Japanese beetles

It is interesting that we have two pests that are metallic green that cause so much widespread problems in the gardening world. They are also both invasive pests too.

For the purpose of this column today, I am not going to go into any detail about Emerald Ash Borer except that the Minnesota Department of Ag is keeping an eye on it. We know that for Lyon County the closest place that has it is Sioux Falls, S.D. It is also in Stearns County and Martin County, with all of Iowa being also quarantined for it.

This week the metallic green insect that we are focusing on is Japanese beetles. If they are in your community, I would like to hear about it and possibly get some pictures of the insect that you are looking at too. There are some look-alikes that we need to make sure what you are looking at is, indeed, Japanese beetles. They are in Marshall around the Legion Field Park area, golf course and the fairgrounds. They are confirmed through the Minnesota Department of Ag to be here. They can be easily identified. They are approximately 1/3 to 1/2 inch long with metallic green head and thorax (the area behind the head) with copper-brown wing covers. The sides of abdomen have five white patches of hairs, and tip of abdomen has two patches of white hair. The grubs of Japanese beetles can be identified as well but not as easily as the beetle. They are C-shaped, white to cream-colored grubs with a distinct tan-colored head. Legs are easy to see. Japanese beetle grubs look like other white grubs and can only be positively distinguished by examining the pattern of spines and hairs on the underside of the tip of the abdomen.

Adult Japanese beetles are destructive in the garden and with field crops. They eat just about everything. They feed on flowers, fruit and leaves of plants. They will eat everything on a leaf except for the stem and veins of the leaf. Healthy, mature trees and shrubs can tolerate a lot of feeding without significant, long-term injury. If gardeners have younger plants or a plant or tree that is not doing well, gardeners should make sure to treat or pick off the beetles. Japanese beetles can kill a plant that is sick or not growing well.

Japanese beetle grubs are also causing problems in the gardens but particularly in our lawns. The grubs eat grass roots which means the grass plants cannot take up water and nutrients to remain healthy. If gardeners have a severe grub feeding area in the lawn, the grass will die and leave large patches of dead grass. Grubs are not the end, however, to the lawn. A good healthy lawn can withstand a certain amount of feeding without having large dieback. If gardeners see moles, voles, skunks and sometimes even raccoons digging in the lawn, it is because they are feeding on grubs in the lawn. The presence of these animals does not necessarily mean that Japanese beetles are there but it could also mean that there are other grubs present such as June beetles.

According to the U of M Extension, “products containing pyrethrins (e.g. Pyola) are somewhat effective contact insecticides provided they are sprayed directly on the insect. Repeat applications are necessary. Avoid spraying bees and beneficial insects as these products are toxic to these non-pests as well.”

Hahn and Weisenhorn with the U of M Extension, “Neem oil is effective for several days but repeat applications are necessary. Neem oil helps deter Japanese beetles but is less effective when large numbers are present. This product is low risk to bees and other beneficial insects. Bacillus thuringiensis galleriae (e.g. BeetleGone, BeetleJus), derived from soil bacteria, is moderately effective against Japanese beetle adults, giving one to two week’s protection. This product is not toxic to bees and other beneficial insects. Bacillus thuringiensis galleriae is only available from online sources.”


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