Japanese beetle traps—a reconsideration

Japanese beetle traps—a reconsideration

28th December 2016348Views17Comments
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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.

Posted by

James Roush
on July 15, 2013 at 8:13 am, in the category Guest Rants, Ministry of Controversy.


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  1. 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.

  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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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!

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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

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