Just when you thought the summer garden was looking pretty good, along comes a spoiler to ravage the roses and attack the foliage of other favored plants, leaving it in lacy tatters. Japanese beetles -- they show up in midsummer like a dinner bell rang.
These shiny, iridescent beetles are easy to spot since they gather in appalling numbers to help themselves to a meal at your expense and lose themselves in sexual orgies aimed at increasing their numbers. It’s party time for one of the top offenders among garden pests.
Since showing up on the East Coast of America in 1916, these beetles are now all too common from Maine to Alabama and west to the Mississippi. First identified in New Jersey on a shipment of iris roots (another Garden State “first!”), the Japanese beetle is making its way to the West Coast at the rate of five to ten miles a year. It’s a slow-motion invasion and the bugs are winning.
Individual beetles can fly as far as two miles in search of something tasty, so even if you are taking defensive measures there’s no guarantee they won’t drop in for a bite. JBs attack more than 300 plant species and those with strong fragrance in direct sunlight are preferred -- which is why your roses are high on the hit list.
The first beetles emerge in early July and after seeking out a favored food plant release a “come and get it” scent known more formally as a congregation pheromone to summon a few hundred dining companions. Newly emerging females wear a separate sexy perfume to draw lusty males to their side for a little hanky-panky in the heart of your hapless flowers.
These well-known scent lures have been exploited by manufacturers of pheromone-baited traps on the premise that you can draw the beetles to their doom. Hah! It’s a losing proposition since these traps will attract MORE beetles to your property than plants alone. You don’t want them in the garden near vulnerable species. In fact, the best (if somewhat underhanded) strategy is to give a few to your neighbors.
The beetles’ incessant rounds of feeding and mating continue through the summer with females descending to the soil every day or two to lay eggs – up to 60 each by mid-August. The adult beetle, living only 30 to 45 days, is the most visible but more ephemeral incarnation of this pest. For most of their life cycle, JBs take the form of fat, shiny white grubs that feast on your turf, damaging American lawns at the rate of $235 million a year.
Most control measures are directed at the grubs and the best time to do something about them is approaching in August and September, when the ugly things are actively feeding near the surface. Two countermeasures are milky spore, a bacterial agent, and parasitic nematodes, a tiny worm.
These environmentally correct weapons aren’t as effective as one would hope, alas. Milky spore needs to build up to critical mass over two to three years and is most strongly recommended for new lawns established for fewer than eight years. (Older lawns will already have populations of organisms that attack JBs.) Researchers at the University of Delaware and elsewhere have found that nematodes are only marginally effective in targeting the beetle plague.
Chemical grub poisons are available as well, but use of these must be weighed against the potential harm to beneficial soil fauna, not to mention pets, children and wildlife. If you are going to wage chemical warfare, it’s a good idea to check in with your local office of the Rutgers Cooperative Extension Center for recommended products and dosages.
You can hope that natural enemies including crows, starlings, catbirds, robins and grackles will frequent your place and dine liberally on the beetles. Hope as well for the appearance of another biological control released in the Northeast in the 1920s.
The spring tiphia wasp burrows down to JB grubs and attaches an egg to their abdomens. When the wasp egg hatches in about a week, it pierces the grub and eats it from the inside out, leaving nothing behind but the empty shell of the its head. Yes, it’s a grisly fate but frankly, I don’t wish the JBs well.
Handpicking Japanese beetles is a sport without end for the tireless – drop them in a bucket of soapy water. Beetles also can be done in by any number of sprays but care must be taken with these, especially to avoid harming pollinators like the threatened honey bee.
You’ll have fewer beetles if you avoid growing things they love, like roses, hollyhocks, fruit trees and grapes, but that’s a fallback position many won’t accept. I often just cut off badly affected rose blossoms and wait for the next flowering cycle in late August and September when the adult beetle have died. There’s no completely good way to go if you have a JB problem. Like it or not (I don’t), they’re here to stay.