Japanese beetles (Popillia japonica) are iridescent green beetles that carry a big threat because they feed on a wide variety of plants. The trick with this small insect is to catch it early! We have 10 tips on how to prevent Japanese beetles from eating your plants this season!
What Are Japanese Beetles?
Japanese beetles do not discriminate when it comes to what types of plants they feed on, though they do have favorites (like roses). In fact, they are classified as pests to hundreds of different species. They are one of the major insect pests in the Eastern and Midwestern United States, causing monumental damage to crops each year.
Prior to the beetle’s accidental introduction to the United States in the early 1900s, the Japanese beetle was found only on the islands of Japan, isolated by water and kept in check by its natural predators. In 1912, a law was passed that made it illegal to import plants rooted in soil. Unfortunately, the failure to implement the law immediately allowed the Japanese beetle to arrive in this country.
Most entomologists agree that the beetles entered the country as grubs in soil on Japanese iris roots. In 1916, these coppery-winged pests were first spotted in a nursery near Riverton, New Jersey, and by 1920, eradication programs were dropped; the beetle proved to be too prolific and widespread.
Japanese Beetles are ½ inch in length with distinct metallic blue-green heads. They have copper-colored backs, tan wings, and small white hairs lining each side of the abdomen. Japanese beetles usually feed in small groups.
Prior to becoming adult beetles in late June, they are 1-inch-long, white, c-shaped grubs that live in the soil and feed on the roots of many plants. Often, these grubs are a problem in lawns.
Once adults, they only live 40 days, but they are voracious. Japanese beetles attack plants in groups, which is why damage is so severe. Most feeding starts in mid-to-late June in the North and mid-to-late May in the South.
Signs of Japanese Beetle Damage
Japanese beetles feed on a wide variety of flowers and crops (the adult beetles attack more than 300 different kinds of plants), but in terms of garden plants, they are especially common on roses, as well as beans, grapes, and raspberries. Here’s what to look out for:
Unhealthy, Brown Patches in Lawn First come the Japanese beetle grubs (larvae), which damage grass when overwintering in the soil. The grubs feast on the roots of lawn grasses and garden plants. This can cause brown patches of dead or dying grass to form in the lawn, which will pull up easily thanks to the weakened roots.
Skeletonized Leaves and Flowers Then come the adult Japanese beetles which chew leaf tissue from between the veins, leaving a lacy skeleton. You’ll know right away when you see leaves that are “skeletonized” (i.e., only have veins remaining). (Mexican Bean Beetles can also leave foliage skeletonized, though, so be sure to identify the beetle by their appearance as well.)
Control and Prevention
To manage Japanese Beetles in yards and gardens, the trick is to start early! Once these insects are present in large numbers, the problem is very difficult to manage. Here are 10 tips to prevent beetles:
Start at the grub stage! BTG or Bacillus thuringiensis galleriae (e.g., products such as grubGONE!) uses naturally-occurring soil bacteria. This bacillus ONLY targets Japanese Beetle grubs before they become destructive adults. The grubs ingest the spores as they feed in the soil. It is as effective as a chemical pesticide but is NOT toxic to beneficial insects, such as bees and pollinators; it will not harm people, pets, animals, or plants. Once ingested, BTG kills grubs for several weeks, during the time they are most active.
Hand pick early: Look for beetles in mid-to-late June or early July. In the early morning or evening, when beetles are more sluggish, knock beetles into a bucket of soapy water. Yes, it’s time-consuming, but it’s also the most effective way to kill these pests. Just be diligent. When you pick them off, put them in a solution of 1 tablespoon of liquid dishwashing detergent and water, which will cause them to drown. Do this daily, as the chemical trail of beetles attracts more beetles.
Protect with row covers: Use row covers during the 6- to 8-week feeding period that begins by mid-to-late June in the North and mid-to-late May in the South. Row covers will keep the pests out, but they will keep pollinators out, too; be sure to remove netting or covers if your crops need to be pollinated. Do not cover plants in bloom that require pollination (i.e., fruits); after the plant sets fruit, then cover it with netting.
Spray Neem oil if early:Deter feeding by adult beetles by spraying plants with Neem oil, which contains potassium bicarbonate. The adult beetles ingest a chemical in the neem oil and pass it on in their eggs, and the resulting larvae die before they become adults. Neem oil is effective for several days, but repeat applications are needed, especially after rainfall. Neem oil is not effective once you have beetles in large numbers. Neem is low risk to bees and other beneficial insects but can be harmful to fish and aquatic life, so don’t use it near lakes, rivers, and water.
Do NOTuse traps. They attract beetles, but research has proven that more beetles fly toward the traps than are caught. You’ll be pulling beetles in from all over town and end up with a worse problem.
Choose the right plants: There are some that Japanese Beetles seldom damage, such as boxwood, clematis, chrysanthemum, conifers, daylily, geranium, ginkgo, Japanese tree lilac, forsythia, common lilac, magnolia, red and silver maple, oak, white poplar, redbud, rhododendron, and yew. See our list of the Best and Worst Plants for Japanese Beetles. Dispersing the beetle’s favorite plants throughout the landscape, rather than grouping them together, can also help.
Introduce natural enemies: You can also attract native species of parasitic wasps (Tiphia vernalis or T. popilliavora) and flies to your garden, as they are predators of the beetles and can be beneficial insects. They will probably attack the larvae, but they are not very effective in reducing the overall beetle population.
Use companion plants: Avoid attracting beetles with companion planting. Try planting garlic, rue, or tansy near your affected plants to deter Japanese beetles. (Roses love garlic is a popular expression.) All herbs and other aromatic plants make wonderful companions. Scented geraniums (Pelargonium), rue (Ruta), feverfew (Tanacetum), parsley (Petroselinum), and thyme (Thymus) all may help ward off Japanese beetles as well as aphids. Try ornamental and culinary sage (Salvia), anise-hyssop (Agastache), Russian-sage (Perovskia), lavender (Lavandula), yarrow (Achillea), oregano (Origanum), catmint (Nepeta) and calamint (Calamintha). Four-o’clocks (Mirabilis) and larkspur (Delphinium) are said to act as decoys by attracting rose-loving Japanese beetles to eat their poisonous leaves, but they do not kill the beetles.
Beware of insecticides:If you wish to use insecticides, speak to your local cooperative extension or garden center about approved insecticides in your area, and be aware of whether you’re killing the very pollinators that bring fruit and flowers!
For example, Chlorantraniliprole (Acelepryn®) provides two to four weeks of protection and is low risk to bees. But most dusts or sprays are highly toxic to honeybees, native bees, and other pollinators. If application of these materials to plants is necessary during the bloom period, do not apply during hours when bees are visiting the flowers (late morning through mid-day). Apply sprays in the morning, never in full sun or at temperatures above 90ºF. If your plants start to wilt, rinse the leaves immediately with clean water.
Nip rose buds and spray rose bushes: Note that insecticides will not fully protect roses, which unfold too fast and are especially attractive to beetles. When beetles are most abundant on roses, nip the buds and spray the bushes to protect the leaves. When the beetles become scarce, let the bushes bloom again. Timeliness and thoroughness of application are very important. Begin treatment as soon as beetles appear, before damage is done.
Catherine Boeckmann loves nature, stargazing, and gardening so it’s not surprise that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann