Oystershell scale Lepidosaphes ulmi
IDENTIFICATION – If you think shellfish are growing on the branches and trunks of your woody shrubs and trees, you’re not seeing things. Those hard, dull-brown, 1/8-inch-long caps are oystershell scale. Like all scale, they feed on sap, which can weaken or even kill a plant if their infestation is heavy.
LIFE CYCLE – Scale start as eggs, hatching in late May or early June. In their juvenile form, called crawlers, they’re the size of a pinhead and very mobile. Within a few hours of hatching, they spread out, settle down, insert their mouthparts into the plant and never move again. Within a week they’re covered with their oyster-shaped, waxy shells.
CONTROL – The shell resists insecticides. But in winter, when plants are dormant, you can smother scale with a solution of dormant oil. Spray the infestation with 8 oz. of oil to 1 gal. of water. For light infestations, remove the pests from the bark with a fine, plastic scrubbing pad. Be careful not to damage the bark. Treat crawlers with a mixture of 4 oz. lighter weight summer oil to 1 gal. of water. You can buy these oils at garden centers or home improvement stores. If a branch is completely covered with scale, your best bet is to remove the entire branch.