Expert shares how to counter Japanese beetles, the nemesis of turfgrass

Japanese beetles look pretty on the outside, with their shell of iridescent green and their copper-colored outer wings. Yet that beauty belies a destructive force, as the insects have no natural predators in the U.S., while their voracious appetites can devastate more than 300 species of North American plants, from turfgrass to roses, grapes and hops.

Virginia Tech entomologist Thomas Kuhar answered questions about Japanese beetles and shared the back story of these deceptively lovely pests.

Q: What are the risks Japanese beetles pose to home gardens and to crops?

“The immature beetle larvae, or white grubs, primarily attack the roots of grass crops from turf to corn, but also may attack the underground plant parts of many vegetable crops such as potatoes, sweet potatoes, and more.  The adult beetles can be equally damaging to other crops such as roses, fruit trees, grape leaves, and many vegetable and ornamental plants especially those flowering.”   

Q: Are Japanese beetles an invasive species?

“Yes, the Japanese beetle is an invasive species. The first report of this beetle within the United States occurred in 1916 in a nursery in New Jersey. As shipments of flower bulbs from Japan were reported to contain white grub larvae a few years prior, researchers believe this may have been how they arrived in the country. The insect has been slowly moving in all directions in North America for over a century. It’s been established in Virginia for many decades, and it is the most important pest of turfgrass.”

Q: How has climate change affected the Japanese beetle population?

“As beetle larvae move deep in the soil in late fall to hibernate during the winter months, they typically can withstand colder climates. It could be that a warmer winter climate may enable greater larval survival in the colder regions of North America. On the other hand, Japanese beetle larvae are very susceptible to dry conditions during late June and early July, as this is a period in their life cycle when eggs are hatching, and tiny neonates are near the soil surface. If climate change leads to more droughts during late June and July, then there will certainly be a negative impact on white grub survival and subsequent Japanese beetles flying the following spring.”

Q: What can a homeowner do to control Japanese beetles?

“Garden supply stores will usually sell three types of control products for Japanese beetles: beetle traps; synthetic insecticides; and natural, or organic, insecticides, which contain either essential oils, soaps or neem oil. None of the natural insecticide products work very well against Japanese beetles in my experience, although they can help with other pests like aphids.  The Japanese beetle bags or traps contain a potent combination lure that draws beetles to the trap. However, putting them near your garden can make things worse by drawing beetles in the neighborhood to your location.  The most effective control is synthetic insecticides that have beetles on the label.  These are often pyrethroid type insecticides. You may need multiple applications during the heavy infestation period in the summer.”

Q: Are there any natural defenses against Japanese beetles in the U.S.?

 “There is an attractive and very common wasp called the blue-winged wasp that buzzes around the surface of lawns that is an important biological control agent of Japanese beetle white grubs.  The wasp is about 1 inch long with dark blue wings and an orange-colored abdomen with two yellow spots. The wasps find and sting the white grub larvae and deposit their eggs within. Their own larvae will devour the white grubs.”

Q: Are there any other surprises when it comes to these beetles?

“Japanese beetles are not established in the western states.  So, if someone comes from California to Virginia, for example, they are often interested in this pretty beetle that they see everywhere — while if you’re from the east coast, you’re often surprised as this insect has become such a common nuisance and economic pest.”  

About Kuhar
Thomas Kuhar is a professor of vegetable entomology in the Virginia Tech College of Agriculture and Life Science’s Department of Entomology. His goal as a researcher is to help implement sound integrated pest management practices that can enhance the profitability and sustainability of crop production in Virginia, as well as improve food and environmental quality by minimizing the use of toxic pesticides. Read more here.

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