Japanese beetles can do a lot of damage to your plants in a short period of time. In some cases, they completely defoliate trees and shrubs (which means there are no leaves left at all) in just a few weeks.
That’s why you want to protect your trees and shrubs from Japanese beetles before they have a chance to attack. In this article, we’ll explain some defense strategies to keep Japanese beetles out of your garden in the first place.
- 5 ways to prevent Japanese beetles on trees and shrubs
- Trees and shrubs that attract Japanese beetles
- Japanese beetle-resistant trees and shrubs
- About Japanese beetles
- FAQ about protecting trees and shrubs from Japanese beetles
- Get a head start on Japanese beetle protection
5 ways to prevent Japanese beetles on trees and shrubs
1. Wipe out Japanese beetle grubs
When Japanese beetles first hatch, they’re not beetles yet. They’re tiny white worms called grubs or white grubs. Those grubs live in soil and feed on grass and plant roots, leaving brown patches of lawn and dead plants in their wake.
Even though grubs are a menace to lawns and smaller plants, they won’t affect your trees and shrubs. If you exterminate the grubs before they mature into adult beetles, you won’t ever have to worry about damage to larger woody plants.
How do you get rid of grubs? Here are a few suggestions:
- Beneficial nematodes
- Milky spore
- Neem oil
- Attracting birds
- Lawn-aerating sandals
When should you get rid of grubs? If you want to prevent an attack of adult Japanese beetles in the summer, you have to exterminate grubs in early spring. There’s a second wave of grubs in early fall, too, and you can exterminate them to prevent more grubs and beetles next year.
For a more in-depth explanation on controlling grubs, see How to Get Rid of Grubs in Your Lawn.
2. Floating row covers
Farmers often use floating row covers for their crops, but you can use them in your garden, too. Floating row covers are made of lightweight cloth that lets light, water, and air reach your plants but blocks out pests like Japanese beetles.
Here’s how to install floating row covers to protect your trees and shrubs:
- Step 1: If you plan to fertilize your plants, do so before covering them.
- Step 2: Drape the fabric over your plant. Leave plenty of fabric trailing on the ground for anchoring.
- Step 3: Double over the edges of the fabric before anchoring. This makes the fabric less likely to rip out of the ground in heavy wind.
- Step 4: Bury the doubled-over edges of fabric in the soil or use landscape staples to anchor the row cover to the ground. If using staples, use several on each side of the tree or shrub.
- Step 5: When you’re done, the cover should be loose to leave room for plant growth.
Install row covers over your plants during the adult Japanese beetle’s active period of early June through late August.
Downsides of row covers: Row covers aren’t for everyone. For one, if summer is your plants’ season of interest, it would be a shame to cover them up. For another, row covers keep out pollinators along with pests, so you can’t use them on plants that need pollination.
3. Space out plants that attract Japanese beetles
There are some trees and shrubs that Japanese beetles always flock to, such as roses, raspberry bushes, and crabapple trees. We’ll talk more about plants that attract Japanese beetles later.
The point is, if you must have roses or another favorite food of Japanese beetles, plant them far apart from each other. A whole bed of rose bushes is a beetle infestation waiting to happen, but one rose bush on its own is much more manageable.
Even better, plant resistant plants (more on those later) around any plant that particularly attracts Japanese beetles. Space out their favorite plants to break up their feeding grounds and make your garden less inviting.
4. Natural Japanese beetle repellent sprays
Take advantage of natural smells that repel Japanese beetles and use them to make your own repellent sprays.
Here are two simple repellent sprays easy enough for anyone to make:
- Cedar spray: Mix a few ounces of cedar oil into 5 gallons of water and pour the solution into a spray bottle.
- Garlic spray: Crush at least six cloves of garlic (more if you want a more potent spray). Soak the crushed garlic in 1 gallon of boiling water overnight. After the garlic has had time to steep, strain out the garlic pieces and pour the water into a spray bottle.
Apply your homemade repellent spray to your plants every few days from early June to late August to keep Japanese beetles away. Re-apply the spray any time rain washes it off your plants.
5. Preventive pesticides for Japanese beetles
Don’t reach for chemical insecticides right off the bat to prevent Japanese beetles. These chemicals contribute to water pollution and reduce populations of beneficial insects such as butterflies and bees, so you should only use them as a last resort if other methods fail.
Residual pesticides: If you decide you need chemical pesticides to keep Japanese beetles away from your trees and shrubs, purchase a residual insecticide with one of the following active ingredients:
- Carbaryl or acephate: Apply every 1-2 weeks
- Pyrethroids (including bifenthrin, cyfluthrin, lambda cyhalothrin, and permethrin): Apply every 2-3 weeks
- Chlorantraniliprole: Apply every 2-4 weeks
Residual pesticides come in different forms, including liquid sprays, dusts, and granules. How you apply the treatment and how much you should use varies for each individual product. Always follow the instructions on the label carefully.
