Hydrangea Scale insect
Hydrangea scale is a relatively new pest in the UK.
It is not endemic to Hydrangeas as it has also been known to attack other woody plants including Acer and Prunus.
Although it is not certain, it has probably been introduced through the importation of plants from abroad.
As the insect sucks sap from the foliage and stems, the plants tend to lose their vigour or even suffer from premature leaf drop.
It manfests itself in early summer when the mature scales deposit their eggs on the stems and underside of the leaves.
The white fibrous waxy coated eggs form in oval patches on the leaves, the coating /shell remains on the plant even after the insect has hatched and moved on.
The newly hatched pale yellow scale insects are less than 1mm long, they turn brown as they mature.
In late summer the nymphs move to the stems where they overwinter before maturing in late spring.
Removal of heavily infested plants is the only option.
Contact pesticides containing fatty acids will control young scale nymphs if applied thoroughly to the underside of leaves as they hatch.
Several applications may be necessary.