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Japanese beetles have plagued Eastern U. S. gardens for decades. Meanwhile, dimly aware of the insect’s destructive potential, Minnesota gardeners battled lesser pests. Sadly, the age of innocence is ending here, especially in the Twin Cities area.
The beetles have been present in Minnesota for several years but populations have been local and numbers relatively low. This year many Twin Cities area gardeners encountered them for the first time and numbers exploded in some places. More of the same is likely to come.
The bronze and green beetles are known to feed on 300-400 kinds of plants. Rose, linden and grape are among their favored hosts. The beetles are gregarious, often gathering in clusters on chosen plants with remarkably destructive effects. Beetle activity peaks mid-July through mid-August.
Controlling the beetles is so difficult that enduring some damage will probably be the only option for most gardeners. However, measures can be taken to reduce their numbers.
Recommended
Hand Picking: Knock beetles into a pan of soapy water.
Barriers: Cover prized plants with mesh or netting.
Plant selection: Choose plants that do not attract the beetles.
Insecticides: Use products labeled for beetle control.
Not Recommended
Milky spore: This bacterial product has been used to control Japanese beetle grubs in turf. It hasn’t been proven effective in Minnesota. The beetles are strong fliers so grub control doesn’t guarantee a beetle free garden.
Traps: Research shows that traps often attract more beetles than they trap, making a bad problem worse.
Here are links to more information about Japanese beetles and ways to control them:
http://www.extension.umn.edu/distribution/horticulture/DG7664.html
http://www.ca.uky.edu/agc/pubs/ent/ent5/ent5.htm