Check for Viburnum Leaf Beetle Feeding

Be on the lookout for the viburnum leaf beetle (Pyrrhalta viburni). You may not have seen it, but if you have an arrowwood or cranberrybush viburnum (Viburnum dentatum or V. trilobum), odds are that you have seen the damage. (Koreanspice viburnum, V. carlesii, is not affected.)

The Culprit

Viburnum leaf beetle was first reported in the Chicagoland area in 2013. Within two years, it was regularly reported to the Morton Arboretum. There is only one generation each year, but it can kill a healthy viburnum in as little as two to three years. You will likely see the leaf damage before noticing the insects however, generally beginning in May. The leaves will take on a skeletonized appearance.

The adults, somewhat nondescript greenish-brown beetles, lay eggs in the fall at the ends of viburnum twigs. They chew a cavity into the stem, lay three to eight eggs, then cover the cavity with the chewed-up wood. A female can lay up to 500 eggs in a season. The larvae are tiny upon hatching, perhaps an eighth of an inch long and are pale green to pale yellow with black dots and lines along the sides and back.

Upon pupation, which occurs in mid-June in zone 5, they will crawl down to the ground and in about ten days emerge as adults. They will continue to feed from mid-summer to frost, resulting in major damage to your plants.

The Treatment

It is critical to start looking for feeding damage in spring. Check the leaves carefully – you may not notice early damage from a distance. Treatments are most successful when started early as the larvae are more susceptible than adults. Look carefully on all sides of the plant as they will often hide under the leaves.

As they say, an ounce of prevention is worth a pound of cure. Planting resistant species, such as Koreanspice viburnum, leatherleaf viburnum, and doublefile viburnum will save you the headache of worrying about treatments as they generally survive even if attacked. Native viburnums like blackhaw and nannyberry are only moderately susceptible and are also good choices. While they may suffer some damage, they are rarely destroyed by beetles.

Arrowwood viburnum and cranberrybush are highly susceptible to VLB and should not be planted.

If damage is noted, remove the leaves. Bag them and put them in the trash. No composting! After the leaves have fallen, it should be easy to see the infested twig tips. These can also be removed from October until April.

Finally, for severe infestations, a product containing Spinosad can be used. Spinosad is an insecticide derived from bacteria found in soil. Spinosad is effective against many common garden pests, even those that are traditionally very hard to kill, while not affecting many of the more beneficial garden bugs. (It is toxic to bees while wet, however, so application is best made in the late evening or very early in the morning.) Of course, always follow label instructions for use and use proper protection when using any chemicals in the garden.

Viburnums are a wonderful addition to any landscape with clusters of flowers ranging from pink to white and with lovely fall foliage. With just a little bit of care and vigilance, they will flourish! Learn more about insects, both beneficial and bad, at extension.illinois.edu/insects. Also, check out the University of Illinois Extension Horticulture YouTube Channel for videos on other horticulture topics.

Donna Mack is a University of Illinois Extension Master Gardener serving Kane County.

Have a question for the Master Gardeners? Residents can contact the Kendall County Master Gardener volunteers on Monday, Wednesday, and Friday from 9 a.m. to 12:30 p.m. by calling 630-553-5823, stopping in at 7775B IL Route 47, Yorkville, or emailing uiemg-kendall@illinois.edu. For helpful hints on what to include in your email, please visit go.illinois.edu/HelpDeskMGdkk.