Magnolia scale (Neolecanium cornuparvum) has been very prevalent in mid-Michigan for the past two years. Midland has seen heavy infestation of many, if not most older magnolia trees. This insect is a "soft" scale and soft scales have been increasing rapidly over the past couple of years. Other common soft scales are cottony maple scale and lecanium scale. The magnolia scale is in the lecanium scale family.
This is one of the largest scale insects, with females being 1/2 inch or more in size. The female develops on the twigs of several types of magnolia tree and after feeding on sap for the summer, gives birth to live young crawlers in late August and September. The crawlers (or nymphs) then settle on the twigs to overwinter and in the spring they mature, then mate and the females settle in one place until they bear young again.
The appearance of hundreds of these large, grayish white insects on the twigs can be quite dramatic. The honeydew they produce as they suck sap and then excrete it on surfaces below can be quite a nuisance. Severe infestation can damage the tree with small branch dieback and defoliation stress as the leaves are deprived of sap flow.
Some control would be theoretically possible with sprays in late fall or early spring to target the nymphs, however, application of pesticides kills beneficial insects that help to control this pest insect. A better approach is with systemic insecticides that kill only the pest insect by poisoning the sap.
BEST CONTROL OPTIONS FOR MAGNOLIA SCALE
DON'T SPRAY MAGNOLIA SCALE!!
Sprays are NOT recommended by us and our arborists. They are often not very effective and they kill beneficial insects while not having much effect on adult female scales. The would require very thorough coverage during the crawler stage, which can be difficult to achieve and again, would have devastating effects on beneficials and possibly other non-target organisms.
Excellent options are available for systemic treatment by homeowners and by professionals:
Homeowners can apply soil applied systemic insecticides to small trees and this can be effective. Dinotefuran and imidacloprid applied as directed on the package will give some control. Dinotefuran is available as Ortho brand tree and shrub insect control. Imidacloprid is available as Bayer brand tree and shrub insect control.
Larger trees can be treated professionally by arborists using the Arborjet system. Systemic acephate, dinotefuran, and imidacloprid can be trunk-injected and have been proven to give excellent control, often for two or more seasons with one treatment.