Japanese Beetles - Know the Enemy

Japanese beetles (Popillia japonica) don't get the publicity they deserve. They're a success story. They can spell Armageddon for some of the most popular plant varieties maintained in backyard gardens, and once entrenched in a neighborhood, are almost impossible to eradicate completely. That doesn't mean you should give up trying, though. Japanese beetles are formidable foes, but they do have their kryptonite. The problem is there are lots of them, and most successful methods for eliminating them take planning. Think of it as warfare. The best campaigns use well-planned strategies that take time and consistent effort to pull off. Once you accept that one bucket of soapy water or a single spray session with a bottle of insecticide isn't going to do it, regardless of your level of gardening skill, it's easier to sign on for the long term fight. Is it painful? Sure. There will be losses. The good news is diligent effort pays off.

They Come from New Jersey?

Don't think you're being targeted by an unfair universe intent on turning your rose bushes into lace doilies. In the U.S. Japanese beetles are so pesky because they have fewer natural predator controls than indigenous species. These pests are Japanese imports (in case you thought the name meant something else), stowaways that made landfall in New Jersey around 1916. In their native land, they aren't nearly the problem they are here. They've been migrating west, gobbling up a smorgasbord of domestic and exotic plant species. Today they're active in 30 states, so at least you're not alone in your frustration and grief over plant losses. JB's are known to feed on at least 275 different plant species, and the annual cost to the turf industry alone is over $460 million. Many of their favorite meals are also popular garden plants.

Japanese Beetles - Know the Enemy

They like it wet

It isn't all bad news. Weather can have an impact on how active Japanese beetles will be in a given year. They are less abundant during hot, dry summers, and reproduce in fewer numbers, so there's some residual bounty in drought years. The flip side is they love warm, wet weather. If you've had a soggy summer, expect problems again next year if you don't do something to control their numbers.

They live underground most of the time
Japanese beetle eggs

By summer's end, it may seem as though you've been battling Japanese beetles forever, but they're actually only active for six to eight weeks. After that, the adults will have laid their eggs and died. Those eggs in your lawn and flowerbeds turn into grubs that will feed on grass and other plant roots over the autumn and winter, and emerge next spring. How much time do they spend in the soil? That would be around 10 months a year.

Mark your calendar

Depending on where you're located, Japanese beetles will surface sometime between Mid-May and mid-July. A good rule of thumb is the farther south you are, the sooner you're landscape is likely to warm up and trigger their appearance. They're pretty predictable, emerging around the same time in an area year after year. If you started seeing them the second week in June, you can plan for next year's assault around the second week in June.

Japanese Beetle Deterrents You Should Know About

There are some natural deterrents to Japanese beetles, and the U.S. Department of Agriculture is even taking steps to help:

Milky Spore and Japanese Beetles
Milky disease, or milky spore, is a bacteria (Paenibacillus popilliae aka Bacillus popilliae) that lives near the surface of the soil where plant roots are abundant. This is also the zone inhabited by freshly hatched or young JB grubs (larvae). If present, grubs will ingest milky spore as they feed on grass and other roots. Within one to three weeks, the bacteria will kill the grubs, multiplying and releasing new generations of beneficial bacteria into the soil. Milky spore is safe around humans and pets, and benign to earthworms, bees and other common beneficial insects. Even though it is often criticized as only working on JB grubs (which seems like enough if your garden is infested), current research suggests milky spore can be effective against other types of white grubs as well.

Although you may have a little milky spore in your soil already, especially if you live in some East Coast regions, you can introduce more, but timing is important. Grubs hatch around August in most locations, and will feed in the "spore" zone until temperatures cool down, after which they will burrow deeper and feed little if at all. Apply milky spore then or before, when the soil temperature is at or above 65 degrees F and likely to remain that way for a while. One big advantage to a successful application is that it will remain viable in the soil for a long time. There is a difference of opinion on this, but some research suggests one application can provide additional JB defense for a decade or more.

Nematodes and Japanese Beetles
Nematodes are another biological defense against Japanese beetles. There are lots of different types of nematodes, a kind of microscopic roundworm.  Some are beneficial because they attack and kill destructive insects, and others are themselves nasty customers and best avoided. Beneficial nematodes are available as a packaged natural defense against Japanese beetles and other soil dwelling pests. Some wood boring pests may also qualify for eradication this way. Beneficial nematodes are known as natural grub killers, and different varieties can have an impressive list of victims. Many products include a blend of different worm types like:

  • Steinernema carpocapsae
  • Steinernema feltiae
  • Steinernema glaseri
  • Heterorhabditis bacteriophora

Working together, they will destroy the bugs you love to hate, like: Japanese beetles (of course), vine borers, bagworms, cucumber beetles, flea beetles, outdoor fleas and weevils. Most beneficial insects like bees, earthworms and ladybugs are unaffected.

Although treatment recommendations will vary by manufacturer, most involve mixing nematodes with water and spraying them on your soil and lawn. You can apply a nematode mixture with a hose or pump sprayer. Because they are alive -- and small, check Amazon or another supplier for satisfaction ratings on any product you're interested in. Common customer complaints include receiving suspected dead batches. This can be hard to determine. A high satisfaction rating is a hedge against receiving dead or dying nematodes.  It's also important to note that different nematode blends affect different pest species, so be sure you choose a product that will kill the range of pests you prefer. Japanese beetles are a favorite for nematode products, so they're included in most blends.

The application window for nematodes is similar to that of milky spore, with the exception of a spring application option that will kill grubs before they emerge, but after they've done plenty of root damage, so not ideal as a single application strategy. Some products recommend twice yearly applications, but once communities of nematodes are in your soil, they'll keep you protected indefinitely unless you kill them with pesticides.

Products are rated for the size of the area to be covered. You can buy nematode products at many local garden centers and online. Be aware this is a seasonal, time sensitive product with a limited shelf life. There are living critters in the bottle or box, so don't make the mistake of leaving them in your roasting hot car or on your porch during an early freeze. It's also a good idea to review the application directions carefully. Some products can be applied multiple times (usually dried varieties), while others are a one shot proposition.

Pesticides and DIY
Japanese beetle grub (larva)
Nematode and milky spore will help reduce JB populations living and thriving on your property, but won't do much to stop visitors that fly in from your neighbor's yard looking for a snack. For that you can use *pesticide (the USDA recommends Malathion, chlorpyrifos, diazinon and trichlorfon), or employ a guerilla warfare tactic by just knocking any beetles you see into a bucket of soapy water and letting their reeking corpses discourage latecomers.

I've discussed these choices in other posts, and they do work. The numbers can be against you, though. Some neighborhoods are so infested that targeted responses like these are overwhelmed. You can kill hundreds of beetles in a day, and tomorrow there'll be plenty more to take their place. Many pesticides require direct contact with the pest, and that requires very regular respraying, which can get expensive and be devastating for beneficial insects like bees (who have a tough enough time of it already).

Staying with a pesticide or soapy bucket program will reduce the impact of an infestation over time and discourage beetles from laying eggs on your property, which is good news for next year.

With regard to the bucket idea, if you start early enough, like in the first few days after JBs emerge, you may be able to keep damage to a minimum. The theory here is that early risers lay down scent markers for others to follow. If your property isn't marked extensively, later swarms may bypass you for more enticing locations.

Grubs make great snacks for chickens. Yum (Cluck)
Japanese Beetle Predators
Beyond adding milky spore and beneficial nematodes, insects have been pressed into service to combat the Japanese beetle problem in the U.S. Two types of wasps, (spring and fall tiphia) have been imported from Korea and Japan respectively by the U.S. Department of Agriculture (USDA) and released in area specific campaigns to control crop losses. These little wasps are among the Japanese beetle's natural predators. Unlike the ladybug and praying mantis, you can't purchase and release your own wasps to patrol the garden, but their introduction to the U.S. may have an impact on JB populations going forward.

You can also look to the skies for relief. Some birds love to chow down on JBs. So, if you're willing to offer up some seed to attract them, and share a few of your garden's earthworms, a little hired help never hurts. Expect mayhem, and hang around for the show. Here are some likely feathered candidates:

  • Blue Jays
  • Bobwhites
  • Catbirds
  • Chickens
  • Crows
  • English Sparrows
  • European Starlings
  • Grackles
  • Killdeers
  • Kingbirds
  • Orioles
  • Purple Martins
  • Robins
  • Seagulls
  • Sparrows
  • Swallows
  • Woodpeckers 

The idea of using pheromones to attract Japanese beetles to a location where they can be trapped and killed sounds great. You don't even have to get dirty in the process. When the USDA and others tested this method of JB control, it didn't do too well, though. Although traps do a good job of attracting beetles, they only secure about 75 percent, leaving 25 percent to roam around your yard. In some cases, you could get stuck with more bugs than you had yesterday, even after the trap kill-off. One clever reader suggested giving traps to your neighbors as gifts, which could have the effect of luring JBs out of your yard and into theirs. If you try it, don't expect a fruit basket at for Christmas (Hanukah, Kwanzaa).

They Want to Live in Your New Landscape

Japanese beetles are more abundant in newer neighborhoods. Yes, it's true. The experts believe this is probably because there's plenty of virgin lawn and tender, immature shrubbery in new residential construction, along with fewer natural enemies around to spoil the fun.

