You have most likely already seen Japanese beetles in your surroundings. They are beautiful, but also formidable leaf chewers! Perhaps they have damaged one of your favourite plants in the past, and you are now anticipating their arrival without knowing what to do. We are here to help you better understand this insect and learn which actions can help reduce its presence and protect your plants.
How Can I Detect Japanese Beetles on My Trees?
When Do Japanese Beetles Arrive in Gardens?
Year after year, Japanese beetles generally start appearing in gardens around early July. There is a good chance they may already be present in your area.
Before that, they were hidden in the surrounding lawn as larvae. You may know them as white grubs. Japanese beetles are one of the different grub species included in this group. These large white larvae feed on roots and can cause significant damage to lawns in spring.
By July, the beetle has reached adulthood. It emerges from the soil with a new diet: the leaves, flowers, and fruit of more than 400 woody and herbaceous plant species.
What Are the Main Signs of Japanese Beetle Damage?
- Japanese beetles are very hungry chewing insects. With their mouthparts, called mandibles, they eat the tissue between leaf veins, leaving only the skeleton of the leaf behind.
- Severely damaged leaves may turn brown and fall off.
- Flower buds may also be eaten, as well as fruit.
How Can You Identify Japanese Beetles?
Physical Characteristics
- The adult beetle is about 1 cm long.
- Its head, body, and legs are metallic green.
- The wings on the top of its back, called elytra, are coppery brown. They protect another pair of wings underneath.
- One final identifying feature of this beetle is the small tufts of white hair visible along the sides and rear of its body.
Where Do Japanese Beetles Feed?
Japanese beetles can be seen directly on the leaves, anywhere on the plant.
At the beginning of an infestation, they are often found near the top of smaller plants exposed to the sun. Gradually, they move down toward the lower leaves. They then tend to move on to larger trees and fruiting plants. Finally, when the leaves are no longer attractive to them, flowers and field crops become their next food source.
When Do Japanese Beetles Feed?
Adult Japanese beetles emerge from the soil in early July and are very active until August in Ontario and Eastern Canada. They are especially active in sunny, hot, and humid weather. They feed less when it is cloudy and almost stop feeding when it rains.
Which Plants Do Japanese Beetles Attack?
Japanese beetles have a very diverse diet, which is why we see them almost everywhere around us. Among trees and shrubs, they are especially fond of lindens, walnuts, mountain ashes, roses, cherries, horse chestnuts, and many other species. In Ontario and Eastern Canada, more than 400 species of woody and herbaceous plants may be part of their food source.
Which Plants Are Japanese Beetles Less Likely to Attack in Ontario and Eastern Canada?
Some plants produce repellent compounds that make them less attractive to Japanese beetles. This is the case for:
- Common Milkweed;
- English Lavender;
- Plants in the allium family, such as garlic, onion, leek, and chives.
The following species are also believed to be less frequently damaged:
- Junipers;
- Dogwoods;
- Common Yarrow;
- Asters;
- Maidenhair Tree (Ginkgo biloba);
- Wild Bergamot and Spotted Beebalm;
- Spireas;
- Labrador Tea;
- Canada Goldenrod and other goldenrods.
How Can You Get Rid of Japanese Beetles?
First, let’s be realistic: getting rid of Japanese beetles completely is an unrealistic goal. There is no method that can eradicate them. Keep in mind that any action you take will only affect the insects currently present on your property. Other beetles may arrive from elsewhere. However, by combining several strategies, it is possible to make your environment less attractive to them and noticeably reduce their presence. Be warned: patience is essential.
Physical Control Methods
When Japanese beetles feed, the plant releases volatile compounds into the air. Other beetles detect these compounds and arrive in greater numbers. For this reason, physical control methods are more effective when applied early in the infestation.
Hand-Picking
- As soon as you see the first beetles on your plants, fill a container with soapy water and take action. Ideally, do this in the morning, when they are least active. Simply push the insects directly into the water. There is no danger in touching them, but wearing gloves can make the task a little less unpleasant.
