This column has talked about how different plants, insects and diseases have developed based on our unseasonal temperatures and rainfall. Last year, I reported on the Viburnum Leaf Beetle larvae feeding towards the end of June. Our accumulation of growing-degree days being so far ahead, the larvae have already been feeding for more than 10 days. Throughout Cook and DuPage counties, homeowners have discovered unfamiliar foliage feeding on their viburnums in the landscape. Surrounding counties are not immune; we just have not found them yet. The Viburnum Leaf Beetle has established itself in the northeast and began to show up in Illinois in the last two to three years. The beetle came to us from Europe.
The adult Viburnum Leaf Beetle feeds on foliage leaving irregular shaped holes, while the larvae stage feed much more heavily as they develop into larger versions before they pupate. They leave the leaf nearly skeletonized. The female beetle does additional damage by chewing into the current season's twig or branch growth to deposit the eggs, which will hatch in 2018. The egg laying activity, in a general way, will resemble the damage done by the female cicada. The adults are not winter hardy and will perish at our first killing frost of the fall. Reports are that heavy feeding and subsequent egg laying can cause plant death if left untreated in as little as 2 to 3 years.
The Viburnum Leaf Beetle favors those viburnums that have a smooth, shiny leaf. In our area, Viburnum dentatum (Arrowwood Viburnum and all of the cultivars associated with the species) seems to be the favorite viburnum of the beetle so far. Those viburnums that have "hairy" leaves do not seem to be attractive to the beetle. Other smooth-leaved viburnums common to our area include American and European Cranberry Bush. Next in line for the beetle is Viburnum lantana (Wayfaringtree viburnum), which also is frequently planted. Down the list further for being moderately susceptible is Viburnum lentago (Nannyberry viburnum).
If the larvae went untreated, the best management practice is to look for and remove any twigs with evidence of the female egg laying activity. This will drop the population of hatching larvae down significantly in 2018. The next best practice would be to treat the newly emerging larvae as soon as possible. Like all insects, controlling them when they are young is much easier, plus less feeding damage will be done to the plants too! Controlling the larvae now, even if lots of feeding has been done, will lesson damage in 2018.