Battling Japanese beetles

Wednesday, June 29, 2011
Paul Schnare shows the damage Japanese beetles can do to plants. (Kristin Eberts)

Over the weekend I was looking at a list of subjects that I had discussed in previous columns. Last summer at this time the topic was Japanese beetles. Normally I don't write about the same subject within 12 months, but I felt that because of all the questions I have been fielding over the last two weeks about Japanese beetles, I should revisit the current invasion.

Japanese beetles were introduced about 1916 into the United States on the East Coast.

They have been moving their way westward ever since. I first heard of their presence west of the Mississippi about four or five years ago. They are here to stay.

There seem to be a lot of misconceptions about the damage they cause, their life cycle and consequently their control. I will try to clear up these concepts.

The current invasion of adult Japanese beetles began about two to three weeks ago. The adults are focused on two things: feeding and mating.

There are more than 400 known plants that have been documented as food sources for the adults. So just because you only see them on roses, thatdoesn't mean they won't move to other plants.

The adults feed on and skeletonize the foliage or flower. One or two beetles per plant will not cause significant damage. Unfortunately, these beetles like to congregate. They produce pheromones (sex attractants) that draw all of the boys and girls from the entire neighborhood to that one plant. A congregation of beetles can destroy a plant in just a few hours.

The adult female will lay 40 to 60 eggs per season in soils around your landscape and in your lawn. These eggs will hatch later this summer and become grubs. These grubs will be eating plant roots during the fall and again next spring.

About April or May the grubs change again and become pupae. These then become adults and the feeding/mating season begins again.

Knowledge of the life cycle is essential for control. Don't think that you can make one application of a product in a season and forget about the problem until next year.

I would suggest that you consider a three-pronged attack on the beetles. First, when you see beetles feeding on plants in your landscape immediately spray the plant/plants with a common insecticide such as malathion, carbaryl, permethrin, deltamethrin or spinosad. Be sure to read the pesticide label. Not all formulations can be used on edible plants.

You also should place traps in your lawn area to draw the adults away from valuable landscape plants. Do not place the traps in your flower/vegetable garden or near your valuable landscape plants. Place the traps 20 to 30 feet away from these plants so you draw the beetles away from your landscape/garden. If you have a lot of beetles in your area, use more than one trap.

Finally, in August apply imidacloprid in a granular form to your landscape planting soils and to your lawn. This insecticide will kill grubs and help reduce the population for next year. You can also use milky spore disease. This product needs to be applied several times over a two-year period in order to get good, long-lasting control.

Again, I would like to suggest that you consider all three forms of control.

These beetles are going to be with us for a long time. In fact, my friend Dr. Sven Svenson told me that he lived in Knoxville, Tenn., when the invasion reached that city. The population continued to climb for several years before it began to level off. I think we will struggle with beetle control for years to come.

Send your gardening and landscape questions to Paul Schnare at P.O. Box 699, Cape Girardeau, MO 63702-0699 or by email to

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