Japanese beetles (Popillia japonica) can be exasperating pests. In the United States and now in some parts of Europe, these beetles swarm and eat geraniums, roses, and fruits. They skeletonize leaves and buzz around in numbers that put Hitchcock’s movie The Birds to shame.
So how do you discourage swarms of destructive Japanese beetles? By learning about them and their lifecycle, then giving them everything they hate. Some of these choices are out of our control (i.e. a really wet spring, cool temperatures year-round) but there are other choices that we can make that can save our valued plants.
Jump to:
- What Do Japanese Beetles Look Like?
- The Japanese Beetle Arrives in the United States
- Know Thy Enemy
- Life Cycle of the Adult Japanese Beetle
- From Eggs to Larvae
- What Kind of Weather Can Kill Off Grubs?
- What Attracts Japanese Beetles?
- Love is in the Air
- The Smell of Food is Also in The Air
- Prepare For Next Year Today
What Do Japanese Beetles Look Like?
Japanese beetles are about the size of a dime, about a ½ to ¾ inch long. They sport an iridescent copper shell with a green head, with six tufts of white hair along their sides under the edges of their wing shells. They’re quite a pretty insect when they aren’t attacking your roses and apples en masse.
The Japanese Beetle Arrives in the United States
In August 1916, two inspectors from the New Jersey Department of Agriculture were at the Henry A. Dreer nursery near Riverton, N.J., inspecting plants for unknown diseases or insects, as they do. Upon finding a dozen unknown beetles, possibly in a shipment of Iris kaempferi (now Iris ensata), a popular garden plant at that time.
The inspectors checked the insects against the insect collections at the National Museum in Washington, D.C. There they found that the specimens were Japanese beetles.
At this time, the beetles occurred only on the main islands of Japan – Honsu, Kyushu, Shikoku, and Hokkaido – and in some Pacific islands, including Hawaii.
In Japan, the beetle was not a garden pest. The main islands at the time were heavily forested mountainous country. Conditions here aren’t great for a hungry beetle that loves sun and heat. Due to the cool weather in the Japanese islands, the beetle took two years to reproduce.
Habitat for the beetle’s larvae (the grubs) was also scarce in Japan. Only Hokkaido had the grasslands that the larvae love, but they also contained natural predators that kept beetle numbers low.
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However, when the beetle showed up in America, those checks and balances didn’t exist. Here, the beetle has:
- Warm weather that allows it to move through a complete life cycle in one year –
- No natural enemies to keep it in check –
- Miles and miles of lawns with juicy, well-fertilized grass roots that larvae love --
- And 400 species of tasty plants serving as a veritable all-you-can-eat buffet.
All these make the United States a paradise for the Japanese beetle!
In very little time, the beetle started spreading. Entomologists, recognizing its destructive potential, jumped into action.
The year after it was discovered, in 1917, the Japanese Beetle Laboratory was established in New Jersey to study these insects and try to slow their spread. But the beetle refuses to comply. It’s been over a hundred years since its arrival, and it still hasn’t stopped spreading. Every year, new areas are added to its range.
Know Thy Enemy
A good tactician understands that, when you have the resources for it, a war should be fought on more than one front. Each front should target your enemy’s weak spots. You just keep hitting and moving, hitting and moving, until you have won the battle or have routed your enemy. And always stay one step ahead of your enemy.
These tactics, though greatly simplified, also apply to fighting diseases and pests in the garden and in the field.
With Japanese beetles, several different methods of control are necessary for the most effective knock-down of these noxious pests.
These tactics include understanding the habits and life cycle of the Japanese beetle.
Knowing what an insect needs to live is key to good control. Then, you can target all these things to kill off a large infestation.
Life Cycle of the Adult Japanese Beetle
A Japanese beetle’s life usually starts on some hot July night, when a female Japanese beetle that has been chomping up apples and leaves and rose blossoms flies down to the grass and works her way down through the grass to the soil. There, she will dig a little burrow two or three inches deep and lay a little cluster of eggs. The next morning, she crawls to the surface to fly back up into the trees, find another mate, and eat a thousand more leaves, and the next night, she’ll lay more eggs in a little burrow.
Each female lays about 40 to 60 eggs. There’s nothing I hate more than an overachieving beetle.
The Japanese beetles themselves die off toward late summer. Unfortunately, the cycle of life continues underground.
From Eggs to Larvae
The eggs have to absorb water in order to grow. They hatch in midsummer. The grubs, or larvae, emerge an inch or two underground, where they begin to feed on grass roots. These white grubs are an inch long when fully grown and rest in a C shape. When you turn over the soil in your garden, you’ll probably find a couple of grubs there in your shovelful of dirt, though some of those grubs might also be for June bugs, chafers, or some other grub from the Scarab family of beetles.
It’s harder to tell Japanese beetle grubs from those of June bugs and other related beetles because the grubs are generally differentiated by the pattern of hairs on their hind ends. Entomology is fun!
During summer, when the ground is warm, the grubs inhabit the top two inches of the ground, feeding on grass roots. By September, these grubs are almost an inch long. In late fall, the cold weather will drive the grubs deeper into the soil. They’ll dig about four to eight inches down and hibernate there through the winter, safe from the worst of the cold.
In spring, the grubs return to the grass roots to feed until they’re plump. In late spring, the grubs pupate, metamorphose into beetles, and pop up out of the soil for their late-spring and early-summer buffet of destruction.
What Kind of Weather Can Kill Off Grubs?
