The Fly That Controls Japanese Beetles

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The Japanese beetle is universally detested by gardeners … but a small fly is now helping to control its population. Source: Judy Gallagher, Wikimedia Commons

Ever since the Japanese beetle (Popillia japonica) first showed up in New Jersey in 1912, apparently brought over from Japan in a shipment of bulbs, farmers and gardeners have been desperately seeking an easy way to control it. And it’s now made it to Europe as well, with outbreaks in Italy, Russia and, since 2017, Switzerland.

When you have Japanese beetles, you know it. The adults seem to make little effort to hide. The metallic green beetles with coppery wing cases gather by the thousands and munch their way through foliage and flowers alike, leaving devastation in their wake. The worst hit plants have no untouched leaves at all, just browning ones with intact veins. And this pest attacks a wide range of plants (read Japanese Beetle Host Plants). To make matters worse, Japanese beetles attack in mid-summer, just when your garden should be at its finest!

And it’s not just leaves and flowers! Underground, their larvae chomp away on grass roots: yes, they’re among the various scarabs whose larvae we call white grubs, so hated by lawn owners everywhere.

In other words, Japanese beetles piss pretty much everybody off!

An Attempt That Fails… Then Succeeds!

Back in Japan, the Japanese beetle has several predators to contend with and, as a result, the population generally remains low and damage is minor.

American researchers tried to capitalize on this by introducing, starting in 1927, a serious of insects that feed on Japanese beetles in its native habitat. However, the first trials seemed unsuccessful. But it turns out at least one insect, a tachinid fly called the winsome fly (Istocheta aldrichi), has adapted better than was thought at the time. (Another Japanese beetle predator, a parasitoid wasp known as Tiphia vernalis, is doing fairly well too, although its range today is less extensive.)

The winsome fly hadn’t done well in New Jersey tests, because in that climate, the life cycles of the two insects barely overlapped. The adult fly—the egg-laying phase—tended to emerge too early and was near the end of its cycle by the time the Japanese beetles emerged in their turn in July. Thus, few JBs were parasitized and it was thought the experiment had failed. However, several decades later, the winsome fly was found again. It had made it further north on its own, to New England, where the cooler springs delayed its emergence enough so the two life cycles overlapped much more effectively. In some areas, in some years, up to 80% of all Japanese beetles that emerge have been found parasitized.

Winsome flies are still on the move! Even as Japanese beetles continue to expand their range in North America (they’re now present in most US states and Canadian provinces), so do winsome flies. They’re now found in most of the Northeastern US states and recently reached Canada where they’re thriving and spreading in Ontario and Quebec.

The Life Cycle of a Beetle Predator

The winsome fly is a parasitoid: it doesn’t just live on its host, it kills it!

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Winsome fly (Istocheta aldrichi). Source: National Collection of Insects, Arachnids and Nematodes

It’s a small grayish fly about 5 mm long, looking much like any other small true fly. It emerges just a short time before the Japanese beetles do and builds up its energy by feeding on flower nectar. When the Japanese beetles appear, the female fly starts laying white eggs on her host’s thorax, just behind its head. They are easily visible, at least if you have your glasses on.

The fly will lay a hundred eggs over the second two weeks of its 4-week emergent cycle. It tends to mostly parasitize female beetles, because they spend much of their time pinned under male beetles trying to mate with them and thus can’t readily escape the fly. Japanese beetles not mating react rapidly when winsome flies are around, quickly dropping to the ground.

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Eggs on the thorax of a few Japanese beetles. Source: blog.uvm.edu

The eggs hatch in about 24 hours. Even if the beetle carries several eggs, only one larva will actually penetrate the body of its victim where it will begin to digest it from the inside. First to go are its flight muscles, leaving the beetle unable to fly. The beetle then goes into protective mode, falls to the ground and buries itself.

The infested beetle dies only 5 to 6 days later, but the fly larva remains in the dead body of its host all winter as a pupa, then the cycle begins again the following summer. There is only one generation per year.

Note that this predation occurs as adult beetles are emerging, before they lay eggs. Since the female JB would have normally laid 40 to 60 eggs, that many fewer beetles will be born the following year!

Winsome flies will also predate, to a lesser extent, other white grub-producing scarabs, like European chafers.

Encouraging Winsome Flies

The winsome fly is not commercially available. You have to wait for it to show up in your area on its own.

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Umbellifers with their small, clustered flowers, are favorites of winsome flies. The blooms above are those of coriander (Coriandrum sativum). Source: H. Zell, Wikimedia Commons

Once they have reached your neighborhood, anything you do to attract a horde of winsome flies to your garden will help. For example, plant many small, shallow-flowered plants, such as umbellifers (coriander, dill, lovage, etc.), crucifers (sweet alyssum, mustard, etc.) and Asteraceae (chamomile, daisies, yarrows, etc.) to attract and feed the flies.

Also, avoid spraying insecticides. They usually affect the fly (our friend) more than the unwanted beetle… plus once you start seeing eggs on Japanese beetles, you need a new strategy: killing them all is no longer that useful.

Less squeamish gardeners—or those that most hate Japanese beetles!—can hand trap them and sort them, letting parasitized beetles go free (remember, they’ll be underground and out of sight in just a few days) and dropping unparasitized ones into a pot of soapy water. I don’t know how you’ll explain this to your neighbors, though!

The Result?

Winsome flies will never completely eradicate Japanese beetles. No wise parasite ever totally eliminates its host: that would be suicidal!

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With winsome flies in the area, you should no longer see such complete defoliation of your plants. Source: www.ontariohopgrowersassociation.ca

Where Japanese beetles occur, you’re unlikely ever to get rid of them entirely, so should choose plants accordingly (see Plants That Japanese Beetles Tend to Avoid). However, by eliminating the most beetle-susceptible plants from your garden, replacing them with ones they dislike and encouraging winsome flies and other predators by supplying nectar-rich flowers, you’ll find the number of beetles can drop significantly in just a few years. Many gardeners in areas where the winsome fly is well established say they can now garden much like they used to before JBs appeared, since the few remaining ones do little damage.

Breaking News!

Shortly after this article was published (like, about 10 minutes later!), I received word that the Montreal Botanical Garden is undertaking a study to determine the extent of the distribution of this parasitoid fly in Canada and asks Canadian gardeners to participate. If you see a Japanese beetle bearing the telltale white egg(s) of the winsome fly, please take a photo and send the information, along with pertinent details (there’s a short form to fill out) to the Entomological Information Service at Space for Life.

Thanks to Sandrine for this information!


The expansion of the winsome fly is therefore very good news for many gardeners and farmers! If combining a choice of JB-resistant plants with a small parasitoid fly can make gardening easy again, who’s likely to complain?20180707C blog.uvm.edu

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Controlling Those #$@&%* Japanese Beetles

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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 100th, 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

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.20160721C

Love Trap for Bugs

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Japanese beetle trap.

A pheromone trap is an insect trap that gives off pheromones (chemical substances similar to hormones) used to attract the insect being controlled. Pheromones can mimic the smell of an insect’s favorite food (fruit, flower, etc.), but more often the pheromones used in traps are a bit more libidinous than that: they imitate insect sex pheromones, normally that of the female pest. Thus, the males travel from afar, attracted by the smell of what they think is an attractive young virgin. Once they enter the trap, though, the males can’t get out and therefore can’t impregnate any females, leading to, at least in theory, a drop in the local insect population.

To make the trap even more effective, it is usually colored yellow, blue, green, or purple, depending on the favorite color of the insect.

A Trap for Each Pest

Pheromone traps are very specific: each is designed to attract a particular type of pest. There is therefore no danger they will trap beneficial insects. You therefore have to purchase a different trap for each insect you’re trying to control.

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Japanese beetle.

The possibilities for pheromone traps are almost limitless, but for the moment, only one is widely available in most areas in North America: the Japanese beetle trap. It is, of course, designed to repress Japanese beetles (Popillia japonica). It actually contains two pheromones: a sex pheromone that mimics the smell of the female Japanese beetle, which therefore attracts male beetles, and another that gives off a floral scent that attracts both sexes.

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Rose chafer.

Because of the floral pheromone, the Japanese beetle trap can also be used to catch rose chafers (Macrodactylus subspinosus), a Japanese beetle relative. However, there is also a specific trap for rose chafers that only gives off the floral scent. It seems to be more difficult to find in local stores.

Also on the market are apple maggot traps, usually shaped like a red ball that resembles a mature apple. Some models include contain a pheromone: a fruit essence that attracts the apple maggot (Rhagoletis pomonella).

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Trap for emerald ash beetles.

While the home gardener has only a limited choice of traps, farmers and foresters have access to a wide range of pheromone traps for just as wide a range of crop pests. You may, for example notice, traps placed in ash trees (Fraxinus spp.) In a park in your municipality. With this trap, authorities try to determine whether the dreaded emerald ash borer (Agrilus planipennis) is lurking in the area.

The Downside of Pheromone Traps

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When the trap is too full, the insects can’t enter.

But there is one major flaw with pheromone traps. While they do indeed attract insect pests to the area, but the latter don’t all enter the trap! Sometimes they simply miss the trap because the scent is carried elsewhere by the wind. Or the trap may already  be full.

Whatever the reason, the result is that pheromone traps often actually don’t reduce insect damage. The insects that didn’t enter the trap, now starving, flock to the nearest available food plant and start to chow down. This is sadly the situation with Japanese beetle traps: they do catch beetles and lots of them, but they also draw more beetles into the sector, so instead of the damage being reduced, it is often worse.

The joke usually proffered is to buy traps and offer them to your neighbors so the beetles will go to their garden instead of yours! That really would work, but I suspect your neighbours would be a bit upset when they find out!

Effective Use of a Japanese Beetle Trap

You can however use the trap effectively if you follow three simple rules:

  1. Place Japanese beetle traps well away from the plants they eat (at least 50 feet/15 m). For example, on a pole in the middle of a lawn.
  2. Empty the traps regularly. Sometimes you have do it every day, otherwise they fill up and new insects can’t get in. Just dump the pests into a bucket of soapy water.
  3. 20150720FCollect beetles daily from nearby vegetation, preferably early in the morning when they are not very active, using a hand vacuum (empty the vacuum afterwards over a bucket of soapy water). Children, especially, seem to find collecting beetles with a vacuum a lot of fun.

If you start using this combined method of insect control at the beginning of the season, you can make serious inroads into reducing the infestation.