- DO NOT KILL beetles that have white spots near their head! These small white dots are the eggs of a beneficial insect coming to the rescue. A small fly from the Tachinidae family, a natural enemy of the Japanese beetle, was recorded in Québec in 2018. It lays its eggs on the thorax of adult Japanese beetles. Once the eggs hatch, the tiny larvae enter the beetle’s body, feed inside it, and eventually kill it. We therefore benefit from not eliminating Japanese beetles that have small white spots on the thorax. Their days are already numbered.
Photo: White spot on the thorax, near the beetle’s head, indicating that it has been parasitized. Sometimes, several white spots may be visible.
- Some people suggest covering the soil with a tarp or wooden boards at the base of plants. Once white grubs become adults, they emerge from the soil. Beetles may then be found under the tarp or boards and collected by hand.
Exclusion Netting
Netting is mostly used to protect plants grown for small fruit harvests. While netting greatly limits attacks from beetles arriving from nearby areas, it cannot exclude adults emerging from the soil. You will still need to monitor your plants and manually remove any beetles that may be present.
Are Pheromone Traps Effective Against Japanese Beetles?
This type of trap uses sexual lures, or pheromones, placed on a plastic structure attached to a container. Beetles attracted by the pheromones are trapped inside the container.
Photo: Unitrap-style pheromone trap
The idea is appealing, but the use of pheromone traps is questioned by many. Several studies have shown that traps can attract more beetles to plants than if no traps were used at all. It is therefore recommended to use them only in areas where infestations occurred in previous summers and to place them far away from the plants you are trying to protect.
If you decide to install pheromone traps:
- Determine the area you want to protect and place the traps downwind of the prevailing winds.
- Several traps will probably be needed.
- Replace the lures frequently, as their effectiveness decreases over time.
- Empty the containers often, since the smell of decomposing insects can mask the smell of the pheromones.
Cultural Control Methods
Females lay their eggs in moist soil. Short grass creates favourable conditions for egg laying. To disrupt their egg-laying potential, you can:
- Turn off outdoor lighting at night.
- Raise your lawn mowing height.
- Replace your grass lawn with a clover lawn. Japanese beetle larvae would stop developing when the ground cover is made up only of clover.
- Keep the soil drier by avoiding lawn watering in July and August. Dry soil greatly reduces egg hatch.
To encourage the presence of predators
- Grow lacy phacelia. This annual plant attracts predatory flies that parasitize beetles by laying eggs on their abdomen or back.
Biological Control Methods
- Beneficial nematodes: Nematode-based products are only effective against Japanese beetles at the larval stage. They must therefore be applied to the soil in spring or fall, during the two feeding periods of the larvae.
- Fungi: Biological products based on Bacillus thuringiensis ssp. are available on the market. They act on larvae and adults after the insects feed on roots and leaves treated with these products.
- Birds: Some birds naturally feed on larvae. If there are no feeders nearby, they will naturally search for larvae in the soil.
Community Action
We have presented a wide range of strategies to help reduce the presence of Japanese beetles. Remember that no single solution can eliminate them completely. Instead, by combining several approaches, it is possible to make your garden less attractive to this insect and significantly reduce the damage it causes.
Every action counts. By adopting good practices in our gardens and acting collectively, we help slow its spread while encouraging a better balance between pests and their natural predators. With patience and an adapted approach, it is possible to enjoy a healthier, more biodiverse garden, even in the presence of Japanese beetles.
Article written by Suzanne Simard, horticultural writer and graduate horticultural technologist, and Audrey Rondeau, Biol., M. Env., Production and Development Director, Pépinière Vert Forêt.
References:
Hodgson, L. (2017). Plantes que les scarabées japonais n’aiment pas. https://jardinierparesseux.com/plantes-que-les-scarabees-japonais-naiment-pas/
Hodgson, L. (2016). Pour contrôler les scarabées japonais. https://jardinierparesseux.com/pour-controler-le-scarabee-japonais/
OMAFRA (2024). Le scarabée japonais dans les pépinières et les pelouses. https://www.ontario.ca/fr/page/le-scarabee-japonais-dans-les-pepinieres-et-les-pelouses
QuébecVert (2024). Fiche technique : Le scarabée japonais. https://www.gestionintegree.quebecvert.com/wp-content/uploads/2024/11/gion_fichetech10_scarabeejaponais.pdf
