From the ground up: Japanese beetles are back and just as hungry as ever

Each adult beetle will lay up to 60 eggs in 30-45 days

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If you donít already know it, I am sorry to be the one to tell you. The Japanese beetles have emerged and if they are not there already, they will soon show up in your yard and garden. Linn County Master Gardener Lisa Slattery provides some guidance for us as we attempt to grow food and flowers despite their presence.

Q: How can I stop Japanese beetles from eating my garden plants and trees?

A: The beetles are back. Japanese beetles will be present for about the next six to eight weeks. Adult beetles are starting to emerge from the ground with new adults continuing to appear through July. Each adult beetle will live from 30 to 45 days. They will lay up to 60 eggs in that time frame, and the grubs will grow to almost 2 inches.

The Japanese beetle was first found in the United States in 1916, and although the eastern United States has a natural control for them which is a soil-inhabiting protozoa, the Midwest does not. These beetles are very destructive eaters since the beetle grub feeds on the roots of grass, and the adult beetle feeds on more than 300 types of plants. They may not kill plants, but they severely damage and destroy the foliage and blooms. They do like some plants better than others: roses, hollyhocks, linden and crabapple trees, willow trees and cannas. They will quickly skeletonize the leaves of a plant.

They are slightly square-shaped copper/bronze with a shiny metallic look. You find them clumping on both foliage and blossoms.

They are tough to control. One way is to hand pick or knock them into a container of soapy water. Itís possible to protect high value plants by covering them with fine mesh netting or screening material with openings in the material less than one quarter inch. Make sure there are no openings for the beetles to enter the netting. Another is to use a contact spray insecticide, which will only reduce damage for a few days with repeated applications necessary. However, insecticides are toxic to beneficial pollinators like bees.

There are several types of beetle traps on the market that use a floral scent or pheromone to lure beetles into a net, jar or bag where the beetles can be contained till disposed of. ISU does not recommend using these since they can attract more beetles than they catch. In heavily infested areas, traps may catch hundreds or thousands of beetles in the course of the summer but thatís a small percentage of the beetles in the area and appears to make no lasting impact on the beetle population or on plant damage.

The foliage of recently planted and high-value trees can be protected with a soil-drench application of a systemic insecticide, such as imidacloprid. But in order to be effective, treatments must be made several weeks ahead of beetle emergence.

Some homeowners will try to eliminate the beetle grub by applying an insecticide to the lawn. This can prevent the grubs from damaging the turf grass but it will likely have little effect on next yearís Japanese beetle population. Japanese beetles are strong, capable fliers and can fly a long distance from where they developed as larvae (grubs.) Since Japanese beetles will fly in from adjacent areas, treating your lawn with an insecticide will likely have little effect on your beetle population next year.


33rd annual Seed Savers Conference, Friday and Saturday, 3094 N. Winn Rd., Decorah. Register at

Cedar Rapids Downtown Farmers Market, 7:30 a.m. to noon Saturday, Third and Fourth avenues and Second and Third streets SE.

Questions on gardening, land use or local foods? Contact Michelle Kenyon Brown, community ag programs manager at Linn County Extension,

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