Japanese beetlemania strikes

  • Article by: DEB BROWN , Contributing Writer
  • Updated: August 31, 2010 - 4:29 PM

Our warm summer seems to have encouraged Japanese beetles, a garden scourge that doesn't really need a whole lot of help.

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Japanese beetles like roses.

Photo: Kevin G. Gilbert, Associated Press

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We've seen Japanese beetles in the Twin Cities area for several decades, but in the past five to six years, their numbers have risen, and their range has expanded. This year, they appeared earlier than usual -- and they're still going strong.

I've fought them for months, but now there's no question who's winning the battle. (Hint: There's one of me, but there seems to be an endless supply of them.)

Japanese beetles are with us year-round, but we see them only when they're feeding in summer. The rest of the year they spend underground. Females lay eggs in the ground, where the eggs overwinter. In spring, the eggs hatch into burrowing white grubs that eat grass roots before they emerge as hungry adult beetles in early July (well, June, this year).

As beetles go, they're fairly attractive: about 3/8 inch long, boxy-shaped, with dark green heads and metallic bronze wings. They're also fairly easy to distinguish. They have little tufts of white hair that run along each side of their body.

Despite their relative good looks, I've come to detest Japanese beetles because they feed both on flowers and foliage, leaving lacy skeletons of leaves and shredded petals in their wake. Unfortunately, roses are among their favorite foods, although they'll attack grape vines as well as many flowering annuals and perennials.

It's easy to spot Japanese beetles. You often can find them mating on leaves, an activity that sends out pheromones, which attracts more beetles to the same plant. In my yard, they congregate on two rosebushes in the front, but have yet to attack another bush 25 feet away.

The good fight

I've been battling the beetles by examining my roses several times a day, then knocking the beetles into a jar of soapy water. They must sense that this is not a happy ending because they either scatter or cling tenaciously to the leaf or petal on which they're feeding. Even so, I usually manage to "harvest" a dozen or more of the little critters each time I check. The problem is, the beetles just keep on coming, no matter how vigilant I am.

Several insecticides are labeled for use on Japanese beetles, but they're not considered very effective, partly because the beetles are extremely mobile. Even if you kill some by spraying them, more will soon fly in to replace them. In addition, the insecticides aren't long-lasting, so they must be reapplied often, particularly after heavy rainfall. Worse, spraying can discourage or kill bees and other beneficial garden insects.

If you want to try a low-impact insecticide, Jeff Hahn, an entomologist with the University of Minnesota Extension, suggests a product containing Neem.

To read more about Japanese beetles, go to www.extension.umn.edu/distribution/horticulture/dg7664.html.

Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota. To ask her a gardening question, call 612-673-7793 and leave a message. She will answer questions in this column only.

WHAT BEETLES WANT

Japanese beetles will feed on hundreds of different plants.

THEIR FAVORITES INCLUDE:

roses

hollyhocks

dahlias

hibiscus

evening primrose

grapes

raspberries

THEY GENERALLY IGNORE:

ageratum

columbine

begonias

coreopsis

coral bells

hosta

impatiens

pansies

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