Japanese maple scale is a major problem in the landscape. I had a Japanese maple almost killed in my landscape by this scale until I got it under control. The following is more information.
Japanese Maple Scale
This scale has become the number one scale in nurseries and landscapes over the last 5 years - so pay attention and watch for this scale. Try to bring it under control before it overwhelms you.
Plants Damaged: Japanese maple scale is showing up in more and more landscapes and nurseries lately. Look for this scale on Japanese maple, Ilex sp., American red maple, dogwood, zelkova, lilac, yellowwood, pyracantha, privet, holly, euonymus, redbud, stewartia, cherry, magnolia, Itea, and Styrax.
Damage: This armored scale feeds directly on plant cells, not in the phloem tissue like soft scale insects. Heavy populations cause a slow decline of the tree. If high populations have resulted in dead branches on trees, prune these out before crawler hatch to reduce the number of scales potentially moving onto other branches and trees.
Life Cycle: The life cycle is poorly understood. The male and females overwinters as immatures and mature in early spring. There are two generations per year. The two generations overlap and crawlers are present from June through October. We will monitor and let you know when crawlers start emerging. Look for the white to gray, narrow oystershell-shaped female covers on twigs and main branches. Use a hand lens to look for the light purple crawlers.
Control: Applications of 1% horticultural oil and Distance should be made when crawlers are detected. This should have about 2 - 3 week residual activity. At that time monitor your plants again to see if you still have crawler activity. The twice-stabbed lady bird beetle provides some biological control of this scale.
Information from the May 22, 2009 edition of the TPM/IPM Weekly Report for Arborists,
Landscape Managers & Nursery Managers from University of Maryland Cooperative Extension http://www.ipmnet.umd.edu/09May22L.pdf