The Japanese beetle invasion
What to do about it
Last summer Japanese beetles were the biggest issue in my garden (and probably yours). In previous seasons I would occasionally spot a few adult beetles lurking on my roses, but damage was minimal and they didn’t stick around. In 2023, however, everything changed as my entire garden experienced a full-scale invasion.
Japanese beetles munched my beans, annual flowers, peas, roses. Virtually no plants were spared. And of course, it wasn’t just my garden. I had hundreds of inquiries from gardeners across the province worried about this invasive pest.
To learn more about Japanese beetles and what we can do to reduce their populations, I turned to Paul Manning, Assistant Professor at Dalhousie University (Faculty of Agriculture).
He explained that Japanese beetles were accidentally introduced to the United States from Eastern Asia over a century ago and eventually made their way to Canada.
“They were first spotted in Yarmouth, Nova Scotia in 1939 and have been slowly spreading across the province,” he said, adding that on average, an invasive population spreads around 10 kilometres each year.
Japanese beetles are easy to identify. The adults are about one centimetre long with copper-coloured wing cases and green thorax and heads. They also have little tufts of white hair along their abdomens.
“The larvae are pale cshaped grubs that live in the soil which look a lot like a small June bug larvae,” says Manning. "And like many other larval scarabs feed on the roots of grass.”
It’s the adults that do the most damage to garden plants. They’re ravenous herbivores that feed on over 300 species of trees, shrubs, perennials, vegetables, herbs, and annuals. They mostly consume leaves, but will also nibble on fruits and flowers (like my roses).
“The adult beetles tend to concentrate feeding on the softer leaf tissue between the veins, resulting in skeletonized feeding patterns,” notes Manning.
Manning thinks it’s likely that 2023 was an exceptionally bad year as we may have had a double population.
“Japanese beetles only reproduce once a year with the eggs (laid in July-august) hatching soon after, and the larva feeding in the soil for at least one year,” he said.
However, he adds that if the larvae have not stored sufficient energy to complete development, they may spend another year in the soil.
“That means we may have had a combination of 2-yearold and 1-year-old beetles emerging simultaneously.” Gasp!
So what can we do to prevent or reduce Japanese beetles in our gardens this year? Pheromone traps are available at garden centres, but research has shown these actually attract more beetles to your yard.
“The main control is hand picking, especially in the evenings,” said Manning, noting the beetles are most active in early afternoon. "It’s best to wait until they’re more lethargic and easier to catch."
He also recommends using floating row covers (combined with crop rotation) overtop vegetable beds. These lightweight covers prevent beetles from accessing your crops and still allow light, water, and air to pass through.
“These are far more effective than synthetic or botanical insecticides,” says Manning, who also suggests growing plant species that are resistant to Japanese beetles. These include lilacs, hollies, hostas, pansies, nasturtiums, begonias, dogwoods, and boxwood.
Other options you’ll find at local garden centres, like Halifax Seed, include BT (Bacillus thuringiensis), a natural soil bacterium that when consumed by grub species causes them to stop feeding, and nematodes, microscopic worms that target grubs.
I’m hopeful we’ll have a single population of Japanese beetles this summer and have fewer issues with this voracious pest. However, Manning believes that with climate change we can expect to see more invasive pests.
“Prolonged periods of cold temperatures during the winter are an important abiotic factor that keeps insect populations in check,” he said. “This means that species that may have otherwise been unable to withstand our winters can achieve this more readily.”
Other invasive pests that have established themselves in the province include hemlock woolly adelgid, emerald ash borer, and beech mining weevil. Manning recommends keeping an eye on your garden and making note of any changes or unusual insects you see.
Niki Jabbour is the author of four best-selling books, including her latest, Growing Under Cover. She is a twotime winner of the American Horticultural Society Book Award. Find her at Savvygardening.com and on social media.