**Oyster Shell Scale** (*Lepidosaphes ulmi*) is a severe, armored scale insect pest with an extremely wide host range, attacking deciduous trees and shrubs, including lilac, apple, dogwood, maple, and many others. The conflict is severe plant stress, branch dieback, and death: the scale feeds by **sucking plant sap** from the twigs and branches, and in high numbers, they form thick layers that completely **girdle** and kill young wood, leading to the gradual decline and eventual death of the host plant.
Taxonomy and Classification
Oyster Shell Scale belongs to the order Hemiptera (True Bugs). They undergo simple metamorphosis. They are an “armored scale,” meaning the protective wax covering is separate from the insect body, making chemical penetration extremely difficult. They are highly successful pests due to their strong protection and parthenogenetic reproduction in many populations.
Physical Description
The protective scale covering is minute, about 1/8 inch long.
- **Appearance (Key ID):** The scale cover resembles a miniature, slightly curved **oyster shell** or comma, dark brown or grayish-brown in color, firmly attached to the bark.
- **Crawl Stage (Key ID):** The only mobile stage is the first-instar nymph (the “crawler”), which is microscopic, yellow-orange, and active for only a few days, typically in late spring or early summer.
- **Damage Sign (Key ID):**
- **Bark Crust:** Twigs and branches appear to be covered in a crusty, bumpy layer of gray-brown scales.
- **Dieback:** Tips of branches dying back to the main stem; overall yellowing and thinning of the canopy.
- **Conflict:** Girdling, dieback, and plant death.
Distribution and Habitat
Oyster Shell Scale is cosmopolitan. Its habitat is the bark of the twigs, branches, and occasionally the trunks of its numerous deciduous host plants in landscapes and nurseries.
Behavior and Conflict
The conflict is dominated by chemical protection and persistent stress.
- **Armored Defense:** The waxy shell protects the feeding insect from chemical sprays, heat, and desiccation.
- **Girdling:** The scales gather in such dense numbers that they effectively block the flow of nutrients and water in the phloem and xylem, leading to the death of the wood past the point of infestation.
- **Chemical Timing:** Control is difficult because the effective window for chemical treatment (the crawler stage) is very short and hard to predict accurately.
Management and Prevention
Control is integrated pest management (IPM), targeting the crawler stage and using horticultural oils.
- **Dormant Oil:** Apply a heavy-weight horticultural oil during the late winter/early spring dormant season to smother overwintering adults and eggs. Coverage must be complete.
- **Summer Oil:** Apply a lighter-weight oil when crawlers are active, as this smothers the mobile stage.
- **Crawler Spray:** Apply a registered insecticide when the crawlers are active (usually in late spring/early summer), which is determined by temperature monitoring (Growing Degree Days – GDD).
- Prune out and destroy (do not chip) heavily infested branches.
Conservation and Research
Oyster Shell Scale is managed as a serious landscape and nursery pest. Research focuses on improving monitoring techniques for the elusive crawler stage, understanding genetic variations in the scale, and enhancing the efficacy of biological controls (like specialized parasitic wasps).