Newly laid Japanese beetle eggs and newly hatched grubs need enough rain to keep from drying out. Years of bad droughts can put a dent in the grub population by destroying the eggs because the eggs must absorb water in order to allow the embryo beetle to develop.
If your lawn is having trouble with grubs, and if you’re able to do this, stop watering your lawn from June to mid-July in order to help kill off Japanese beetle eggs. This is probably a good reason to grow drought-tolerant turfgrasses in your lawn.
The grubs, once they hatch, can tolerate dry soil. The grubs usually go through three instars (that is, growing periods that end with a molt of their skin as they grow too big for it); during each instar, the grub can withstand a huge loss of water in its body. During the first instar, right after they hatch, these grubs can lose up to 44 percent water before they croak.
If the soil temperature drops to -9.4 degrees Celsius, or 15 degrees Fahrenheit, nearly 100 percent of the grubs will die. But this has got to be the soil temperature, not just the air temperature. This drop of soil temperature can be brought on by sudden, extreme changes in the air temperature with no snow cover. (A blanket of snow insulates the soil against these drastic, grub-killing drops – too bad.)
Also, freezing rain causes the water in the soil to freeze, stabbing many grubs with ice crystals. So there is that.
A wet spring will kill off grubs. We’ve had an extremely rainy spring this year, and the May beetles (which also live in the lawn as grubs) and Japanese beetles haven’t been swarming around the way they did last year.
A very dry summer or a terrible freezing winter causes a drop in the local population, but adequate rainfall and favorable temperatures can cause a spike in the numbers of beetles the next year.
An additional note: Beetles are very alkaline creatures, but they have a very broad tolerance to soil pH, so acidic soils don’t seem to bother the grubs at all. Also, too bad.
What Attracts Japanese Beetles?
The answer to this is straightforward: food and, er, mating.
Japanese beetles find food by following certain chemical odors in the air. They also find love, or whatever, through following pheromones in the air. More pheromones means more beetles, and all of them are doing things that would get them banned in Boston. Beetles will fly long distances to find these party trees, as you’ve probably noticed when you go outside and find your apple tree dripping with mating beetles.
Love is in the Air
An entomologist noted that males always approached a female against the wind, and when the wind shifted, they would change their path to follow the trail of pheromones coming off her.
A study found that 50 percent more beetles landed on infested foliage than on uninfested foliage. They’re following the pheromones of their fellow beetles.
So if a couple of beetles are on a plant, you can bet they’ll soon be joined by six hundred more.
This is a good reason to start killing the beetles as soon as they start showing up. If you can bring their numbers down right at the start and keep them down, they’re less likely to fly in.
The Smell of Food is Also in The Air
Certain essential oils, some fruit fragrances, and the smell of fermentation will attract the beetles. They love fruits with high sugar content. They also are lured in by the smell of fermenting fruit on the ground or in the trees. Some entomologists recommend removing rotted fruit to protect the good fruit from beetle attack.
Beetles gather in huge numbers on early apples and peaches, forming clusters in the shape of a ball, feeding until all that remains is a core or a pit. In 1940, two entomologists counted 296 beetles on one apple. The beetles also cluster in balls on foliage or blossoms.
On fruit trees, the beetles prefer fruit that’s been damaged or has been infected by disease over healthy fruit. It’s easier to eat their way into an apple through a hole in the skin or into a peach if there’s a spot of rot.
The same goes for grapes. The beetles ate grapes that were infested with grape berry moth and black rot – even if the grapes were immature. Only when that food source was exhausted did the beetles move on to healthy grapes nearby.
So remember: To discourage Japanese beetles, get rid of rotting fruit, diseased fruit, or fruit with holes in it.
Japanese beetles also attack and defoliate unhealthy trees first before moving on to healthy trees. One study noted that peach trees that were infected with the peach yellow disease and little peach disease were attacked by the beetle, while healthy trees nearby were mostly unscathed.
So be sure to keep your trees healthy. Mulch them with a layer of compost and water it in. Keep the weeds down around the tree so there’s less competition between roots.
Beetles never stay long in one place and constantly move from one plant to another. They can fly several miles in a day. One reason why beetle control is so difficult is their extreme mobility. When you kill off beetles, more beetles move in from elsewhere.
The adults feed on over 300 plant species (lately, I’ve seen that number quoted as being 400).
They chew up the tissue between the leaf veins, leaving lacy skeletons that fall from the tree. So, if you see a lot of lacy leaf skeletons collecting on the ground in May or June, look up – you might see the beetles in the top branches of the tree.
Another note: Beetles will start eating in the top foliage of trees and plants – in places that are exposed to full sun, and work their way downward through the plants.
Note: Beetles don’t like to feed in the shade, so if you have a shade garden, you’re not going to see as much beetle damage.
The most feeding takes place on warm summer days when the sun is out, when the temperatures are between 83 and 95 degrees.
If the relative humidity falls below 60 percent, beetles don’t want to fly, so they stay in place and feed instead. They also do not fly on cool, windy days or on rainy days.
Prepare For Next Year Today
Here’s something you can do right now to get ready for next year’s onslaught. Walk around your yard and write down which plants were attacked by Japanese beetles. Look for damage in trees and large shrubs – they might have attacked some of these, too, without your realizing it.
Put this list onto next year’s calendar in early summer. Then, you can see the list and remember to prepare for the onslaught by looking for Japanese beetles and working to control them as soon as they show up.
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