Japanese beetle traps—a reconsideration


Japanese beetle courtesy of Shutterstock
Japanese beetle courtesy of Shutterstock

To trap or not to trap? That is the million roses question, isn’t it?  Conventional wisdom holds that the use of Japanese beetle-specific traps will increase beetle damage on plants adjacent to the trap sites. You can find that “wisdom” repeated everywhere—on extension articles, Internet blogs, over and over, accepted and final.

Well, friends, ProfessorRoush had a mentor who once said to me “If I wrote that the sky is green in a book chapter of an authoritative text, in 10 years the entire world would be repeating that the sky is green.”  Phrases like “conventional wisdom” just raise my hackles, because if we’ve learned anything from the past millennium, it’s that if we followed “conventional wisdom,” we would still believe the sun revolved around the earth, the New World would never have been discovered by Europeans, and I wouldn’t be trying to garden in the hell-hole of Kansas.

In the throes of anguish after Japanese beetles finally reached Manhattan, Kansas, I set out to look at some of the actual research behind the no-trap recommendation, and I can already tell you that the question is far from settled. Most of the statements that Japanese beetle-specific traps increase plant damage and don’t affect beetle numbers are referenced back to two papers in the Journal of Economic Entomology, 1985 and 1986, authored by F. Carter Gorden and Daniel A. Potter from the University of Kentucky. The papers indeed reach the referenced conclusions, but if you examine the materials and methods of their research you’ll discover the interesting fact that they placed their traps at 1.2 meters above the ground in both studies. I already knew that a more recent study, by Alm in 1996, found that a height of 13 cm above the ground was the most efficient trap height, which just happens to also be the average height that Japanese beetles fly around a garden. The 1985 and 1986 papers, for those metrically-disadvantaged, had their traps at 120 cm, so, in essence, they were expecting these lumbering insectoid rocks to find the traps approximately 10 times farther off the ground than they normally fly. Thus science advances gardening.

I also reviewed a 1998 Journal of Arboriculture paper by Wawrzynski and Ascerno that found that mass trapping over 15 acre area caused a 97% reduction in Japanese beetles within 4 years. Consequently, I really wonder if gardeners haven’t been kept from using the best tools for this particular job. Commercial traps that use both floral attractants and pheromone lures are demonstrably effective—a popular “bag-a-bug” trap performed pretty well in a 2003 report by Alm and Dawson

What does that mean for ProfessorRoush’s garden?  It means that I’m going to buck the conventional wisdom and trap the bodacious beetles out of my garden for a couple of years to see if I can slow down the invasion. Based on the research available, I will place my traps as close as possible to the recommended 13 cm height and I will place them at least 30 feet away from the nearest important plant so as not to attract beetles right onto my roses. I will empty the traps regularly so the dead beetle stench doesn’t drive others away and I will make sure the lures stay attached. I’ll let you know how it goes.

I’ve already caught three hard-shelled fiends that won’t be breeding little beetles for next year. I hope that it is simple logic. Less breeding, less beetles, more roses.


  1. Have you tried any of the biological JP remedies, like nematodes or milky spore?

    My other comment would be that these traps would have to be ruled out for many urban or smaller gardens if a 30-foot distance from any eatable plant is to be maintained.

    • I haven’t yet tried other methods. Right now, the numbers of Japanese Beetles around here are low enough (I’ve found 8 on the roses in two weeks and there are 2-3 more in each trap right now), that I don’t want to affect anything else; there are, and for years, have been lots of grubs in my lawn, presumably June Beetle larva, but could be anything. And, although I have a rare lapse, I try not to use insecticides, natural or otherwise, in my garden.

    • This year I sprayed nematodes over about 1000 sq. feet of garden and lawn containing roses and raspberries. Every previous year I’ve been inundated with Japanese beetles. But this year they are greatly reduced in number. I can kill most of them by hand. I’m going to spray nematodes again before August as there is always a second round of the pests at that time.

  2. James: Here in Pennsylvania, Japanese Beetles have been an annual nuisance for many years – welcome to the club. You’re right, the verdict is still out on the effectiveness of traps. I don’t know about how they fly in Kansas, but here they fly at all levels, as they dive bomb my home all day – 13cm, 13 feet, doesn’t matter. Years ago I hung traps at about 4 feet, and I’d collect dozens of these garden terrorists each day, every day for the length of their “season”. But after I used Milky Spore in my lawn, the population seemed to drop significantly, and it has remained there. Previously, the lawn was loaded with japanese beetle grubs and few are to be found now.

    • The milky spore treatment is great, but, again, it’s only good for the yard its applied in. I hope your neighbors are using it too! The technique might work in mine; I’m surrounded by prairie that presumably will have low numbers of beetles once they get going.

