What are the tiny gray shells crusting my lilac?

Probably oystershell scale, shaped like miniature oyster shells.

When crawlers hatch, typically late May to early June in Minnesota.

Oystershell Scale: Tiny Shell-Shaped Scales Encrusting Lilac and Apple

They're purplish-gray, about 1/8 inch long and shaped like tiny oyster shells.^[1]
As armored scales, they match the bark color and produce no honeydew.^[1]

Look-alike	How to tell them apart
Other scale insects	Scale species differ in timing, so identify the species to know when crawlers are active.^[1]

Hosts include lilac, apple, poplar, birch, ash, cotoneaster, willow and many other deciduous trees and shrubs.^[1]

US map: public records from 19 states since 1990 (GBIF).

Eggs overwinter under the dead mother's scale, and crawlers hatch in late May to early June.^[1]

New eggs are laid in late summer to early fall; one generation a year.^[1]

Infestations crack bark and leave leaves yellow and stunted; heavy ones can kill plants or invite borers.^[1]

Scales feed only on plants; Minnesota Extension mentions no risk to people.^[1]

Check for crawlers by tapping branches over white paper or wrapping double-sided tape around twigs.^[1]

Spray insecticidal soap or horticultural oil when crawlers are active in late May and June.^[1]

Dormant oil doesn't work on species that overwinter as eggs, like this one.^[1]

What are the tiny gray shells crusting my lilac?: Probably oystershell scale, shaped like miniature oyster shells.^[1]
When should I spray?: When crawlers hatch, typically late May to early June in Minnesota.^[1]

University of Minnesota Extension, “Scale insects (Yard and garden insects).” extension.umn.edu/garden-and-home/yard-and-garden/yard-and-garden-insects/scale-insects (accessed 2026-10-06)
iNaturalist, “Lepidosaphes ulmi taxon page (order and family).” www.inaturalist.org/taxa/391478 (accessed 2026-10-06)

Updated 2026-10-14. Built from sourced records. Spot an error? Tell us.