Q. My large magnolia tree has white cotton-like structures on the twigs. Many of the leaves have turned dark and are sticky. If I park my car near the tree, then the car becomes covered with this sticky material. What's going on?
A. According to the Chicago Botanic Garden, magnolia scale is out and about in Midwestern landscapes. Scale insects attack trees by sucking plant fluids and sap. In recent years, Magnolias have become especially susceptible. In severe infestations, entire branches may be coated with scale and have a chalky appearance. Many gardeners do not realize their trees are infected until they notice the leaves turning black with sooty mold that grows on the clear sticky liquid secreted by the scale insects.
These soft-bodied insects appear in early spring as tiny dark specks on the branches of magnolias. The specks are immature scales called crawlers, which move along the branches for a short time until they settle and begin feeding.
Magnolia scale has sucking mouthparts, which it uses to remove large quantities of sap from twigs and young branches. Heavily infested branches can be completely covered by scale and may be weakened or killed by it. Magnolia scale also produces a sticky, sweet substance called honeydew, which attracts ants, yellow jackets and other insects that feed on it. A fungus called sooty mold can grow on the honeydew, resulting in blackened leaves and branches.
Treating a large magnolia heavily infested with scale can be difficult. In my opinion, finding an arborist to treat the tree may be the best option.
Magnolia scales' waxy coating protects them from directly sprayed insecticides, which also would kill the many natural insect enemies of immobile adult scales.
According to Michigan State University, the basic strategy for treating magnolia scale involves the use of horticultural oils to smother the scales, and systemic insecticides to poison the insects as they feed on sap. Summer oils can be applied in spring and late summer. Dormant oils are applied in late fall and very early spring when there are no leaves on the tree.
Systemic insecticides such as Bayer Advanced Tree and Shrub Insect control and Safari are usually applied as soil drenches to the base of the tree. In my opinion, the best time to apply systemics is in late spring after flowering. Unfortunately, systemic insecticides can persist and be present in the tree when it produces flowers, and that can kill bees visiting the flowers.
Arborists also have access to growth regulators that can help control scale insects in a much more environmentally friendly way.
Here is a link to more information about magnolia scale www.mortonarb.org/trees-plants/tree-and-plant-advice/help-pests/magnolia-scale-neolecanium-cornuparvum.
The Plant Medic, written by Ricky Kemery, appears every other Saturday. Kemery retired as the extension educator for horticulture at the Allen County branch of the Purdue Extension Service.