  3. This year I’ve had very few Japanese beetles in my garden, when typically the roses are *loaded* with them. I think I’ve seen six beetles so far this year. We had a long, cold Spring, so maybe they’re just slow getting going.

    If not though and we’re actually spared this year, keep this in mind. If I had used traps last year I would have thought “wow, they really worked”, when in fact it’s some other factor.

  4. Since this is the first year for JB’s in Manhattan, Kansas, it’ll be the years to come that will tell the tale for me. Years where either I’m a) averagely infested with JB’s along with everyone else, b) overwhelmed worse than anyone else, or c) (preferably) not bothered while the town is being eaten alive.

    Time will tell.

    • Japanese Beetles- I found something that actually works!

      Get rid of Japanese beetles. Once quite rare, and only found in the eastern United States, these pesky beetles are now moving west. They attack many different types of herbs and vegetables and are difficult to eradicate once established. Handpick them in the early morning by shaking tree limbs and branches. Bait them by mixing together water, sugar and mashed fruit. Place this concoction in a sunny spot at least 1 inch off the ground. Strain out the beetles every morning. Plant ‘trap’ crops between vegetables and flowers: Japanese beetles favor marigold, borage and evening primrose. If you plant these throughout your garden, they will naturally navigate toward those specific plants.

      When the fight is on to rid the yard and garden of Japanese beetles, milky spore is the organic gardener’s best friend. Milky spore, an organic pesticide, contains a bacteria that selectively targets the white grub larvae of the adult beetle. Once milky spore has been applied, the larvae feast on the plants that have absorbed it and die within a few weeks. As the grubs decay, the spores they ingested are spread back into the soil to continue their work.

      Follow the above suggestions to help you with your organic garden. Think of the benefits you get by gardening the natural way. Maybe the nutrition is your primary concern, or perhaps you are looking for a way to cut cost. Whatever the reason, enjoy taking a bit out of that ripe, juicy watermelon or a fresh, crisp carrot!

  5. We have been blessed for the last three years with very few of the GD f’ing little sob’s. I hear they go in five yr cycles. So far I have squished only 19 and we are 1/2 way thru their season. Blood thirsty, ain’t I? When infested I brush them into buckets of soapy water. It’s funny where you find them. Never on my climbing rose but devour my green bean leaves. I don’t even plant beans until end of July for that reason.

  6. Have you tried row covers, or would it involve too large of an area? It’s true that milky spore and biological controls, such as nematodes, will only affect the area in which they’re applied; however it may be a harmless option that cuts down on the overall beetle population.

    • I don’t know about Tibs, but row covers aren’t an option here in Kansas. I once tried some “low hoop” houses anchored with stakes and pegs. They lasted at 3 days and are probably still floating in the Jet Stream somewhere over Siberia.

  7. Great article!! I gave up on my ‘Show Garden’ climbing rose because the flowers were simply eaten up by the Japanese beetles. I see the #$%s on my raspberries (although not in the thousands) and on a few other perennials. I will await your results.

  8. The JB’s here flock to the very top of my pole beans, which they seem to prefer for procreation. The good news is, I can usually hand pick them off by the pair! They probably never know what hits them when they get squashed.

  9. Had JB last year and about 3 years previous. I am in the Adirondacks. It used to be too cold , -40 on occasion, for such obnoxious thing s to survive. I think they come in on vehicles from other areas. I don’t have roses, again too cold. They like the asparagus fronds which makes them real easy to see to pick off.

    The bug du jour this year though is the the bright scarlet lily beetle. an import (really?) no predators hand picking- easy to see. Egg nests are gross and they are only eating the oriental lily. Google popped them up instantaneously. There is a biologic that slows them down but has to be used when it is dry. It seems like it has been raining for months, lakes and rivers overflow.

    Then yesterday a whole new batch of unknown beetles hatched! milling all over the picnic table in groups, moving like fish in a school. The table is under a paper birch tree, otherwise in lawn area. Not too near any gardens. Regardless there is a feast of opportunity for such critters here. They are third or less the size of the JB grey with orange sorta tweedy pattern very round.

    What are they hungry for?

    • Diane,

      Any chance those new bugs are Oriental beetles? I’m in Dutchess Co and was plagued by Japanese beetles last year. I’ve seen one so far this summer but have been overrun by the Oriental beetles and they are very aggressive. Sometimes they come after me while I’m floating in the pool.

