Getting Rid of Japanese Beetles

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Let’s be honest.  You will not be able to get rid of them all.  But you can lessen the damage these large, brilliantly colored beetles do. You’ll find them feeding in a group on your roses or raspberries, trees or shrubs, munching big ragged holes in leaves, fruits, and flowers on warm, sunny summer days. 

The Japanese beetle – Popillia japonica – is an invasive species that came to the U.S. on the roots of imported Japanese iris.  First seen in New Jersey in 1916, they have been making their way steadily east, north and south.  You won’t often find them in western states – at least, not yet!  They have made their way across the Mississippi River and are being spotted in Minnesota, Kansas, Iowa, South Dakota, Texas, Nebraska, and Oklahoma.

These pests munch on over 350 species of plants in the eastern United States.  Norway and Japanese maple, birch, crabapple, purple-leaf plum, rose, mountain ash, linden, grapes, and basil seem to be their favorites. 

With no natural predators and the ability to attract others of their kind, Japanese beetles can devastate a garden in a short amount of time if populations are high.  So an understanding of this pest’s life cycle and some preventative measures can go a long way in helping control them.

Life Cycle

Japanese beetles spend most of their lives as larva, down in the soil. They feed on the roots of turfgrass, corn, beans, tomatoes, and strawberries. These larva – called grubs – are white and curl into a C-shape. To identify them, look at the last two rows of spines on the underside of the abdomen.  They are arranged in a V.

This species only spends 30 to 45 days as adults.

Adults are oval, about ½ inch long and wide, and have a metallic green body, darker green legs, and coppery brown wing covers.  They have white “hairs” on the sides of their bodies and their abdomens.

During the summer, adult females burrow a few inches down into the soil to lay eggs.  They go back and forth during the day, eating leaves and flowers in the morning and laying eggs in the afternoon. These eggs hatch in a few weeks and the young larvae burrow into the soil and start feeding on roots. 

As the summer progresses, the grubs grow larger and in the fall, they move deep down into the soil, below the frost line, to wait out the winter.  In the spring, the larvae come up close to the surface to feed a little more, then to pupate.  The adults emerge in late spring and early summer.  There is one generation per year in the central United States and two generations in the south.  Further north, it may take two years for Japanese beetles to complete their life cycle.

Japanese beetle grubs thrive in warm, moist soil that has lots or organic matter and lots of tender roots to eat.  But they can survive in any kind of soil.

Photo credits to Benny Mazur

Identifying the Damage They Cause

Grub damage appears in summer as brown patches in your lawn.  Since they eat the roots, you can often roll the lawn up like a carpet.  If you do this, you will see the grubs.  A healthy lawn can tolerate as many as ten grubs per square foot.  Japanese beetle larvae are not the only grubs that eat turfgrass roots – there are several other species.

Adults will fly up to a mile for a good meal.  They prefer roses, birch, elm, raspberry, currant, basil, Virginia creeper, hollyhocks, marigolds, corn, soybeans, grape, linden, apple, crabapple, cherry, plum, and other fruit trees. They feed in a colony, and they are very social – you’ll see many mating pairs.  As they feed, they release a pheromone that attracts other beetles. 

The adults start feeding in late June or early July.  They leave large, ragged holes in leaves and flower petals.  They will skeletonize a leaf, leaving behind only the veins.  Heavy infestations can completely defoliate a small tree, shrub, or rose bush.  When they feed on trees, from a distance, the trees look scorched.

Prevention

There’s not much a gardener can do to prevent Japanese beetles.  As we’ve said, they will fly quite a distance to eat their favorite foods.  The best we can do is create an environment that is less inviting to them.

  • Landscape with less desirable species of plants such as boxwood, juniper, holly, hemlock, magnolia, red oak, box elder, red maple, dogwood, lilac, burning bush, and clematis.
  • Let your lawn go dormant (or at least water less frequently) in the summer.  The females prefer to lay their eggs in moist soils.
  • Mow high – Keeping your lawn at the height of 3 inches discourages the females from laying their eggs there. They prefer shorter grass.