Working with a new landscape creates a good opportunity to choose your plants wisely and prefer options JBs don't like, but even an established garden can benefit from some strategic reorganization.  Some gardeners are surprised when their landscapes suddenly and (almost) inexplicably become JB targets. This can usually be traced to adding attractant plants like roses or blueberries. This list of Japanese beetle preferred plants should help in your planning:

Trees and Plants Japanese Beetle Like:

  • Apple 
  • Apricot
  • Asparagus
  • Beech (common)
  • Blueberry 
  • Birch 
  • Black walnut 
  • Cherry
  • Clematis
  • Climbing hydrangeas
  • Common mallow
  • Coneflower
  • Corn (sweet)
  • Crab apple 
  • Crape myrtle
  • Dahlia
  • Daylily
  • Evening primrose
  • Gladiolus
  • Grape 
  • Hawthorn 
  • Hibiscus
  • Hollyhock
  • Horse chestnut 
  • Japanese maple
  • Larch 
  • Linden 
  • Lombardy poplar
  • Morning glory
  • Mountain ash
  • Norway maple
  • Peach 
  • Peony
  • Plum
  • Raspberry
  • Rhubarb
  • Rose
  • Sassafras 
  • Shasta daisy
  • Soybean
  • Sunflower
  • Sweet corn 
  • Sycamore
  • Viburnums
  • Virginia creeper
  • Willow
  • Zinnia

**I've used common names on purpose. I wouldn't want you to think all other cultivars were safe because I listed a single variety mentioned in the literature. Actually, the research isn't exhaustive. If you have a plant, shrub or tree that fits the general description above, or are planning to purchase one, it's worth doing some research to determine if JBs are drawn to it. After that, you can decide how important the specimen is in your landscape. Be aware that its attraction can vary over the course of a season. A plant that is not impacted much in, say, June, could become attractive after more desirable options have been defoliated by the middle of July.

The attraction of a specific plant may also be affected by the plants around it. Placing a JB target plant, like a rose bush, next to less attractive options could either conceal it somewhat, or make it less of a target if there are tastier offerings elsewhere. It's a bit like serving ice cream with Brussels sprouts or liver. Japanese beetles do have some discretion in choosing where to feed, and if you make your plants less attractive, at least when the pests first emerge, they may bother your landscape less. Japanese Beetles can actually fly long distances for good meal or a mate. They've been tracked five miles, but probably don’t wander that far very often.

Trees and Plants Japanese Beetles Don’t Like Much

  • Ageratum
  • Arborvitae
  • Artemisia
  • Begonia
  • Boxwood
  • Burning-bush
  • Catnip
  • Chives
  • Chrysanthemum 
  • Citronella
  • Columbine 
  • Coral-bells
  • Coreopsis
  • Dogwood
  • Dusty-miller
  • Forget-me-not
  • Forsythia
  • Foxglove
  • French marigold
  • Geranium
  • Hemlock
  • Hickory
  • Holly
  • Hostas
  • Impatiens
  • Juniper
  • Lantana
  • Larkspur
  • Leek
  • Lilac
  • Lily-of-the-valley
  • Magnolia
  • Mint
  • Moss-rose
  • Nasturtium
  • Northern red oak
  • Onion
  • Pachysandra
  • Pansy
  • Pine
  • Poppy
  • Red maple
  • Rue
  • Showy sedum
  • Spruce
  • Sweetgum
  • Tansy
  • Tulip poplar
  • Violet
  • Yew

Some gardeners have taken the plant repellent idea a step further and made topical sprays out herbs or other smelly plants JBs don't like. Some examples are rue, tansy, catnip and mint.

If this was a bad summer for Japanese beetles in your area, you can make next year better. You may never have a completely Japanese beetle free garden, but your plants don't have to look perforated.  You can see it takes some planning and effort, and approaching the problem on multiple fronts is a good idea. Adding nematodes or milky spore to your garden in late summer can help control localized populations, and losing some JB plant draws can reduce problems even further, as can the addition of repellent plants like catnip. Try to place repellents in areas with good air flow. That way their scent will catch JBs in flight, encouraging them to turn around before ever landing on your property.

* Note: Pesticide applied to grass in August should kill many underground grubs still feeding before the arrival of colder weather. This won't be nearly as useful later in the year, and should not be combined with either milky spore or nematode treatment.

** For more information about specific plant varieties that either attract or repel Japanese beetles, please refer to the USDA's "Japanese Beetle Handbook.


U.S. Department of Agriculture. "Controlling Japanese Beetles." National Agriculture Library. 1982.

U.S. Department of Agriculture. "Managing the Japanese Beetle: A Homeowner's Handbook." Animal and Plant Health Inspection Service. Revised 2015.

Ingham, Elaine R. "THE LIVING SOIL: NEMATODES" United States Department of Agriculture. Natural Resources Conservation Service.

McGrath, Mike.  "Milky Spore Disease" Gardens Alive(Q&A). 2006.

Potter, M.F., D.A. Potter, and L.H. Townsend. "Japanese Beetles in the Urban Landscape" University of Kentucky. 2006.

Photo Credits:

Japanese Beetle Blue Flower -  Flickr. Courtesy of User: Espie

Nematode - Flickr. Courtesy of User: Christophe Quintin

Japanese Beetle Treats - Flickr. Courtesy of User: Vanessa Hernandez

Japanese Beetle Grub - Flickr. Courtesy of User: Travis

Japanese Beetle Eggs - Public domain photo. Source: USDA Animal and Plant Health Inspection Service, 'Managing the Japanese Beetle: A Homeowner's Handbook"

1 comment:

  1. I hand pick but think I will add Milky Spore to my regimen.
    Very informative article, thanks.


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