White blobs on underside of leaves are waxy egg masses of cottony hydrangea scale
Young nymphs hatch from midsummer and suck sap from the undersides of leaves
They move to the stems in late summer where they overwinter before maturing in late spring.
Severe infestations will weaken plants
Many natural enemies such as ladybirds and parasitic wasps are active on unsprayed trees, and often control scale adequately for the home gardener.
Remove and burn infested branches and stems.
Kill bugs on houseplants by touching them with a cotton swab dipped in methylated spirits, or simply wipe them off the leaves.