Although we typically don't see serious insect damage on Dawn Redwood (Metasequoia) trees, the insects pictured are Japanese Beetle adults, and the damage to the needles does look like insect chewing as opposed to drought stress or another cause. That said, these trees do appreciate having moist soil, so if it's not already being irrigated periodically, it will probably benefit from being monitored for watering needs during this prolonged drought.

Unless damage becomes more severe, insecticide treatment should not be needed, especially since covering the entire canopy of a tree this size with a contact-type or lower-toxicity insecticide is impractical and may involve hiring a certified arborist.

Miri