Japanese Beetles The Plain Truth About Traps And A Tiny New Winged Hope

Writen by Fred Davis

Every year visitors to our gardens here in Maine bring questions about some problem or other in their gardens. Not surprisingly, many have to do with near-Biblical hoards of Japanese Beetles, and the relationship between burgeoning numbers of these destructive insects and those readily-available pheromone lure beetle traps on merchant's shelves.

Well, what about Japanese Beetle traps? Do they really work? Or are they -- as more and more of us are finally realizing -- part of the problem? You may find my response to the following frustrated gardener's recent emailed plea for help enlightening. . .if not encouraging:

"Help! Japanese Beetles were especially bad this year. We put up three traps and had more beetles than anyone else in our neighborhood! What did we do wrong?"

Some things simply resist explanation, while solutions to other pressing gardening problems are so patently apparent that I'm surprised everyone hasn't already deduced the answer. To get to the core of the problem, during 2004 we conducted a survey of sorts, asking garden visitors about their experiences and thoughts on the subject.

About half of those queried felt there were increased populations of Japanese Beetles that season; the other half said there seemed to be fewer. Not surprisingly, most of the more beetles half had installed commercial pheromone traps in their gardens or on lawns. Japanese beetles literally flocked around those traps!

If anything can be faulted for increased numbers of these rascals in the garden, it certainly wasn't the above gardener. If trap manufacturers had bothered to print a simple advisory on the label, more folks would have been in the less-beetles half. Seems to me it wouldn't have taken rocket science to anticipate the hoards that would be drawn to precisely the wrong spot. . .the banquet table!

You see, beetles are attracted to traps from hundreds of feet away. Putting those traps in the midst of a garden is tantamount to sending out engraved invitations to the salad bar. True, many of them will eventually get caught in the trap but not before they inflict serious damage to nearby expensive or treasured landscape plants. If that's been your experience, you would be far better off buying traps for your up-wind neighbors to hang in their yards! Next year, if you feel you must purchase traps, locate them upwind and away from your garden (we found that 75' to 100' was an optimal distance for our circumstances). Just determine from which direction the prevailing wind comes and locate the trap there. A light breeze will carry the pheromone scent across your yard, beetles will detect the odor and fly out and toward the direction of their demise. Simple. . .cleverly deceptive. . .and notably effective.

Finally a predator! - A Tiny Winged Hope. Imagine, for a moment, that spring is about to burst upon the gardening scene. Adult Japanese Beetles (Popillia japonica) emerge from the ground, take wing, and begin a search for our roses, grapes, hollyhocks, maples, and Virginia creeper vines. . .not to mention mates! Since their introduction from the Orient in about 1916, these wretched little jewel-like insects have been the bane of gardeners and market farmers. The only marginally-effective controls during much of that time were hand-picking, application of BT (Bacillus thuringiensis) to lawns and mowed areas, or the spread of dangerous chemicals.

Now a new "star" has emerged onto the gardening scene: Istocheta aldrichi, a small Tachinid fly whose primary goal in life is to locate small beetles upon which to glue their eggs. It was actually introduced in 1922 specifically to parasitize -- and therefore control -- other destructive agriculturally-important insects. Their general appearance is much like common houseflies, the difference being that tachinids nearly always remain outdoors and feed chiefly on nectar, honeydew and, occasionally, pollen, deposit only eggs, and have extra bristle-like hairs on their abdomen. Common houseflies, on the other hand, have much shorter abdominal "hairs," seek out malodorous garbage, rotting carrion, smelly outhouses, and leftover food scraps upon which to deposit their live-born larvae ("maggots"). Tachinids have more or less recently developed a taste for Japanese Beetles. Here's how they get the job done:

*Tachinid flies over-winter as pupae in the long-dead shells of their beetle hosts, then emerge as adult flies during early June in the Northeast.

*Adults feed on the products of flowers (causing no damage in the process), locate a mate, then immediately begin the search for an appropriate host (mostly newly-emerged beetles) upon which to firmly glue from one to three eggs.

*Each female tachinid can deposit about 100 eggs over a fourteen day period. The deposited egg hatches within 24 hours and the resulting tiny larvae quickly burrows into the beetle's abdominal cavity where it proceeds to consume the insect .

*The now-parasitized beetle almost immediately falls to the ground and buries itself, where it perishes in a few days, usually without having laid its eggs. Compare that brief unproductive existence to the normal 28- to 40-day destructive lifespan of non-parasitized beetles.

*A few days after the host beetle dies, pupation (the next step in tachinid development) occurs, a stage that generally lasts about 1-1/2 to 2 weeks in ideal environmental and weather conditions, followed immediately by emergence of a second generation of these beneficial predator flies.

*Newly emerged adult tachinid flies repeat the sequence all over again until cooling fall weather signals the last generation of the season to remain in the ground until the following spring.

There are a great many types of tachinid flies that "specialize" in parasitizing a wide variety of insects including caterpillars like cabbage loopers and moth larvae and, of course, beetles. Field studies have consistently shown that from 40 to 50% of Northern New England Japanese Beetle populations are currently being parasitized by this very efficient little fly. So, follow these two very important new "rules":

If, while scouting your garden for Japanese Beetles, you spot one or more small, 1 mm whitish dots glued just behind their heads, don't -- repeat: don't -- destroy them. I know that's going to be difficult for most gardeners, but it's been shown that those beetles very shortly fall to the ground to be quickly destroyed by an even more aggressive foe than us. . .and those beetles will never have the chance to lay any more eggs or harm your cherished plants!

Also important is avoidance of toxic chemicals -- or even natural insecticides -- that might destroy or hinder the highly-desirable activities of just about any outdoor, nectar-feeding fly. It's all part of keeping a good thing going. . .and being a responsible gardener!

Fred Davis is a Master Gardener, Master Composter, lecturer, and long-time nurseryman. He and his wife, Linda, own and operate a popular perennial nursery in Palermo, Maine, and maintain a no-frills gardening information website at: http://www.HillGardens.com/