Viburnum leaf beetles (Pyrrhalta viburni), are a serious threat to many species of viburnum. They are voracious feeders with a long feeding season. Both the larvae and adults feed on viburnum leaves. The larvae skeletonize the leaves while the adults chew oblong holes in them. If their numbers are great enough, they can completely defoliate a viburnum shrub, causing dieback that can lead eventually to death. 

Identification and Damage

Eggs: The eggs are laid in groups of about 8, in small, excavated cavities on the underside of young branches. They appear as 1 to 2mm bumps that are brownish black, contrasting sharply with the bark colour (Cornell University rev. 2018).

Larvae: The first instar larvae are about 1 mm long, greenish yellow to off-white, and lack spots. Second- and third-instar larvae are yellowish-brown with black spots along their backs and grow up to 10 to 11 mm (3/8″) (Cornell University rev. 2018). They feed mostly in groups on the underside of leaves when young but begin appearing on tops of leaves as they grow older.

Pupae: In early to mid-June, the larvae crawl down the shrub, and pupate in the soil.

Adults: In early July, adults emerge from the soil. They are yellowish-brown in color brown and about 4.5 to 6.5 mm (1/4″) (Liesch PJ rev. 2019). The females are slightly larger than the males. They persist until the first killing frost.

Damage: Larvae skeletonize leaves while adults chew oblong holes in them. The feeding damage is very similar to Japanese beetle damage. Heavily infested shrubs can be entirely defoliated.

Life Cycle

Viburnum leaf beetle eggs overwinter in small, excavated cavities on the underside of young viburnum branches. Note: the eggs require a chilling period of approximately five months (Gyeltshen J. & Hodges A. 2006, rev. 2020 citing Weston and Diaz 2005). In spring (late April or early May) the larvae emerge from the eggs and begin feeding on the newly expanding foliage. Larvae go through three instars stages growing a little bigger each time even their colour and markings change. During this stage they can do extensive damage skeletonizing the leaves. Once they reach about 10 to 11 mm long (3/8″), they crawl down the shrub and pupate in the soil. The pupal stage lasts for about 10 days ((Gyeltshen J. & Hodges A. 2006, rev. 2020 citing Murray 2005).

The adult beetles emerge from the soil in late June to mid-July and resume feeding on viburnum foliage. At the same time the males and females’ mate and the females begin laying their eggs in small, excavated cavities on the underside of young viburnum branches, at night. The cavities are placed in a row with about 8 eggs placed in each cavity. The female then seals the hole with a “cap” made of chewed bark and excrement held together by a special sponge like cement (Cornell University rev. 2018). Egg laying continues until first killing frost. All in all, an adult female lay about 500 eggs. From egg hatch to adult takes just 8 to 10 weeks. There is one generation per year.

Native Range

“They are native to and found throughout most of Europe. In Canada, they are currently found in many areas of Ontario and the Canadian Maritime Provinces. They have recently been discovered in British Columbia. In the United States, they have been reported in parts of New York State, Maine, Pennsylvania, Vermont, and parts of Ohio” (Cornell University rev. 2018).

Control

Plant resistant varieties: Doublefile, Judd and Koreanspice viburnum offer some resistance while viburnum like arrowwood, European cranberrybush, American cranberrybush are highly susceptible (Liesch PJ, Rev: 2019). For a more complete list of susceptible and resistant species.

Prune off egg-infested twigs: during dormant season. This is probably the single most effective measure you can take to limit beetle populations. 

Hand picking adults: Knock them into a dish of warm soapy water. Like Japanese beetles they drop to the ground when disturbed so hold your bowl under them.

Tanglefoot® A sticky barrier applied to the base stems may keep some larvae from crawling to the ground to pupate.

Encourage beneficial insects: lady beetles, spined soldier bugs, assassin bugs and green lacewings all feed on viburnum leaf beetles at one stage or another.

Hold off watering during the pupae stage: The pupae need moist soil to complete their transformation into adults.

Insecticidal Sprays:

• Organic: Dormant oil spray during dormant season can be very effective. Insecticidal soap and Spinosad work well on young larvae stages, (Spinosad is currently under review in Canada and not available for home gardeners).
• Chemical: acephate, bifenthrin, carbaryl, cyfluthrin, deltamethrin, lambda-cyhalothrin, and permethrin (Liesch PJ, Rev: 2019).

Summary

Viburnum leaf beetles are a formidable foe to many species of viburnum. All life stages, except the eggs, feed voraciously over almost the entire growing season and as such they can cause extensive defoliation of these beloved shrubs. A heavy feeding 2 or 3 years in a row usually means the loss of the plant.

References:

Cornell University rev. 2018, ‘Viburnum Leaf Beetle’, Retrieved on Feb. 3, 2021 from: http://www.hort.cornell.edu/vlb/

Liesch PJ, Rev:2019, UW Insect Diagnostic Lab, Retrieved on Feb.3, 2021 from: Viburnum Leaf Beetle – Wisconsin Horticulture https://hort.extension.wisc.edu

Gyeltshen J. & Hodges A. 2006, rev. 2020 University of Florida, Featured Creatures http://entnemdept.ufl.edu/creatures/orn/beetles/viburnum_leaf_beetle.htm

The Morton Arboretum n.d., Viburnum leaf beetle, Retrieved on Feb. 4, 2021 from: https://www.mortonarb.org/trees-plants/tree-and-plant-advice/help-pests/viburnum-leaf-beetle

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