As Japanese beetles are spreading throughout Iowa and populations are increasing, more and more gardeners are dealing with these very hungry garden pests. Japanese beetle adults feed on a wide variety of plants. Iowa State University Extension 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.
What are the Japanese beetle's favorite food plants?
Adult Japanese beetles have been documented to feed on the foliage, flowers and fruits of more than 300 different plants. Their top favorites are grape, cherry, apple, rose, raspberry and linden. A link to a list of the Japanese beetle's most- and least-favored woody plants can be found at the following website: www.ipm.iastate.edu/ipm/hortnews/2010/7-14/japanesebeetle.html
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.
Will treating my lawn for white grubs reduce the population of Japanese beetles on my plants next summer?
It would be nice if life could be that simple, but the relatively small area you can treat (compared to the grassy sites in the surrounding area) will not have any impact on the following year's adult population. Japanese beetle adults are very strong and capable fliers and may travel long distances from where they developed as larvae in fence rows, roadside ditches and other grass sites, to where they are feeding. You are likely to have beetles next summer whether you treat the grub stage in your lawn or not. Controlling one life stage does not preclude potential problems with the other.
This does not mean you should not treat the turfgrass. If desired, high value turf can be protected from the root-feeding larvae by treating the soil with a preventive insecticide in a timely fashion and according to label directions. Most of the available insecticides must be applied before mid-August to be effective as preventive treatments. Treating for grubs will protect the turfgrass. It will not prevent beetles from feeding in your landscape the following year.
What is the life cycle of the Japanese beetle?
In Iowa, adult beetles emerge in mid-June through July. Japanese beetles are similar to other Junebugs in appearance and 3/8 inch long and 1/4 inch wide. The head and thorax are shiny metallic green and the wing covers are coppery red. A distinguishing feature of the beetles is five tufts of white hairs on each side of the abdomen.
Japanese beetle larvae are typical white grubs. The larvae are in the soil from August until June where they feed on plant roots (especially turfgrass) and organic matter. The grubs are C-shaped and approximately 1.25 inches long when full grown.
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 to 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.
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