Saucer magnolia (Magnolia x soulangiana), star magnolia (M. stellata), cucumbertree magnolia (M. acuminata), lily magnolia (M. liliiflora)
Description & Symptoms
Magnolia scale is one of the largest of the soft-scale insects that attacks trees by sucking plant fluids and sap. It is most noticeable in late spring and summer when it appears as large, whitish lumps, up to ½ inch long, attached to the branches of magnolia trees. 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. In early spring, magnolia scale appears as a barely visible dark speck which soon grows into a mature insect.
Timing & Life Cycle
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. Each crawler grows into a waxy, whitish lump about ½ inch long. The mature scale lays eggs over the summer, which hatch into a new generation of crawlers in September. Magnolia scale overwinters as crawlers, which begin to move again when temperatures warm in the spring.
Severe and repeated cases of scale can cause the death of individual branches and reduce a tree’s overall vigor.
Treatment & Solutions
Magnolia scales’ waxy coating protects them from directly sprayed insecticides, which also would kill the many natural insect enemies of immobile adult scales. Applying horticultural oil in the early spring and late summer is the best treatment. Horticultural oil will smother active and recently settled crawlers. When applied in the early spring, horticultural oil is called dormant season oil. It must be used when the temperatures will stay above freezing for 24 hours and before leaf or flower buds start to open. It is important to read and carefully follow instructions when applying horticultural oil.
For more information about the use of selected systemic insecticides to control scale, call the Plant Information hotline at (847) 835-0972.