We agree that this looks like a very heavy population of Euonymus Scale, which has probably been building for several years, even if they weren't causing overt symptoms. Rather than older generations (though that is also possible, to some extent), the white rice-shaped covers are male scale and the brown covers that are more oystershell-shaped are females, both of that species. You are correct that drastic pruning is one option, and that sprays in a plant this dense (and with a population this high) are unlikely to be effective enough to bother with. Pruning or, if you're open to it, plant removal and replacement, would be our recommended course of action.

While treatment with certain insecticides might work eventually, it will take time (potentially over a year) to get their numbers under control, and may expose other insects to pesticide in the process. Plus, it won't impact the aesthetic issue of having the bare interiors of the shrubs being more visible, which is largely a consequence of pruning. (Pruning encourages a denser outer layer of foliage, which further shades the interior leaves, causing them to drop faster than they would otherwise naturally be shedding anyway due to age.) By drastically pruning the plants close to ground level for the sake of scale suppression, the "reset" will also help improve the shrub's density as they regrow, at least for a time. Euonymus of this type (probably variety 'Manhattan', given its abundance in landscapes around our region) are often trimmed where they don't fit, aside from the aesthetic choice of having a more manicured look from pruning, and this can mean the problem may repeat itself if they can't reach close to their full 8-10-foot mature height and width without getting in the way of a sidewalk, driveway, blocking a view, etc. Your description of a 15- to 20-foot width per shrub is surprising, as they don't often get that wide, even if unpruned.

Euonymus is very over-planted, though to be fair, so are a handful of other commonly-grown evergreens used as screens or hedges, like cherrylaurel and certain hollies. While every plant can potentially develop several pest or disease issues (and several types of scale can attack various other shrubs too), a mixed-species planting will at least reduce the risk of a future outbreak of a given pest or disease affecting the entire planting equally severely. Granted, it may look a bit less uniform, but it won't necessarily be wild and unruly, either, if you prefer a manicured look. We can offer some ideas if you prefer to do a replacement rather than a trimming.

If you do trim, you should be able to do so now; avoid any time in autumn, as regrowth may not have time to harden-off in preparation for tolerating winter freezes. Otherwise, pruning would typically be done in mid-spring or so. While a few beneficial insects like parasitoid wasps and certain types of ladybug can kill scale insects, they are not numerous enough to rely upon once a scale population gets this large. They also can't be purchased for release, at least as far as we know, and most will probably disperse and leave the yard one released as adults anyway. By avoiding the use of other insecticides, like broad-spectrum treatments for various pests or aerial mosquito sprays, natural populations of those scale predators can be preserved. (Even if you avoid them yourself, a neighbor who has such treatments done can unfortunately detriment your yard's beneficial insect diversity also.) Having an array of flowering plants in the landscape available as a nectar source for these beneficial insect adults can also help to keep populations of these tiny wasps and ladybugs around in the landscape for suppressing future scale before they reach outbreak proportions.

Miri