaving a skeleton of only the veins, which turn brown. When enough leaves have been skeletonized, the tree can take on a brown appearance, and eventually the skeletonized leaves fall off.
Most landscape plants will not be killed by Japanese beetle attack during a single growing season, but with a significant loss of the leaf surfaces, there is less starch and sugar being produced and stored for the next years growth. Repeated attacks over a period of years may weaken trees, shrubs and perennials.
In the Northfield area, Japanese beetle activity begins in June and peaks in July and gradually lessens as summer transitions to fall. If infestations are low, hand picking the beetles off your plants may help somewhat.
Another approach can be spraying with an insecticide, and repeating applications frequently. A drawback of insecticides is that they may harm bees, and other pollinator insects. If you apply insecticide, spray at times when bees are less active such as late day, just before sunset.
Japanese beetle traps are being sold and used, but may have questionable effectiveness. Some reports indicate that while traps may fill up with the beetles, they may be actually drawing more beetles then ever to your are with their attractive scents that are released.
In summer following mild winters with good snow cover, Japanese beetles will survive the winter in greater numbers, and infestations will be greater. In years with harsher winter temperatures, and poor snow cover, more beetle grubs will die in the ground where thy spend most of their lives, and numbers of adults that emerge in June/July will be reduced. It is quite likely that Japanese beetles are here to stay, at least until some good biological controls are discovered, which I don’t think has happened.
A good apprrach may be to gradually switch over a period years to landscape plant materials that are of little interest to the Japanese Beetles. In future blogs, I will discuss some of the varieties that suffer little or no damage from these pests!