The damage looks like it could be caused by weevils, which of course fly in rather than bother to traipse in through the DE. What's your general location?

That's not swimming pool DE, is it?

Perhaps root weevils.

Or black vine weevil, depending on the location. Jean and I both agree that the damage is clearly that of leaf weevils of some sort.

"The damage looks like it could be caused by weevils, which of course fly in rather than bother to traipse in through the DE."

Actually, adult root weevils - of any species - do not fly. And they are a difficult pest to control, requiring attacking on both the larval and the adult front. DE can be effective however weevils are quite tenacious in how they access plants, so adults are relatively facile in avoiding powdered pesticides. Trapping adults (a simple and rather entertaining exercise) and using beneficial nematodes to kill off the larvae is the recommended approach if you want avoid chemical spraying. Otherwise, acephate is effective.

The good news is the leaf damage is entirely cosmetic. The larvae can be damaging as they eat plant roots but they rarely do sufficient damage to harm established shrubs. The big concern with larvae is in nursery containerized plants.

Controlling vine weevils

Thanks rhizo_1 , Jean and gardengal48 ! I'll research those weevils. Flying insect makes sense since the tops get attacked quickly and viciously. The DE is specifically for pest control, not pools.