  10. Professor, have you tried planting peonies adjacent to your roses? I read this somewhere years upon years ago, tried it – and it actually does work! Does it eliminate them entirely? No. But boy, does it chop down the population. As I understand it, this is how it works: You know those little wasps you see around the peonies just before the buds open? They’re a parasitic wasp. They go below ground at the end of their life cycle, lay their eggs, and when their larvae hatch, they feed on the Japanese beetle larvae. Very neat, totally organic and really effective. Just my two cents worth, anyhoo…..

    • Thanks Susan. I already have a number of peonies in the garden so the wasps have a welcome home. And to Bittenbyknitten, I also have hollyhocks, right next to the roses, but the Japanese Beetles here leave the hollyhocks alone for some reason.

  11. I, too, garden in an urban area, so traps are no good for me. I used Milky Spore 15 years ago & have reaped the benefits to this day. I believe the fact that my neighbors either use chemicals or only have a few lollypop shrubs to the fact that I have no intruders from over my borders….I’ve also used organic techniques with whatever version of IPM I can manage under these tight quarters. I had tried the traps 16 years ago, hanging them on 18 inck hooks they came with & my entire garden, flowers & veggies, was decimated. My hope for you is that your results are much better!

  12. Great piece and discussion! I found my first JB on a rose last week and am waiting to see what kind of damage/population I have, since we just moved to Minnesota from Seattle. Keep sharing, everyone.

    • No Thank You! I’m already questioning whether a nighttime digger I’ve had an occasional problem with is an armadillo. They’ve been reported to the Nebraska border and one caused over $2000 damage recently to my son’s car (he hit it on the highway).

  13. I live in south central Tennessee and have found the JB’s to be the bane of my existence. They attack everything. Nothing ever seems to be totally safe. The first 2 years here I used traps and it seemed like even tho I killed close to a million a year, the damage by those not trapped was horrid. I had them hung at what seemed like the JB’s flight path height. I emptied them daily and washed out the collection bucket to keep the smell down. But I could not even walk outside without getting hit in the head with the buggers.

    Then I decided to not use the traps this past 3 years. I hand pick and give them straight to my chickens who gobble them up like M&M’s. I had a lot less damage on my most important plants, such as the grapes, elderberries and green beans.

    It seems that this year, across the board, people are noticing less JB’s out there. Who knows why or if it will continue? It would be nice to find out that there is some predator eating the grubs or a disease that is weakening the population. But I am sure they are not permanently done for and hand picking will continue to be one of my daily summer gardening chores for many years to come.

    But even tho the JB’s are less in number, we have had a ten-fold increase in the number of Green June Beetles and they are like JB’s on steroids. And I am not going to hand pick them and I know that my chickens wouldn’t be able to eat the monsters anyway.

  14. If you set up multiple traps in a way that triangulates around an area you want to protect and empty them frequently, and the decision of where to place them is on or near less desired specimens (or evergreens whose needles are difficult for the beetle to digest) then trapping can be effective. Scouting for the early beetle scouts at first emergence and dropping them into soapy water can also minimize damage. Letting your turf go dormant in the hot, dry season rather than watering it reduces beetle numbers, since their eggs require moisture to hatch.

    Btt (tenebrionis) strain is supposed to be coming soon for homeowners as a turf treatment. Selective to Coleoptera, as Btk (kurstaki) is to Lepidoptera, and Bti (israelensis) is to Diptera. As far as I know, Btt is only available in formulations for farmers at this time. Bt products must be consumed by the insect to have an effect, so most effective on early instars, and minimal impact on non-target species.

  15. I think there are fewer JB this year because of the severe drought last year. They eggs need moist soil in July-August-September to flourish which they certainly did not get last summer.

    I think if a gardener has plants that attract JB there can be no harm in trapping: since they are attracted already, they may as well be in the trap rather than on the plants doing damage.

    I hand pick morning and evening too whenever I have time since I believe that every individual picked is one less around to reproduce.

    I remove all rose blooms after the first JB sighting until the beginning of August. They don’t have a chance to look good anyway, and the plants get a rest for the duration, and bloom like crazy in late summer and August when they are especially appreciated.

  16. Jap beetles population varies greatly from year to year. For the average gardener they still place the trap right next to the roses…………………..and they wonder

    The TROLL

  17. Professor Roush,

    As a proud-born son of the Kansas Flint Hills I would like to extend my deepest condolences for you being subjected to our unique brand of political, climatic and economic violence. Its hard a hard enough to get used to when you’re born right into the thick of it. People here in Eastern Washington tell me that its so windy and the soil is so rocky… ha. I just smile along and rake all my chunks of basalt aside. 🙂

  18. I really wonder if gardeners haven’t been kept from using the best tools for this particular job. Commercial traps that use both floral attractants and pheromone lures are demonstrably effective—a popular “bag-a-bug” trap performed pretty well

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