Oystershell scale is an armored scale insect whose host range spans over 130 plant species. In Iowa, they are most frequently reported on dogwood and maple. In 2025, numerous reports have been submitted on both host plants.
Oystershell scale has a bizarre life cycle. Adult females create a hardened cover and permanently attach themselves to stems, twigs, and other plant tissues. Under these hardened covers, eggs are deposited in the fall where they are protected from the elements, and hatch in the following spring once temperatures warm. Eggs hatch into a white to tan crawler stage that is mobile. These crawlers find a new place on the plant to feed until development is complete. Females will create a hardened covering under which her life will expire after depositing more eggs.
Oystershell scale can cause branch/twig decline or death, and in severe cases (particularly younger trees), death of the entire plant can occur. Due to how well these hardened scale coverings attach to plant tissue, oystershell scale often appears much more severe on plants than they truly are (i.e., many of the visible scale are probably dead, and potentially multiple years old). Thus, it's important to scout for the active crawler stage to determine the severity of the population. Scouting for crawlers should happen in late May to early June by looking at branches for the immature crawler stage. They are tiny, so you can aid in scouting by tapping branches onto a sheet of paper or wrapping some branches/twigs in double sided sticky tape, checking it frequently. Some oystershell scale have two generations per year, so scouting again in July or August may also be beneficial.
If few crawlers are found, it's possible that the infestation looks more severe than it actually is. If many crawlers are seen, it can be a justification to proceed with management, particularly if the tree is experiencing decline unattributed to other factors.
For all population levels, you can consider:
- Pruning out heavily-infested branches if it is otherwise safe for the tree
- Scrubbing off visible scale with a scrubber sponge (preferably before egg hatch in the spring)
For higher populations, you can additionally consider:
- Applying horticultural oils before bud break
- Contact insecticides such as permethrin, only when crawlers are active
- Systemic insecticides in the most severe cases. Only Dinotefuran is effective against oystershell scale.
In all cases, protecting natural enemies by limiting insecticide use, and maintaining tree vigor can assist in managing oystershell scale.
If you need a confirmation on the identification of oystershell scale or other insect pests in your home garden or landscape, contact the Plant and Insect Diagnostic Clinic.
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