Control 

Japanese beetles are most vulnerable to insecticides in the late summer when they are young grubs. More mature grubs are harder to kill.  Eggs and pupae are invulnerable and the adults just fly away.

Using an integrated pest management system, it is up to each gardener to determine if the population is large enough and the damage severe enough to warrant control, especially if you are considering to use pesticides.

Photo credits to William Garrett

Cultural Controls

Keeping your plants properly watered and fertilized can help them survive a Japanese beetle attack.  You can interplant natural repellents like tansy, catnip, chives, and garlic around susceptible plants.

Japanese beetle populations lessen in years with dry summers.  As we mentioned, let your lawn go dormant (or don’t water it often) and let the grass grow to 3 inches high. 

Mechanical  Controls

Keep the adults off your plants.  Row covers can be used to protect crops from them.

Hand pick –This simple method is probably the best, but you have to do it every day.  With their large size and daytime activity, it’s easy to find these pests and simply pick or shake them off.  Then, drop or shake them into a bucket of soapy water. 

The smell of dead beetles repels other beetles, so if you leave the remains of the ones you handpicked near your prized rose bushes, others will stay away.

You might find pheromone traps for sale – but be very careful if you plan on using them!  The traps attract the beetles, but they don’t all go into the trap. You’re just inviting hundreds of them to dine on your garden.  Pheromone traps are mostly used to determine the size of the population.

Biological Controls

An imported species, Japanese beetles have no natural enemies in the United States. 

However, robins, catbirds, and cardinals will eat the adult beetles, and so will spiders and assassin bugs.  Starlings and crows, moles, shrews, skunks, and raccoons eat the grubs, but they tear up your lawn as they dig for them.

Heterorhabditis bacteriophora is a microscopic soil nematode that feeds on soil-dwelling larvae – including Japanese beetles.  As long as you have a grub population, this beneficial predator will live in the soil, but they will die if there is not a food source for them. It’s recommended that H. bacteriophora be reapplied every year.

Milky spore is a bacterial disease that homeowners can purchase to kill Japanese beetle grubs in lawns.  It takes a few years for milky spore to build up a population and it only kills Japanese beetle grubs.  If you have other lawn grubs, you will need a different control.   Milky spore also requires consistently warm soil temperatures to grow, so this might not be the best option if you live in a northern area.

Bacillus thuringiensis (Bt) is a naturally occurring bacteria that is available for purchase.  There is a strain of Bt that is specific to Japanese beetles and is applied to the soil when they are in the grub stage.  It does not work on adults.

The Spring Tiphia and tachinid flies are two helpful creatures.  These parasitic wasps lay their eggs on the adult beetles and the hatching larvae feed on them.  The adults are attracted to the nectar of forsythia, peonies, Queen Anne’s lace, cilantro, and clover.  If you provide these plants, these beneficial wasps will come to your garden.

Chemical Controls

Using horticultural or neem oil sprays will interfere with the adults’ ability to feed.  If you chose this method, spray every 7 to 10 days.

Many pesticides marketed for grub control are also toxic to birds, fish, and other aquatic animals, honey bees, and earthworms.  Use these products with care.  You want to time your pesticide application for when the grubs are the most vulnerable – that’s when Grandiflora Hydrangeas are in full bloom and when Yucca filamentosa is blooming.   Queen Anne’s Lace (Daucus carota) is another vulnerable stage indicator plant.  

It’s important to know that a grub control product will reduce the population of Japanese beetle larva in your soil and your yard for the next season.   But it does nothing for the beetles that fly in from other gardens.

Sometimes our efforts to get rid of an insect problem create other problems.  Our gardens are living things, with good bugs and bad bugs, good plants and weeds, and good and harmful fungi and bacteria.  Adjusting our expectations and finding a balance and a tolerance level is a better approach than trying to kill every single bug in sight.

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About the author: Alaine has been working way too hard in horticulture since 1992, beautifying golf courses, resorts, and hotels. She is a part time landscape designer who works full time caring for a 28,000 square foot public garden. At home, she maintains her own 400 square feet plot. Alaine lives in northern Illinois - zone 5b.

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