Are you able to send us photos of the suspected scale? Diagnosis is difficult otherwise and determining which exact scale species they may be will allow us to make more specific recommendations for control.

If they are too small for the camera to focus on, you can try setting a ruler next to them so it will have a more contrasting surface to use for autofocus.

Miri

Hello Mike,

Yes, this is indeed scale - particularly Cottony Camellia Scale based on its appearance and hydrangea host plant. (It has a much wider host range than just camellias.)

The simplest solution to avoid pesticide use is to trim off most or all infested growth. While drastic, an established hydrangea should rebound well, though you may not see flowers this season depending on how you trim and what variety this plant is. You can either remove foliage only (as this is where the scale are feeding) or trim entire stems down. The former may save some flower buds and the plant might leaf back out faster from dormant leaf buds.

Otherwise, a low-impact pesticide, horticultural oil, is recommended. Few beneficial insects make use of hydrangeas (pollinators will only visit those with fertile flowers like lacecap varieties, not mophead types with sterile flowers only) so their exposure would be minimal or a non-issue. For any cicadas sitting on the plant, just remove them, as it's only direct contact with the spray (being coated in it) that could be hazardous for them. When it's dry, any cicadas that crawl onto the shrub later won't be harmed.

Two windows of opportunity present themselves with oil sprays to suppress the scale. In June, the crawlers (juveniles) hatch and are quite vulnerable to oil treatments, though more than one spray will probably be needed to get any eggs that hatch late. In winter, dormant stems that may harbor overwintering older juveniles can be treated again, and the oil concentration can be slightly higher (called a "dormant rate" or "dormant oil") since there is no foliage to damage and no active pollinators to avoid. Any time oil is used, though, it must coat the plant surfaces as thoroughly as possible - especially leaf undersides - since it cannot work on insects it does not contact. This is why cutting back growth may be useful, since it will greatly reduce the number of surfaces left to treat for the summer-timed sprays. Do not make applications during temperatures above 85 degrees, and follow label directions about how to dilute (if needed) and when to re-apply.

https://extension.umd.edu/resource/cottony-camellia-scale-shrubs

https://extension.umd.edu/resource/soft-scales-trees-and-shrubs

Miri