Yard and Garden: Japanese Beetles



AMES, Iowa – Japanese beetles first emerged around the end of May in some parts of Iowa. As populations increase, gardeners around the state are asking how to deal with these very hungry garden pests. Japanese beetle adults feed on a wide variety of plants. Iowa State University Extension and Outreach specialists answer questions concerning these difficult to control pests. To have additional questions answered, contact the experts at hortline@iastate.edu or call 515-294-3108.

Japanese beetles are devouring my roses.  What can I do? 

Control of Japanese beetles is difficult. Persistence, diligence and repeated efforts often are necessary because new beetles emerge every day over a period of several weeks.  

Handpicking may be feasible when dealing with small beetle populations. Remove beetles early in the morning while temperatures are cool and the beetles are sluggish. Collect or shake beetles into a bucket of soapy water and discard.  

It’s also possible to protect roses and other high value plants from Japanese beetles by covering them with fine mesh netting or screening material. (Openings in the material should be less than one quarter inch.) Make sure there are no openings for the beetles to enter the enclosure.  

Insecticides are another option. Spray roses with a garden insecticide, such as carbaryl (Sevin) or permethrin, to reduce damage for a few days. Repeated applications will be necessary to maintain control. Avoid spraying rose flowers, as many insecticides are toxic to bees. 

Can Japanese beetles be effectively controlled by using traps?

In a word, NO. Several kinds of traps are available that use a floral scent and/or sex attractant to lure beetles into a net, jar or bag where the beetles can be contained till disposed of. In heavily-infested areas, traps may catch hundreds or thousands of beetles in the course of the summer. Unfortunately, this is a small percentage of the beetles in the area and makes no lasting impact on the beetle population or on the plant damage experienced.

The use of traps is not recommended. Research conducted in Kentucky and elsewhere found the traps do not control moderate to heavy infestations. The traps may attract more beetles than they catch and actually add more beetles to the yard than would occur otherwise.

In isolated locations far away from other Japanese beetle infestations, and in very lightly-infested areas, trapping may provide some benefit. Otherwise, traps will not make a difference.

How do I control Japanese beetles on my grapevines?

In home gardens, small numbers of Japanese beetles can be controlled by handpicking. The best time to physically remove Japanese beetles is early morning when the beetles are sluggish. Collect or shake beetles into a bucket of soapy water and discard.  

If handpicking is not feasible, applications of an insecticide, such as carbaryl (Sevin), malathion, or permethrin, can reduce Japanese beetle damage. Repeated applications will likely be necessary because of the short residual effect of the insecticides. Be sure to observe the harvest waiting interval when applying insecticides to grapes and other edible fruits. 

Japanese beetles are defoliating my linden tree. Will they kill it? 

Japanese beetles feed on more than 300 different plants. However, lindens are one of their favorites. Defoliation of well established, healthy lindens (and other trees) is usually not fatal. Defoliation is most harmful to recently planted trees (those planted in the last two or three years) and trees in poor health.  

The foliage of recently planted and high-value trees can be protected with a soil-drench application of a systemic insecticide, such as imidacloprid (Bayer Advanced Tree and Shrub Insect Control and other products). To be effective, treatments must be made several weeks ahead of beetle emergence. 

How long do Japanese beetles feed on plants in the yard and garden? 

Japanese beetles are present for about six to eight weeks every summer. Adult beetles usually begin to emerge from the ground in mid-June and new adults continue to appear through July. Each beetle lives from 30 to 45 days.

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Photo by Clemson University - USDA Cooperative Extension Slide Series