Japanese beetle (Popillia japonica)

No, they are not easy to control! Japanese beetles (Popillia japonica) have been the scourge of the North American garden world ever since they were accidentally introduced to New Jersey in 1916 (happy 100^th, Japanese beetle!) and have been spreading steadily ever since. They are established throughout much of Eastern North America and are now found in all states east of the Mississippi. In Canada, they got to Nova Scotia rather early, but only recently moved into Quebec, Ontario and New Brunswick in a big way. Even so, they tend to be very localized: you may well have a major problem with this insect in your town while nobody in the neighboring village has ever even seen one.

The Japanese beetle also moved into Italy in recent years and it is feared that it will spread throughout Europe over time.

Bad Eating Habits

Japanese beetles will skeletonize leaves.

Japanese beetles have been discovered eating over 400 different species of plants, from perennials, annuals and vines to trees and shrubs (but rarely conifers). The adult beetles chow down on leaves, flowers, and fruits. They often skeletonize leaves, often leaving only the veins, then the leaves turn brown and fall off. From a distance, a severely affected tree may look like was scorched by fire. After the beetles stop feeding, the affected plants usually produce new leaves and therefore don’t suffer as much from the infestation as do their human owners, but several years of defoliation can certainly weaken a plant or even kill it.

Japanese Beetle Host Plants

Here’s a link to a list of over one hundred of the Japanese beetles’ favorite food plants.

At Closer Look at Japanese Beetles

Really rather handsome, don’t you think?

The adult is a plump beetle about ½ inch (13 mm) in length with a metallic green head and body and copper brown wing covers. They also sport six rather spiffy tufts of white hair along each side of wing covers. All in all, it’s a mighty handsome beetle… if you’re into beetles, that is.

Various stages of Japanese beetle grubs.

The larva is a white grub that lives underground. It’s one of the famous white grubs, the larvae of May/June beetles and various chafers, that do so much damage to lawns. It is C-shaped and has a brown head and a cream-colored body. To tell if you’re looking a Japanese larva or one of the other white grubs, you’d have to carefully study the arrangement of the hairs on its rear end, which I know you’re not going to do. At any rate, you don’t want any of these grubs in your lawn and the same techniques used to control one will get to the others.

The adults emerge in late June or July and feed on low-growing plants at first, then migrate to the top of trees, eating their way down. Even when they reach shrubs, perennials and the like, they tend to stick to the top of the plant, as they prefer sun to shade. In fact, you won’t find much Japanese beetle damage in a forest: it just isn’t their thing,

After a few weeks of making your plants look like they were repeatedly blasted with a shotgun, the female begins laying eggs in the ground, almost always in lawns or grassy fields. She lays a few eggs every few days over a 6 to 8 week period, burrowing down a few inches. When the grubs hatch, they feed off grass roots. In the fall, they dig further down, 6 inches (15 cm) or so, to keep from freezing in the winter. They’ll go even deeper in really cold climates, about 14 inches (35 cm), although that isn’t always enough and in some areas, a really cold winter can seriously deplete the population.

The grubs, now quite large, move back nearer to the surface up in the spring and do most of their damage then, leaving dead brown patches on the lawn. Then they pupate for a few weeks and emerge as adults to start another cycle.

Two Important Details

First, although Japanese beetles may have you mostly wailing about damage to your hibiscus and cannas, you have to understand that they are very much linked to lawns, and grass lawns at that. Without large and abundant lawns of grass for their grubs to feed on, they wouldn’t be numerous enough to be a major pest. In Japan, where they are native, other forms of plant far outnumber lawn grasses and as a result Japanese beetle are just a minor pest. In North America, with it’s endless green carpets of nothing but grasses, much more space is devoted to lawns than to other cultures, resulting in huge numbers of grubs turning into equally numerous and very hungry adults that have to concentrate on what little other above ground vegetation you left have to offer. It’s a bit like you invited 20 people to the salad bar, but 90 showed up. There ain’t going to be much left over!

Mommy beetles prefer to lay their eggs in lawns growing in full sun and in sandy soil. The patchier and more open the lawn (often the case in sandy soils), they better they like it. They don’t particularly like dense green lawns or tall grass. Already if you just cut back on mowing in July and August, letting the grass grow to 4 to 8 inches (10-20 cm), rather than scalping it in the typical suburban fashion, it helps discourage the females from laying eggs. Of course, they can and do fly, but still tend to remain fairly near their favorite sandy lawns.

Japanese beetles are gregarious: you rarely see just one.

Equally important to understand, though, is that Japanese beetles are extremely gregarious. They are aggregate insects, drawn to other Japanese beetles by their scent. You rarely see just one, but rather dozens, often 2 or 3 or more right on top of each other copulating. And that can be good news for a gardener. If you are diligent in your control at the very beginning of the season and keep the numbers low, the remaining JBs may quickly move on to smellier pastures.

Control Methods

People, this isn’t a war and you aren’t going to win it. But what you can do is to keep the population low enough that little damage is done. And here are some possibilities.

1. The single best practice is to gradually remove, over a number of years if necessary, the most severely affected plants and to replace them with plants they like least. This is not only amazingly effective, but also self-sustaining. No food plants, no FBs: it’s as simple as that. The problem is that many of their food plants are big trees, something you’re unlikely to want to remove… until you’ve seen them defoliated 3 years in a row. That may change your mind!

