Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
What’s eating my plants? If it’s an iridescent green and brown beetle, you’ve got an infestation of Japanese beetles.
What are Japanese Beetles?
As their name implies, Japanese beetles (Popillia japonica) are originally from Japan. They are not a problem there because they have natural enemies that keep their population in check. Japan is an island surrounded by oceans so the beetles weren’t able to spread further until the advent of global trade.
It is thought that Japanese beetles entered the United States in 1912 in soil around iris that had been imported from Japan. The beetles have no natural enemies here so they spread rapidly around the US and Canada.
Japanese beetles are about ½ inches long with iridescent colors. The heads are green and the body is brown. They reproduce once a year in temperate zones, every two years in colder zones. They have four life stages: egg, larva, pupa and adult. The females dig shallow burrows in the ground and lay up 40 to 60 eggs per year. The eggs hatch within two weeks and the resulting larvae feed on the roots of your lawn, killing your grass. You can actually roll up the brown patches in your lawn because the roots are completely gone. You won’t see any larvae though. They have moved on to the green parts of your lawn and are consuming those roots.
The growing larvae molt and become the familiar C-shaped grubs (pupa) that you find in the ground. The grubs hibernate underground during the winter and emerge in the spring as the soil warms up. Within 4 to 6 weeks, the grubs pupate into adults. The adults only live 30 to 45 days but it is long enough to do tremendous damage to your landscape.
Adult Japanese beetles feed on the foliage of a wide range of plants. They skeletonize the leaves meaning that they eat everything except the veins and stems. They start at the top of the plant and work their way down to the bottom. To make matters worse, as they eat they emit a pheromone that attracts other Japanese beetles.
How to Kill the Grubs
A good way to prevent the adults from feasting on your landscape is to kill the grubs before they pupate into the adult beetles.
- Drench your lawn with soapy water (2 tablespoons of dishwashing soap per gallon of water). This will force the grubs to the surface of the soil where they will be promptly consumed by birds. Spray this mixture on your lawn once a week until no more grubs emerge.
- Inoculate your lawn with Milky Spore disease, which is caused by the bacteria Bacillus popilliae (brand names Japidemic Doom or Milky Spore). The larva consume the spores as they feed on the roots of your lawn and then die of the disease. It takes two to three years of applications before there are enough spores in your lawn to kill all of the larvae so this treatment can be expensive. If you are willing to invest in this solution, the spores live on indefinitely in the soil.
- Introduce parasitic nematodes (Heterorhabditis bacteriophora ) into your lawn. These are microscopic parasitic roundworms that carry bacteria. When they encounter a grub, they inoculate it with the bacteria which then multiplies in the grub’s body. The nematodes then feast on the bacteria, killing the grub. The nematodes should be applied at night and the lawn kept moist to keep them alive.
How to Kill the Adult Beetles
When I was a child, my great aunt swore by her method of getting rid of Japanese beetles. Each morning she would go around her yard with a glass jar filled with gasoline looking for the beetles. When she found one on a leaf, she would nudge it into her jar, killing it. Everyone else in the family laughed at her but she was right.
- Handpicking Japanese beetles is the best way to get rid of them because, as noted above, while they are feeding on a plant, they are emitting a pheromone that attracts other Japanese beetles to that plant. By removing them from the plant, you remove the attraction for other beetles. Of course, nowadays we would use soapy water which is less expensive and more environmentally friendly than gasoline.
- Use fruit cocktail, another environmentally friendly solution to killing Japanese beetles. Open the can and leave it to ferment in the sun for a week. Then place it in a pail. Fill the pail with water up to the rim of the can. The fermented fruit will attract the beetles who then drown in the water.
Don't Use Japanese Beetle Traps
The joke that the best place for a Japanese beetle trap is in your neighbor’s yard is true. Traps baited with pheromones attract a lot of Japanese beetles, however most never make it into the trap. Instead, they stop and feed on nearby plants. Setting the traps will only lure more beetles into your yard, increasing the damage to your landscape.
Try Companion Planting
You can try to discourage Japanese beetles from feeding on your plants by surrounding them with plants that repel them such as garlic, tansy, catnip and chives.
Japanese beetles are highly destructive. They have no natural enemies here in the US but they can be controlled using methods that are effective and environmentally friendly.
Questions & Answers
Question: We live on two acres. A half acre is swarming and covered in nettles. There are a lot of them, from morning until dusk. How do I get rid of so many?
Answer: You could try rototilling the area with the nettles both in the fall and again in the spring after the ground thaws. This will kill the grubs that are overwintering in the soil.
Question: The Japanese beetles have started feeding on my birch tree this past week. I had considered applying a grub mixture earlier this season. Since I accept the fact that I don’t have an invisible fence around my home, what is the likelihood of destroying the grub phase and eliminating Japanese beetles in my yard if neighbors or surrounding areas don’t treat?
Answer: Unfortunately, there is no way to eliminate Japanese beetles in your yard if they are present in the surrounding area. The best that you can do is to make your yard uninviting to them by planting herbs like tansy, catnip, garlic and chives which will repel most of the beetles. If you don't want to apply soap, milky spore or parasitic nematodes to your lawn, try inviting birds into your yard by providing a water source such as a birdbath and trees and shrubs for them to perch and build their nests in. Birds like robins love grubs. I've even seen woodpeckers digging for grubs in my yard.
© 2017 Caren White