They attack by land and by air.
Japanese beetles are one of our country’s most destructive insect pests, tunneling underground to eat turf roots and flying through the air to find leaves to devour. The USDA estimates that we spend $460 million battling them each year. In this article, we’ll learn more about these pests and what we can do to combat them.
WHEN DO JAPANESE BEETLES APPEAR?
In the South, Japanese beetles emerge from the ground in the middle of June. In the northern states, they may not appear until early July. The cooler the climate, the later they’ll appear.
WHERE ARE JAPANESE BEETLES FOUND?
Japanese beetles were accidentally introduced to the United States before plant inspections were mandatory. They were first spotted at a New Jersey nursery in 1916. They have since invaded most of the eastern states.
Japanese beetles are well-established from Maine to Georgia, west to Arkansas, and north to Minnesota. Relatively small populations exist in South Dakota, Texas, and Mississippi. Conditions in Florida and Louisiana haven’t given the beetle much of a foothold, however.
In the western states, summers are generally too dry for Japanese beetles to complete their lifecycle. Residents there take Japanese beetle sightings seriously and deal with them proactively.
WHAT DO JAPANESE BEETLES LOOK LIKE?
Adult Japanese beetles are slightly less than a half-inch long, with metallic green bodies and shiny copper wing covers. Five white tufts protrude from their lower halves on each side.
HOW DO JAPANESE BEETLES FEED?
Adult Japanese beetles like to feed on warm sunny days. They’ll eat flowers and fruits, but they most commonly eat leaves, chewing around the tough veins and “skeletonizing” them. They prefer foliage in full sun. Beetles will often start at the top of the plant or Tree and work their way down.
Japanese beetles tend to feed in groups. Damaged leaves release chemicals that attract more beetles to feed. The beetles themselves also produce pheromones that lure other Japanese beetles in from great distances to feed and mate.
Feeding is most intense for about four to six weeks during peak season.
WHAT DO JAPANESE BEETLES EAT?
Roses are one of Japanese beetles’ favorite foods, and you can look for them there when they first emerge. There are several hundred other species of plants that they’ll also eat. As for Trees, Japanese beetles favor the following:
- Crape Myrtle
- American Elm
- Horse Chestnut
- Hydrangea, Tree form
- American Linden
- Japanese Maple
- Norway Maple
- Mountain Ash
- Ninebark, Tree form
- Plum (especially Purple-Leaf)
- Lombardy Poplar
- Rose of Sharon, Tree form
- Weigela, Tree form
WHAT TREES DO JAPANESE BEETLES AVOID?
Japanese beetles tend to leave the following Trees alone:
- Shagbark Hickory
- Freeman Maple (such as Autumn Blaze®)
- Red Maple
- Oak (most)
- Sweet Gum
- Tulip Poplar
HOW MANY EGGS DO JAPANESE BEETLES LAY?
A female Japanese beetle lives about 30 to 45 days as an adult. During that time, she is an eating, mating, and egg-laying machine. She’ll lay between 40 and 60 eggs during that period. She’ll burrow down a few inches into the soil beneath turf to lay her eggs either individually or in small clusters.
WHAT DO JAPANESE BEETLE LARVAE LOOK LIKE?
Japanese beetle eggs hatch underground, and the grubs eat organic matter and the roots of turf grasses. The grubs are white and C-shaped with orange heads. By the end of the summer, they have reached their full size—about an inch long.
WHAT HAPPENS TO JAPANESE BEETLE GRUBS OVER THE WINTER?
Japanese beetle grubs do not mature the same year they’re hatched. Rather, they spend the winter as grubs. They simply burrow down deeper in the soil when winter approaches and hibernate during the cold months.
When the soil warms in spring, they resume their activity. Soon, they pupate—like a caterpillar does when it turns into a chrysalis. A couple of weeks later, they emerge from the ground as adults and start the cycle anew.
WHAT KIND OF DAMAGE DO JAPANESE BEETLES DO?
Japanese beetle adults generally do only cosmetic damage to Trees by eating some of their leaves, though severe infestations can weaken Trees by removing a great deal of chlorophyll. Japanese beetle grubs can create dead spots in lawns from concentrated feeding on roots.
DO JAPANESE BEETLE TRAPS WORK?
Japanese beetle traps work by baiting the beetles with artificial pheromones. This lures them into a hanging bag and traps them there. These traps are very effective. Once, in a heavily infested area, around 10,000 beetles were found in a trap after one day!
Although the traps are considered quite effective, research done at the University of Kentucky indicates that they can worsen the problem by attracting additional beetles into an area. If these beetles don’t wind up in the traps, they’ll end up on the plants around them. In conclusion, these traps will give you an idea of how many beetles you’re dealing with, but they can be counterproductive when it comes to controlling them.
HOW, THEN, CAN JAPANESE BEETLES BE CONTROLLED?
Adult Japanese beetles are difficult to kill with organic pesticides. Safe, low-toxicity remedies like insecticidal soap and hot pepper sprays might work on soft-bodied insects like aphids, but they don’t work so well on beetles, which are encased in their own suits of armor. More heavy-duty chemical pesticides are necessary to circumvent their defenses. However, these chemical solutions can threaten bees and other valuable organisms. Instead, we recommend gathering up any offenders and dropping them in a jar of soapy water. Beetles are sluggish in late afternoon, so you might be able to make a dent in the population if the problem isn’t too severe.
When it comes to dealing with the grubs, there are other methods you can try. A beneficial nematode called Heterohabditis bacteriophora can be applied to the soil in areas where you detect grub damage. This has shown some success in controlling them. You might also hear of something called “milky spore disease,” which has had less than impressive results.
Those willing to use a chemical solution will want to look into chlorantraniliprole, also known as Acelepryn G®. It’s also sold under the name GrubEx®. When a Japanese beetle population was discovered in Cedar Mill, Oregon, in 2017, the Oregon Department of Agriculture applied Acelepryn G® to all 2,400 residences there. That the product was deemed fit to use on an entire town is a fairly good indication of its low toxicity to people, pets, and wildlife.
COPING WITH JAPANESE BEETLES
As these pests in the US lack the natural predators they would have in Japan, their numbers have exploded, and they do millions of dollars of damage each year. However, this doesn’t mean that you can’t have a thriving outdoor space! Choose Japanese beetle–resistant Trees and plants, handpick the insects as soon as they appear, and use low-toxicity pesticides when necessary.