2. As soon as the adults start to appear (late June, early to mi-July), start hand harvesting them. Do so in the morning, when they’re still sluggish. Just knock them into pail of soap water: you don’t actually have to handle them. Or harvest them with a portable vacuum. This can work amazing well if you start early and keep at it. This is the second best method of controlling them.

Japanese beetle pheromone trap.

3. Use Japanese beetle pheromone traps. They’re widely available and quite effective… if you use them correctly. They contain two pheromones: a sex pheromone that mimics the smell of the female Japanese beetle, which attracts male beetles, and another that gives off a floral scent that attracts both sexes. But you have to make sure you place these traps far from any of their favorite plants. You see, the odors waft over a considerable distance, drawing them from afar, but only a handful make it into the trap. So put the traps at least 50 feet (15 m) from their favorite plants. And empty the traps daily if they start to fill up. (Sometimes they’ll catch hundreds a day.) For more information on pheromone traps, read Love Trap for Bugs.

4. You could also try spraying. Do so early in the morning, before pollinators and other beneficial insects are around. For an organic choice, you could use insecticidal soap, neem or pyrethrum. Avoid the dishwater detergent: it’s toxic to many plants. There are also systemic chemical pesticides you could theoretically use that make the whole plant toxic from the tip of its roots to the tip of its stem, but do you really want to live in an environment where you don’t dare have kids over for fear of poisoning them? Many of the best systematic pesticides are no longer offered because of their great toxicity or only professionals have access to them. Rightly so, in my belief.

5. Mix clover into your lawn or even replace your lawn grasses with a clover lawn. Neither grubs nor adults will touch clover. Egg-laying mommy JBs will tend to go elsewhere when the lawn is mostly clover.

Don’t direct any lighting to the lawn: it will show the beetles where to dig!

6. Avoid lighting lawns and flowerbeds at night. The light attracts Japanese beetles just at the right time for egg laying: it’s like telling them: dig here! Use just enough lighting for safety purposes and try not to direct any towards the lawn itself.

You can find beneficial nematodes in most garden centers.

7. You can also treat your lawn with beneficial nematodes – microscopic predatory worms – from mid-August to early September, when the grubs have just begun to hatch and are still vulnerable. Nematodes are sold alive, but refrigerated: you have to keep them cool until you apply them. Give the spot a thorough watering the day before you apply them, then apply them as a liquid spray (shake the container frequently). And water again to settle them in. Keep the soil moist for 4-7 days. Full sun when you spray them can kill them, so it’s best if you apply them on a gray or cloudy day, or early in them morning so they can burrow out of sight before the sun gets hot.

8. There is a bacterial disease you can apply. It’s called milky spore disease (Paenibacillus popilliae, formerly Bacillus popilliae). It is specific to Japanese beetle grubs and will harm no other insect. You apply it a bit like nematodes (but follow the specific instructions given). It has a greater persistence than nematodes, which tend to be rather fragile, and can remain active in the soil for up 20 years or more! However, it takes 2 to 3 years before the amount of spores in the soil builds up to the point where you start to see a difference, so you have to be very patient. Milky spore disease has never been approved for Canada and is not available there.

9. There is also a special form of BT called BTG (Bacillus thuringiensis galleriae), a bacterium that is specific to beetles, chafers and their ilk, but has not yet been approved for use in North America. It is effective against all stages of Japanese beetles, both grubs and adults, and seems very promising.

Shrews are interesting Japanese beetle predators.

10. Encourage the presence of natural predators (toads, birds, shrews, moles, etc.) or release ducks or chickens in the area. Skunks also eat the grubs… but do a lot of damage to the lawn while looking for them. Two natural predators of Japanese beetles from Asia have been released in the United States, the predatory fly Istocheta aldrichi and the tiny wasp Tiphia vernalis, and are spreading on their own. I don’t believe you purchase them at this time, but if they have reached your neighborhood and you want them to work for you, you’ll have to avoid using insecticides.

11. When it comes to vegetables and small fruits, you can get really good protection by simply covering them with a floating row cover. Apply it early in the season, before you seen any adults.

Castor bean is said to poison Japanese beetles: they munch on the leaves, then keel over!

12. You could try poisoning your beetles with toxic plants. They’re said to be susceptible to the flowers of the zonal geranium (Pelargonium x hortorum) and bottlebrush buckeye (Aesculus parviflora) and the leaves of castor bean (Ricinus communis) and four-o’clocks (Mirabilis jalapa). Of course, this treatment will only work if the beetles nibble on them, so you’ll be sacrificing these plants to the cause. These trap crops are not going to solve your JB problem, but it can be quite satisfying to watch the beetles drop off the plants mentioned, land on their back and twitch drunkenly. At best they’ll combine with other methods to help lower the population.

12. Other plants are reputed to repel Japanese beetles, particularly alliums (onion, leek, garlic, chives, etc.), rue (Ruta graveolens) and tansy (Tanacetum vulgare). The idea is to plant them around susceptible plants and they should keep the beetles at bay. However I hear both positive and negative results with this method, which is why I put it last. Besides, tansy is a very invasive, hard-to-control plant. Not something you’d want to let loose just anywhere!

Few Perfect Solutions

As mentioned, you won’t really be able to totally control Japanese beetles once they have found your neighborhood. The best you can do is to reduce their numbers to more acceptable levels. Only replacing beetle favorites with plants they simply don’t eat will really solve your problem completely… but by carefully combining different methods, you can reduce the population to the point where the beetles become more a small inconvenience than a scourge.

I’ll let you choose the methods you feel will work best for you.