Alder flea beetles are particularly active this summer as they go about their business of skeletonizing leaves on red alder trees. I have seen this come and go over the years, and generally flea beetles are not a serious threat.
Flea beetle flare-ups in the forest usually run their course after 2-3 years, after which their population crashes – similar to western tent caterpillar infestations. Healthy alder trees with good vigor are usually not seriously affected. Stressed alder trees – particularly those in dense thickets – may die after 2-3 consecutive years of flea beetle infestation. If you like your red alder trees and want to promote “good vigor” the best way is to 1) avoid growing alder on poor alder sites and 2) keep trees well-spaced to ensure large healthy crowns.