Vacillating About Viburnums

Pam MacDonald – Halton Master Gardener

Native Viburnum species are beautiful and popular choices for gardeners who wish to attract wildlife to their gardens. Cardinals, Eastern Bluebirds, Robins and Cedar Waxwings are some of the birds that will visit Viburnums to enjoy the berries from late summer into fall. Viburnums also act as a host plant to a variety of lepidoptera species including: Spring Azure Butterfly, Holly Blue Butterfly (Celastrina argiolus). Sami cynthia (a large silkmoth), Daraspa choerlius (a sphinx moth), and Hemaris thisbe (the Hummingbird Clearwing moth) . Insectivore birds feed on these larvae, and many more birds harvest the caterpillars of the lepidoptera as food for their young. Finally, the nectar produced by the beautiful flowers is enjoyed by both bees and butterflies.

So why vacillate about growing viburnums? Unfortunately, several species both native and exotic, are susceptible to damage from the Viburnum Leaf Beetle (VLB).

History

This Eurasian pest entered North America in 1947. It was first detected in the Niagara Peninsula and the first-established breeding populations were recorded in 1978 in the Ottawa-Hull area. Since then, VLB has spread across North America. Its expansion southward seems to be limited by a requirement for the eggs to experience an extended period of cold to advance to the first instar (stage of development). The VLB was detected in Michigan and Wisconsin in 2019 and is being monitored by extension departments at universities in both states.

Read More. Michigan State University Extension’s article What’s eating my viburnum and how can I stop it? provides a good summary and a list of less susceptible species. Note that the pesticide recommendations do not apply to Ontario.
Cornell University took an early lead on the study, tracking, and development of control strategies for VLB. Visit the Cornell site for videos and a wealth of information.

Impacts to the host

The Viburnum Leaf Beetle feeds exclusively on this genus of plants. An infestation can be devastating because both the larvae and adult beetles feed on the leaves. The larvae defoliate the plant at the beginning of the season, then the adult beetles defoliate the plant during the second half of the growing season. Plants under this double attack cannot recover in the growing season. Consecutive years of heavy defoliation will result in dieback and eventual death of the host plant.

Life cycle

Detection and control

Eggs are laid in fall usually on the current year’s growth. Look for egg sites in late fall or winter when it is easier to examine stems and remove twigs on which eggs have been deposited.

“The adult female lays up to 500 eggs on viburnum twigs and small branches by excavating deep, rounded pinhead sized egg cavities in a straight row on the under surface of the terminal twigs. After filling the egg cavity with five to eight eggs, the female closes the opening with a lid or “cap” made of excrement and chewed bark held together by a mucous secretion that hardens upon exposure. The cap not only protects the egg from predation, but also absorbs water to maintain the humidity.” (Featured Creatures. U. of Florida Entomology and Nematology Dept.)

In spring, examine underside of leaves for the first instar of yellowish larvae. They are tiny so a hand lens may be needed to see them. The second and third instar are darker in colour and have black spots. Feeding activity moves to the top of the leaf during the later instar stages. When it’s time to pupate, larvae climb down the stems rather than dropping to the ground

CAUTIONARY NOTE: As a last resort, some might consider using a product that creates a sticky barrier (Tanglefoot for example) around lower stems to intercept the adults and reduce numbers. However there are reports that other wildlife, including beneficial insects and pollinators, birds, and small mammals, can be inadvertently trapped. Read more about the concerns regarding sticky traps HERE before considering taking this step.

At maturity, female beetles are larger than males. Adults will drop to the ground or fly away when disturbed so the time to interrupt the cycle is at the egg or larval stage.

Biological Controls

Cornell University recommends control strategies that include biological agents (see “Read More” above). Encourage beneficial predatory insects by planting a wide diversity of plant species to create insect habitat near the viburnums as well as avoiding using any pesticides.

Here are some of the beneficial insects that prey on Viburnum Leaf Beetles.

OMAFRA assessment

The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) provides a more recent and local assessment of the impact of VLB in Ontario. The following is taken from the OMAFRA webpage last updated in February 2021. .

	Exotic Species	Native Species
Preferred Host	Viburnum opulus, European highbush cranberry and its selections
Moderate Damage	V. lantana, wayfaring tree viburnum	V. rafinesquianum, downy arrowwood
Slight Injury		V. dentatum, arrowwood viburnum V. trilobum, American highbush cranberry viburnum
Injury from adults under laboratory conditions		V. acerifolium, maple leaf viburnum V. lentago, nannyberry viburnum

OMAFRA http://www.omafra.gov.on.ca/english/crops/facts/vlb

Should I choose a viburnum?

Are native viburnums a reasonable choice today for gardeners wanting to support wildlife in their gardens? In my opinion, the answer is YES most definitely, though vigilance and a willingness to accept some damage is required. Here is a statement in 2016 from Brian Eshenaur, Senior Extension Associate, New York State Integrated Pest Management Program, who has been involved with the Cornell project on VLB since 1999:

“The viburnum leaf beetle hit us hard in the Rochester [New York] area [around 2000]. During those first few years in which the beetle population peaked, most of the susceptible native species like arrowwood, that were growing in wooded areas, were killed. Some landscape plants succumbed to the defoliation then too. At that time I would not have recommended planting a susceptible species like the Cranberry bush viburnum. Now however the populations of the beetles are down significantly and it is safe for us to plant species again like cranberry bush and arrowwood viburnums. They’ll get a little bit of damage but nothing lethal. Why did the populations go down? It seems with all the very susceptible native plants that were around, [they] initially allowed the populations to reach unnaturally high levels and the beetles moved into landscapes annually. With those food sources gone, the populations declined. Also, and maybe more importantly, predator insects, and nematodes that affect the larva in the soil have built up and found the Viburnum leaf beetle as a food source!
This insect went through southern Canada around 1985 and they have experienced a similar situation. Now it’s okay to plant susceptible species again, just expect a little leaf feeding.”
Brian Eshenaur ManagingViburnum Leaf Beetles

If you decide to plant some native viburnum in your garden, I do believe there will be grateful mother birds who will repay you with their songs. You may also enjoy watching the video below from Kim Eierman (environmental horticulturist and ecological landscape designer) who interviews Dr. Doug Tallamy (author of Bringing Nature Home and Homegrown National Park) on his understanding of the benefits of planting Viburnum species in your garden.