Scale insects. Rub them off.

Have you noticed movement? I was thinking that it could a form of scale
(cottony maple leaf scale) that is treatable using Bayer with an active
ingredient called Imidicloprid or insecticidal soaps or insecticidal oils (neem oil).

This pest usually surfaces in late May every year. The cotton acts as a sac where eggs will hatch in the summer. Many of these products will work best when they are applied after the eggs hatch.

You may want to apply those things next year too but start spraying ahead of time. Say 2-4 weeks before you first noticed the problem.... they like to return back home.

Crushing the adults now would interrupt the annual cycle. There aren't that many present, a utility brush should make short work of them. A lot quicker than all the rigamarole involved with mixing and applying an insecticide.

Imidicloprid is a systemic that will stay in the plant tissues and kill pollinators that visit the flowers. I would try one of the other options first.

im wondering based on the history provided.. if they turn to dust .. whether this was an issue a year or two back.. and all she is seeing is the mummified remains of an old problem ...

unless you can find active scale.. whats to treat???

hard to tell from the pix.. but the plants might take to a hard rejuvenation pruning .... but that would be a topic for the shrub forum ...

ken

ken, this group of scale insects only bears the white egg sacs when very much alive, active, and bursting with viable eggs.

Gail, since you have forty large plants, you may find it necessary to resort to a systemic pesticide to get this bad infestation under control. You can do this with little impact to foraging pollinators if you apply after the flowers are spent.

But aren't the flowers of Hydrangea macrophylla sterile and completely devoid of anything of interest to bees and butterflies? I just don't remember. But if so, that would make the pesticide issue moot.

There are always some fertile flowers on Hydrangea macrophylla, even the mopheads. They are just not very visible or showy and tend to be obscured by the sterile florets.

Thanks, gg.