What’s Eating My Viburnum?

If the leaves of your viburnums look like they’ve gone through a leaf shreader, you can be sure they’re under attack from the viburnum leaf beetle (Pyrrhalta viburni), This small, elongated beetle is native to Europe and Asia. It was accidentally imported into North America in the 1940s, but only began to spread widely in the 1970s. It’s now common throughout Northeastern North America and is now spreading on the West Coast as well.

Viburnums are abundantly found in the wild, but are also popular garden shrubs, grown for their clusters of (mostly) white spring flowers, often beautifully textured leaves and colorful fall berries. There are some 150 species, mostly from the temperature regions of the Northern Hemisphere, with a few species straggling down into Africa and South America at higher latitudes. They belong to the brand-new Adoxaceae family, recently separated from the Caprifoliaceae (the honeysuckle family).

Recognizing the Damage

If your viburnum is suffering from viburnum leaf beetle, you’ll know it: it skeletonizes leaves, leaving only the veins. Both adults and larvae feed on viburnum leaves, ensuring a long season of continuing damage.

The beetle’s primary host is the guelder rose or European highbush cranberry, Viburnum opulus, especially the highly popular snowball bush (Viburnum opulus ‘Roseum’, also sold under the name V. opulus ‘Sterilis’), with its rounded balls of flowers. However, it will also attack other viburnums, including American highbush cranberry (V. trilobata, syn. V. opulus americanum) and arrowwood viburnum (V. dentatum), two native North American shrubs. If a shrub is attacked severely every year for 2 or 3 years, it may die.

Life Cycle

The larvae hatch around May from eggs that overwintered on twigs of the host plant and immediately begin to eat young viburnum leaves. They look like small yellow to pale green caterpillars covered with black dots, measuring only 10-11 mm long. In early to mid-June, they drop to the ground at the base of the shrub and begin pupating just below the soil.

Towards the end of June or in July, the adults emerge, climb back onto the shrub and start eating the leaves in their turn. They are gray-brown elongated beetles, 4.5 to 6.5 mm long, and recall a lily leaf beetle in shape. They keep feeding well into autumn, until the first killing frost.

Throughout the summer, females lay eggs in single file on stem tips, mostly on lower branches, covering them with a little dome of chewed bark and excrement. A single female can lay up to 500 eggs per season.

Conventional Treatments

If you really want to stop a viburnum beetle infestation, you really have to be proactive. When the shrub is leafless, therefore between late fall and early spring, take a pair of pruning shears and inspect all the stem tips, removing and destroying those that bear the characteristic straight row of brown bumps, very literally nipping the infestation in the bud.

In early spring, you can also try a dormant oil treatment, but selective pruning is more effective.

If you missed these opportunities and you find yourself dealing with a shrub with skeletonized leaves (the most common situation), there are other treatments you can try.

Spraying with an insecticide containing insecticidal soap and pyrethrin will help, but it works best at the beginning of the season, when larvae are small. Adults are harder to reach by spraying, as they often drop to the ground when humans approach, making them harder to reach. You’ll have best luck if you spray early in the morning while they aren’t yet very alert.

Alternatively, try shaking infested branches over a soapy bucket of water. This is also best done early in the morning.

The Laidback Gardener Treatment

That’s all fine and good, but can you really see yourself clipping away at branches each fall or spraying again and again each summer, because you’ll find that even if you do successfully control them one year, new beetles will soon fly in and start a new infestation. If the infestation is becoming intolerable, be pragmatic and just yank out and destroy the shrub. Then plant something that is insect resistant.

Of course, viburnum beetles only cause minor to moderate damage on some viburnums (see Moderately Susceptible below). This is one of those cases where I suggest applying the 15 pace rule. If you can’t see the problem at 15 paces, it’s not worth treating. If it remains very visible at 15 paces… well, I suggest “esthetic surgery”.

Here are two examples of tolerable levels of infestation.  I’ve been growing an Onondaga viburnum (V. sargentii ‘Onondaga’) for years and, under my conditions at least, the damage is barely noticeable. Furthermore, the infestation removes minor year after year. I never therefore felt the need to treat the problem. As for my variegated lantana viburnum (V. lantana ‘Variegatum’, although listed as moderately susceptibly by experts, it it has viburnum beetles, the damage must be really minor, as I’ve never noticed it. So I just let it grow

Remember though that these same shrubs might have a much more serious problem under your conditions. Whether to tolerate or to yank is therefore up to you!

Susceptibility of Different Species to Viburnum Beetles

Most Susceptible

1. V. opulus (guelder rose, including snowball bush) zone 3
2. V. dentatum (arrowwood viburnum) zone 2
3. V. trilobum (American highbush cranberry) zone 2

Moderately Susceptible

1. V. acerifolium (mapleleaf viburnum) zone 3
2. V. x carlcephalum (fragrant snowball) zone 5b
3. V. lantana (wayfaring tree) zone 2b
4. V. lentago (nannyberry) zone 2
5. V. rafinesquianum (Rafinesque viburnum) zone 2
6. V. prunifolium (blackhaw viburnum) zone 4
7. V. sargentii (Sargent viburnum) zone 3

Resistant

1. V. x burkwoodii (Burkwood viburnum) zone 5b
2. V. carlesii (Koreanspice viburnum) zone 5b
3. V. x juddii (Judd viburnum) zone 5
4. V. lantanoides, syn. V. alnifolium (hobblebush) zone 2b
5. V. plicatum tomentosum (doublefile viburnum) zone 5b
6. V. x rhytidophlloides ‘Alleghany’ (lantanaphyllum viburnum) zone 4a
7. V. rhytidophyllum (leatherleaf viburnum ) zone 6b
8. V. sieboldii (Siebold viburnum) zone 5