‘Eating machine’ beetles have gardeners on high alert

    On Thursday, June 13, I noticed some damage on several leaves of a highbush cranberry along our driveway. The following Friday, June 21, I passed by the same shrub and discovered all of its leaves had been reduced to skeletons. Our yard has been invaded by viburnum leaf beetles, an imported European eating machine.
    Viburnum leaf beetles (Pyrrhalta viburni) are native to Eurasia and introduced to North America in the 1940s. They have spread throughout Canada and New England and are making their way across the northern United States. They destroy native viburnums, since both the adult beetles and their larvae feed on the plants at the same time.
    Both our highbush cranberries and our arrowwood viburnums (Viburnum opus var. americana and V. lentago) are infested. These are the favorite targets of the beetles, especially when the shrubs grow in the shade, which is where I first discovered them.
    The beetles are hard to spot since they’re only a quarter-inch long and tan. The adults, which will emerge from the soil in the next few weeks, feed on the viburnum leaves, then excavate chambers for their eggs in the new growth that appear as dark spots down the stems. A single female can lay 500 eggs.
    The larvae are yellowish-green caterpillars with black dots along their sides and black dashes across their back. They’re also tiny, about a third of an inch long after they’ve molted a couple of times. They feed on the underside of the viburnum leaves.
    Once the caterpillars hatch, they can be killed with insecticide. Putting something sticky like Tanglefoot around the bottom of the shrub stems can also stop them from migrating down the branches and entering the soil, where they pupate. Once the beetle larvae are in the soil, it’s almost impossible to kill them.
    Homeowners with any native viburnums should consider treating them this winter, either by checking for signs of beetle eggs on the new growth and pruning it out (destroy cuttings by burning or chipping) or by applying horticultural oil, an organic product used by fruit growers to smother insects that overwinter on trees as eggs. Spinosad, which is extracted from soil bacterium, is another organic remedy, although it should not be used when the shrubs are in flower since it is toxic to bees.
     Southern populations of native viburnums may survive, however, since the insect eggs need five months under 23 degrees to become viable. Right now the Wisconsin DNR says viburnum leaf beetles are active only in Milwaukee and southern Ozaukee Counties, but it no longer recommends planting highbush cranberry or arrowwood viburnum.
    We’ve sprayed existing caterpillars and cut back the damaged branches on our shrubs. Now on alert, we hope to control the infestation. This winter we’ll use horticultural spray on all our viburnums since we spray our fruit trees. And we’ll prune out what we can. A link to a good article about the beetle with photos is on the Port Garden Club website at portgardenclub.org.

 

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Wisconsin’s largest paid circulation community weekly newspaper. Serving Port Washington, Saukville, Grafton, Fredonia, Belgium, as well as Ozaukee County government. Locally owned and printed in Port Washington, Wisconsin.

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