One of the most notorious garden pests of the summer is the much reviled Japanese Beetle. Gardeners spend much of their summer swatting, trapping, stomping, and fretting over the damage this pest causes.
Popilla japonica, a stowaway from Japan, was accidentally introduced into the U.S. in 1916, and when it encountered no natural predators, populations grew to overwhelming proportions inside of 50 years. They remain an annual nightmare for gardeners and farmers.
Anyone who’s ever been subjected to visits by these creatures for a few weeks in the summer knows exactly what I’m talking about. In a bad year, they’re seemingly everywhere, eating their way through Rose foliage and other tasty plants, and leaving holes in fruit. In fact, Japanese Beetles feed on over 300 plants in 80 families and are very difficult to get rid of.
Your lawn is a Japanese Beetle’s winter home
If you’re reading this between August and May, our flying friends are living in your yard’s soil right now. Japanese Beetles in larval stage, called grubs, live less than a foot beneath your lawn. As the weather warms, the grubs slowly work their way towards the surface, where they feed on the roots of your grass for 4-6 weeks as they grow into adults. Unchecked, they can kill wide swaths of lawn, as the weakened grass loses its ability to survive heat stress.
When the grubs grow to adults, they emerge from their winter home in the soil to attack a wide assortment of plants and trees, including roses, raspberries, pear trees, maple trees, dahlias, grapes, apple trees, hollyhocks, lilacs, and holly. And then there’s the THUNK sound made by their heavy green metallic shells as they smash into and bounce off windows and aluminum siding. They’re like clumsy, drunken visitors from another planet. Fortunately, Japanese Beetles have a short lifecycle of just one generation.
Why did their populations grow so quickly? The American lawn. Since we have such an addiction to well watered turf grasses, the Japanese beetle larva has plenty of food and lodging over winter. Big lawns aren’t common in the pest’s original home of Japan – or many other places in the world for that matter – and providing this undisturbed winter shelter encourages a high survival rate.
How to get rid of Japanese Beetles naturally
As always, insecticides are definitely not the way to rid your plants of Japanese Beetles. Garden centers will usually recommend products containing carbaryl (like Sevin, Adios, Carbamec, and Slam), but carbaryl is incredibly poisonous for birds, bees, pets, anything living in your soil, local waterways, fish, and YOU (it’s notorious for causing skin burns and irritated eyes).
But there is hope, as animals like skunks, shrews, moles, and birds feed on the grubs. In early spring you may find small holes in your lawn, made by wildlife searching for the young beetles – they’re apparently a pretty tasty morsel. Unfortunately, there are few natural predators for the adult beetles, which is why they remain prolific. There are no effective natural repellants for adult Japanese Beetles.
Milky Spore Powder is the gold standard
When adult Japanese Beetles mate, the female burrows in the soil and lays her eggs. The larva, knowns as grubs, incubate in the top few inches of soil and emerge the following year as adults. So basically, your lawn is full of Japanese Beetle larva if you haven’t taken measures to control the grubs.
The best organic control for Japanese Beetle grubs is Milky Spore powder, which contains a very specific bacterial spore, Bacillus popillae. When ingested by the grubs, the bacteria turns their internal fluids milky white, hence the name. The grubs die of this disease, and when they do, more milky spore is produced, which creates a self-perpetuating cycle. Milky Spore lasts for many years in your soil, and it’s completely safe for wildlife, pets and you. An application on your lawn in early fall, followed by a good watering in, is all that’s needed (grubs are more likely to ingest the spores in fall when they’re feeding heavily).
Milky Spore Powder is only effective on grubs between July-September, according to the University Of Minnesota Extension. Outside of that window, grubs are too large to be affected by the bacteria.
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Pick adult Japanese Beetles off of plants by hand
Japanese Beetles are slow and clumsy, so picking them off your plants or shooting them with a soap and water spray solution is very effective. When they fall, you can easily crush them with your hand or foot.
Geraniums may help deter Japanese Beetles
In 1929, it was discovered that a natural control for adult beetles is, believe it or not, geraniums. The beetles feed on the geranium leaves and flowers, which puts them into a narcotic state for 12-18 hours. They lay on their backs while enjoying their altered state, which makes them susceptible to predators and easy for gardeners to collect and crush. Some beetles ingest the irresistible geranium leaves so often that they die from it.
But don’t interplant geraniums around your roses, thinking that this will deter the pests. Because Japanese Beetles love geraniums so much, interplanting will create the opposite effect and attract so many beetles that your roses will become infested. Plant a patch of geraniums in a sunny place far away from the plants you want to protect – it’s believed that the sun has an intensifying effect on the narcotic substance in the geranium leaves.
Japanese Beetle traps should not be used
Japanese Beetle traps are a disaster. Sure, the traps catch tons of beetles. But since the bait is a pheromone, the beetles are attracted from up to 1 mile away. Not only are you catching the adult beetles from around your home, but you’re catching the entire neighborhood’s. And those beetles who flew in from down the street and didn’t get caught in the trap feast on your plants and trees. I’ve had better success deterring Japanese Beetles without the traps.
Natural predators of Japanese Beetles
Tiphia wasps
Most adult Japanese Beetle control is performed by other “beneficial” insects and the USDA considers the spring Tiphia wasp the most effective. This is another reason not to use insecticides, as you’ll kill the beneficial insects too.
After mating, female spring Tiphia wasps burrow into the soil, searching for grubs. When the female finds one, she paralyzes it while she attaches one of her eggs to its abdomen. The beetle grub serves as a food source for the egg and after it hatches, the larva continues to feed on the grub until the grub dies. The female wasp can normally parasitize 1 to 2 grubs daily in this manner and can lay a total of between 40 and 70 eggs over her lifespan of about 30 days.
Fortunately, the spring Tiphia wasp is not aggressive towards humans and will not normally sting people, since they’re only 1 cm – 2 cm long. Trees such as Tulip Poplar, Chokecherry, Norway Maple, American Elm, Forsythia, Firethorn, and Pine are great host plants for these wasps. Plant one of these trees on your property if Japanese Beetles are a consistent problem.
Winsome flies
Winsome flies actually lay eggs in adult Japanese beetles. When the larvae hatch, they burrow deep into the beetle and begin feeding. This sends the beetle into a frenzy and it buries itself in the soil where the larva continues to feed until the beetle dies. Then the fly overwinters in the hollowed beetle shell. The adult Winsome fly emerges the following spring to find more Japanese Beetles to lay eggs on.
I almost feel bad for the Japanese Beetles after writing this last part. Almost.