BY ELAYNE SEXSMITH

Ask a gardener this time of year how their garden is doing, and you’ll often hear horror stories of damage and dismay brought on by Japanese beetle infestations. Listening to gardeners at the Johnson County Fair Attended Displays and other MG events we heard many of these stories that often ended with pleas for advice as to how to banish these beetles. With the worst of this year’s damage still in our memories, I want to share my experience in successfully managing the adult Japanese beetles in my yard—something for you to consider as you start planning your garden for the coming year.

Truthfully, if the Japanese beetle (Popillia japonica) wasn’t so destructive, they might be considered attractive. You know them when you see them—they’re small (less than ½-inch long) with an iridescent green head and thorax with coppery brown wing covers. The adult beetles emerge from the ground in late June to early July and begin feeding immediately destroying some of our favorite garden plants. And it isn’t your imagination, these beetles are rarely seen alone – they are often found in large clusters mating and feeding and ultimately skeletonizing the leaves on their favorite trees, shrubs and plants and eating fruits and flowers. It turns out that the leaves they damage emit an odor that is attractive to other Japanese beetles which invites more of the metallic green beetles to join the party. And while healthy, mature plants can typically withstand the damage, young and weaker plants can be permanently disfigured and future production impacted.  

Some background before I share my success in dealing with this year’s infestation in my backyard. The Japanese beetle was inadvertently introduced into the United States in a nursery shipment arriving in New Jersey from Japan in 1916. While not a major pest in their home country, conditions here were ripe for their success as there are no major predators to keep them in check. As a result, the Japanese beetles have been steadily making their way across our country. They were first reported in Iowa in 1994. As of now, they’ve traveled as far as the middle of the country and are present in ¾ of Iowa counties.

So many gardens, so much damage. The beetles can seem relentless—they feed on over 350 species of plants and that long list includes many gardeners’ favorites. In my yard zinnias, cosmos, dahlias, roses, raspberries and blackberries have all suffered from beetle damage. And the damage isn’t limited to what we see above ground—the larvae and grub form of the beetles live in the soil where they feed on turf and plant roots, damage that is further compounded as raccoons, skunks and even crows dig into the ground to feed on the grubs. How does one control these adult rose-destroying menaces? One straightforward approach is to manually pick the insects off plants in the morning when the beetles are less active and drop them into soapy water. People with smaller gardens and stronger stomachs have success with this. And while insect traps and lures are temptingly simple to use, there seems to be a common understanding that these may attract more beetles to your yard than they trap. Neither of these approaches appealed to me and I was hoping to avoid the use of traditional insecticides in my dog-friendly yard so I was delighted to read about the success that some gardeners were reporting with some new Bt-based products.

So … here’s my experience. After investigation, I purchased beetleJUS!^® online early in the gardening season to have on hand so I could respond quickly to the first Japanese beetle sighting in my yard (the week of June 23 in my rural Iowa City garden). beetleJUS!^® contains Bacillus thuringiensis galleriae in a powder that is to be mixed with water and then sprayed on the foliage and flowers of the favored plants. The product is certified by the National Organic Program for use in the production of organic food. The label indicates that it is not to be applied within 300 feet of any habitats of threatened or endangered moths and butterflies and has a caution statement to keep out of reach of children. The spray does not kill the beetles on contact; the beetles must eat some of the treated foliage and then they die.

We timed our first use of the product around rainstorms, wanting to be sure that the beetles had adequate time to ingest the Bt product before rain washed it off. We went out the morning after that first application and the beetles were gone—yes, gone. We could still see a few beetles flying around the berry bushes, but their numbers were small and those beetles soon disappeared as well. Altogether, we sprayed 3 times this summer and we were very pleased with the results. We quickly learned to make certain we applied the product to the actual blooms, not just the foliage, or the Japanese beetles would continue their destruction of those flowers. Each time we used the minimum amount of product called for in the instructions, applying with a hand-held pump sprayer. We felt the process was easy and effective. We have a sizeable yard with a number of plants that the Japanese beetles find irresistible. The cost of the product for the season (roughly $40) was acceptable and allowed us to enjoy the fruits of our labor.

While there is no guarantee that you’ll have the same experience as we did, I believe the Bacillus thuringiensis galleriae is definitely worth exploring. And as you’re looking on the internet you might also want to study the lists of plants that the Japanese beetles don’t find appealing—investing in plants they don’t find attractive is a worthwhile gardening strategy.

And it’s interesting to note that Japanese beetles are not immune to the impact of climate change in our gardens. Model projections based on current and future climate conditions predict that the beetle’s northward expansion could extend into Canada while moving its southern-most range simultaneously. Unfortunately, in all these projections the Japanese beetle remains viable in Iowa.

Resources

1. Japanese Beetle, Iowa State University Extension and Outreach, Horticulture and Home Pest News, last reviewed July, 2021.
2. Japanese Beetles in Yards and Gardens, University of Minnesota Extension, authors Jeff Hahn, Extension entomologist, Julie Weisenhorn, Extension horticulturist, and Shane Bugeja, Extension educator, Reviewed in 2020.
3. Japanese Beetle, University of Wisconsin-Madison Extension, Authors R. Chris Williamson, P J Liesch, and Vijai Pandian, Last Revised December 2020
4. Kistner-Thomas, Erica Jean, The Potential Global Distribution and Voltinism of the Japanese Beetle (Coleoptera: Scarabaeidae) Under Current and Future Climates, Journal of Insect Science, March 2019, Department of Agricultural, Agricultural Research Service, Midwest Climate Hub, National Laboratory for Agriculture and the Environment, Ames IA.
5. beetleJUS!^® for Ornamental and Vegetable Pests, Manufactured for Gardens Alive!^®, Phyllom BioProducts Corporation.