Viburnum leaf beetle
Pyrrhalta viburni
An invasive beetle that skeletonizes viburnums twice a year — larvae in spring, adults in summer — and kills an established shrub in two or three seasons.
How to know it
The larva is a small yellow-green grub with rows of black spots, feeding in groups on the underside of the leaf in spring. The adult is a plain brown beetle 5 mm long, active from midsummer. The most useful identification is the egg site: neat rows of small brown bumps, capped with chewed bark, along the underside of the current year twigs.
What you'll see
- Leaves skeletonized to a lace of veins in late spring, worst at the bottom of the shrub.
- A second round of ragged holes chewed in the leaves from midsummer, by the adults.
- Rows of small raised brown caps along the underside of young twigs, visible all winter — the egg sites.
- A shrub defoliated twice in one year, refoliating weakly and going into winter exhausted.
- Complete death of an established viburnum after two or three seasons of this.
Where it comes from
One generation a year, in two damaging halves. Eggs overwinter in pits chewed into the underside of the twigs and hatch in spring; the larvae feed in groups for about a month, drop to the soil to pupate, and the adults emerge in July and feed until the frosts, laying as they go. The twig egg sites are the vulnerable point in the whole cycle.
Established across the northeast, the Great Lakes and the Pacific northwest and spreading. Arrowwood, European cranberrybush and other smooth-leaved viburnums are hit hardest; hairy-leaved and leathery-leaved species are markedly resistant.
Dealing with it
- Prune out and burn the egg-laid twigs between October and April. This is by far the most effective control and it can be done at leisure with the leaves off.
- Look for the rows of brown caps on the youngest wood and cut those twigs back to clean growth.
- Spray insecticidal soap on the larvae in spring while they are small and clustered on the leaf undersides.
- Encourage ladybirds and lacewings, which take the larvae in numbers.
- Water and mulch a defoliated shrub so it can afford to refoliate.
Keeping it out
- Inspect twigs every winter and remove the egg sites — a defoliation prevented is a shrub saved.
- Plant resistant viburnums: Koreanspice, doublefile, leatherleaf, burkwood and Judd viburnums are all largely passed over.
- Avoid arrowwood and European cranberrybush where the beetle is established.
- Buy clean nursery stock and check the twigs before planting.
- Keep the shrub vigorous, though a healthy plant is only slightly better able to survive this than a stressed one.
What it goes for
Viburnums only, with arrowwood, American and European cranberrybush and possumhaw the most susceptible.
Who hunts it for you
- Green lacewingChrysoperla carnea Its bristly larvae — "aphid lions" — impale aphids, mealybugs, thrips, and mite colonies, eating hundreds a week. Adults feed on nectar and pollen from small open flowers and overwinter in hollow stems left standing. Grow to recruit it: Cilantro,Cosmos,Dill,Sunflower,Yarrow
- Ladybird (lady beetle)Coccinellidae The gardener’s favorite ally: adults and their alligator-shaped larvae each eat thousands of aphids, plus scale, mealybugs, and other soft-bodied sap-suckers. Grow flat, open flowers for the adults and never spray a colony that ladybird larvae are already working. Grow to recruit it: Calendula,Cilantro,Cosmos,Dill,Fennel,Yarrow
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com