Tony O’Neill, gardener and author of the popular “Composting Masterclass” and “Your First Vegetable Garden,” combines lifelong passion and expert knowledge to simplify the art of gardening. His mission? Helping you cultivate a thriving garden. More on Tony O’Neill
The first Japanese beetle (Popillia japonica) report in the United States was in a nursery in southern New Jersey in 1916. In Japan, natural enemies keep this attractive yet destructive beetle population in check.
The Japanese beetle has become a significant plant pest hazard to American agriculture and turf. Adults skeletonize the leaves and consume the fruit of several types of fruit trees, ornamental trees, shrubs, vines, and vegetable crops, while larvae feed mainly on turf roots.
Although Japanese beetles (Popillia japonica) are most prevalent in the east of the United States, they can be found in nearly every state and part of Canada.
Adults have a head that is metallic green, bronze wing coverings, and measure between a third and a half an inch long.
Along the side of the abdomen, there are five white spots.
Managing Japanese beetles can be challenging, requiring integrated pest management (IPM) approach.
With each female Japanese beetle able to lay 60 eggs per season and limited natural predators, your community’s Popillia japonica population can rapidly get out of control.
Controlling the Japanese beetle should be a community IPM implementation effort.
There is currently a quarantine to prevent the spread of the Japanese beetle. The following states are affected:
Japanese beetles found outside these states must be reported to the county extension office.
So, let’s review what that means. What works, and what worsens the problem of Japanese beetle infestations? We’ll review the life cycle, breeding habits, and how to stop the Japanese beetles in their tracks in your neighborhood.
Japanese Beetle Life Cycle
Japanese beetle grubs spend the winter underground in the soil of lawns and other grassy areas. Grubs migrate up to the soil’s surface in the spring to finish their feeding and pupate into adult beetles.
In late June or early July, adult Japanese beetles begin to emerge from the earth, and they can cover several miles in search of food.
Although some may continue to remain active into September, adults usually feed in July and August. Beetle-damaged leaves release scents caused by feeding that draw more insects (like sharks to blood).
This frequently leads to dense populations of beetles feeding and reproducing on one type of plant while surrounding, equally appealing species are only marginally affected.
Males are attracted to the sex pheromone the females excrete. Throughout July and August, females will burrow one to three inches underground at various intervals to lay as many as 60 eggs in total.
A year-on-year beetle population drop may result from reduced egg survival rates in dry soil conditions.
The grubs primarily feed on lawn roots after hatching (about two weeks). During the summer, grubs go through three distinct growth phases (instars), getting bigger with each one.
The nearly adult, full-sized (third instar) grubs dig deeper into the ground when the soil begins to chill in the fall, where they spend the winter.
Plant Damage Caused by Japanese Beetles
Feeding injury can entirely defoliate plants, making trees and shrubs appear close to death.
Populations appear to fluctuate, going up and down and increasing in some years while decreasing in others – probably caused by dryer weather patterns or a fungus that kills some grubs.
Japanese beetles can be very abundant in some years and less in others. Either way, having some Japanese beetles isn’t the worst thing that happens to your garden, as there are several ways to deal with them. Generally, Japanese beetle damage is only cosmetic and will not kill plants.
Managing Adult Japanese Beetles
Adult Japanese Beetle Threat Identification
Start looking for beetles in your garden and yard in late June or early July. When they first appear, begin management.
Lessening the number of beetles on plants should result in fewer beetles being drawn to them since damaged leaves attract more beetles.
Managing Japanese Beetles Using Non-Chemical Option
To reduce feeding damage, check your plants daily and eliminate any bugs you see. Remember that leaves with beetle damage exude compounds that draw more beetles.
You can lessen new beetles drawn to your plants by removing damaged leaves.
Sunrise and sunset are the better times for Japanese beetle hunting when the beetles are still chilly and lethargic. Pick them off by hand and drop them into a pail of soapy water.
In rare circumstances, it is possible to shield plants from beetle attacks using fine netting, but covering plants in bloom is not advised since it will keep pollinators from getting to the plants. Instead, manually remove bugs until the plant has finished flowering and is beginning to set fruit before covering it.
Japanese Beetle Traps
Japanese beetle traps use synthetic female sex pheromones and a blend of chemicals with a strong floral odor to attract beetles. The problem is that the mix works to attract them, but the trap is less effective at managing the invited population of Japanese beetles. Bottom line: don’t use traps; they don’t work.
Japanese Beetle Plant Preferences
Japanese beetles consume a wide variety of plants; however, some they rarely harm. Below is a guide provided by USDA’s Animal and Plant Health Inspection Services (APHIS).
|Japanese Beetle Resistant Woody Plants
|Japanese Beetle Susceptible Woody Plants
|1. Red maple
|1. Japanese maple
|2. Norway maple
|4. Apple, crabapple
|5. Tulip poplar
|5. Virginia creeper
|6. Plum, apricot, cherry, peach
|7. Pin oak
|9. American mountain-ash
|16. Northern red oak
|16. Beech Fagus
|17. Black walnut
|19. Lombardy poplar
|Japanese Beetle Resistant Herbaceous Plant
|Japanese Beetle Susceptible Herbaceous Plants
|4. Common mallow
|7. Pennsylvania smartweed
|9. California poppy
|10. Sweet corn
|15. Cardinal flower
|18. Showy sedum
|19. Red raspberry
|20. Violet, pansy
Homeowners can use parasites, nematodes, fungi, or other biologically based techniques to manage Japanese beetle populations.
Biological control agents remain longer in the environment than insecticides, even though achieving the same outcomes takes longer. Additionally, they don’t harm the soil biota as pesticides do.
Nematodes that consume insects search for grubs in the soil, injecting with a mutualistic symbiont bacteria that consume the grub tissue.
The nematode then consumes the bacteria and the grub. Steinernema glaseri and Heterorhabditis bacteriophora are the two nematodes that are most efficient against Japanese beetle grubs.
The latter is offered for sale commercially. Remember that nematodes are living organisms with a somewhat high oxygen need.
Bacillus thuringiensis (Bt)
Bt is a naturally occurring soil bacterium typically used as a microbial insecticide. The Bt strain registered for the Japanese beetle is only used in the grub stage.
Bt is a stomach poison and must be ingested to be effective. Apply it to the soil as you would insecticides.
Managing White Grubs (Japanese Beetle Larvae)
It’s doubtful that controlling Japanese beetle grubs will lessen the population of adults on landscape plants.
Beetles may fly a long way to their preferred adult-feeding plants after they emerge from untreated grass patches. To avoid harming lawns, only treat white grubs.
This is the popular term for the spores produced by the bacteria Bacillus papillae. As grubs consume the spores and die, they release 1-2 billion spores back into the soil, further protecting the turf.
Healthy turfgrass that is not under stress can generally tolerate ten grubs per square foot.
The biological IPM strategies referred to above are effective on white grubs.
Using Pesticides on Japanese Beetles
If you have followed my posts, you will know I’m not keen on using pesticides as they negatively affect the soil biota, often causing more long-term harm than any pest could cause.
Healthy soil produces healthy, resilient plants. Insecticides destroy microorganisms essential for healthy soil.
Check with your local extension office if insecticides are essential to your IPM.
To hand-pick Japanese beetles, go out early in the morning and place a container that has soapy water in it underneath the area where the beetles are.
They tend to drop off the plant when disturbed, so if you start to pick them off, they will drop into the soapy water and drown.
If you do decide to use traps, place them away from the plants you want to protect so that you draw the beetles away from them. Don’t be surprised if you attract more than you bargained for.