Apply residual pesticides throughout the beetles’ active season of early June through late August for effective prevention.
Systemic pesticides: Instead of a spray that goes on top of plants, systemic pesticides work from the inside out. You inject these pesticides into the soil or directly into the trunk itself so that the plant absorbs the poison and passes it along to Japanese beetles munching on its leaves.
Look for these active ingredients in systemic pesticides for Japanese beetles:
Systemic pesticide treatments like these are best left to pest control professionals. In fact, some systemic chemicals are only available for pros.
Trees and shrubs that attract Japanese beetles
Be smart about your plant selection if you’re serious about preventing Japanese beetles in your garden. Some plants are more likely to attract them than others.
According to the United States Department of Agriculture, these are the trees and shrubs most often infested with Japanese beetles:
- Japanese maple
- Norway maple
- Virginia creeper
- Red raspberry
- Pin oak
- American mountain-ash
- Black walnut
- Lombardy poplar
Avoid planting these trees and shrubs if you live in an area prone to Japanese beetles. If you already have some of these plants in your garden, keep a close eye on them in summer. The sooner you spot Japanese beetle damage, the better.
Japanese beetle-resistant trees and shrubs
Just like there are plants that Japanese beetles particularly love, there are trees and shrubs they tend to avoid. When planning your garden, choose as many of these plants as possible to make your yard less attractive to Japanese beetles.
The USDA’s list of Japanese beetle-resistant trees and shrubs includes:
- Red maple
- Tulip poplar
- Northern red oak
There’s no guarantee that Japanese beetles won’t attack these plants if other food sources are scarce. However, these trees and shrubs are less likely than others to host a Japanese beetle infestation.
About Japanese beetles
The more you know about Japanese beetles (scientifically known as Popillia japonica), the better you can combat them. Learn to identify these pests based on their appearance and signs of damage. Plus, you can figure out the best time to apply pesticides based on their life cycle.
Japanese beetle appearance
If you have an infestation of adult Japanese beetles, you’ll probably see them crawling around on your plants in the mornings and evenings.
Here’s what they look like:
- Size: About ½ inch long
- Shape: Oval-shaped body with hard, shell-like wings
- Coloring: Coppery wings and a metallic green head with white hairs along the sides of the body
Japanese beetle damage
What does a plant look like after Japanese beetles attack it? Some trees and shrubs are left with no leaves at all.
When Japanese beetles eat leaf tissue, they eat around the veins. What’s left is the “skeleton” of the leaf, which looks like a lacy pattern.
Japanese beetle active season
If you want to know when to expect Japanese beetles, you have to understand their life cycle.
In spring, they emerge from underground as larvae, when they’re called grubs. Grubs live in soil and eat grass roots, which often results in brown patches of lawn.
Grubs grow into adult Japanese beetles who leave the soil in early June, sometimes as early as mid-May in the South. They feed on plants and lay eggs for about 40 days, then die out by late August, although some might live until early September.
The beetles’ summer eggs hatch as a second generation of grubs in early fall. By winter, the grubs will go back to living deep underground, only to emerge again the next spring. And the cycle continues.
Japanese beetle behaviors
The most important thing to know about Japanese beetle behavior is that they eat in groups. Any time one of them eats a leaf, it releases a pheromone that alerts other beetles to the presence of a food source and attracts them.
That means that even if you only see one Japanese beetle on your plant, more are on the way. They sometimes travel from several miles around looking for food.
FAQ about protecting trees and shrubs from Japanese beetles
No. For most plants, Japanese beetle damage is only a cosmetic issue. However, young plants and plants with other health issues could potentially die from excessive Japanese beetle feeding.
Fruit trees and vegetable plants that Japanese beetles have fed on can still produce, but you should expect a lower yield than usual.
If Japanese beetles are already feeding on your trees and shrubs, here are the most plant-friendly ways to get rid of them:
—Hand-picking them off your plants and throwing them in a bucket of soapy water.
—Make a natural insecticide.
—Lay a drop cloth around plants to collect beetles easily.
—Attract natural enemies of the Japanese beetle, such as birds and parasitic flies.
For more information on these and other methods of Japanese beetle control, see our complete guide on How to Get Rid of Japanese Beetles.
Japanese beetles hate many of the same scents as other insects, such as:
Get a head start on Japanese beetle protection
To protect your trees and shrubs from Japanese beetles, you have to think ahead. Take measures (like the ones described above) to prevent these voracious pests before you ever see a sign of them.
Since adult Japanese beetles feed in summer, the best time to think about prevention is spring. Exterminate grubs when they first emerge in early spring, then begin spraying repellents or pesticides in May.
Have you tried everything, and you still can’t break the cycle of grubs and Japanese beetles on your own? Reach out to a pest control professional for help.
Main Photo Credit: Richard Malo | Pixabay