The magnolia scale is an occasional and widely scattered pest problem in Iowa on magnolia shrubs. Over the past 20 years infestations were reported in Des Moines west of downtown (The Art Center vicinity) and Beaverdale. This summer we have heard of infestations in Des Moines, Davenport and Jefferson.
Magnolia scale is the largest scale insect in Iowa. Sap feeding by the scales causes stress to heavily infested plants and can result in stunted growth, yellowish discoloration of the foliage and branch dieback or death of the plant. This scale insect produces large quantities of honeydew (sugary excretion) that makes the leaves shiny, and supports the growth of sooty mold, a fungus that causes the black color. Wasps (and sometimes flies, bees and even butterflies) will visit scale-infested plants to feed on the sweet honeydew.
To control magnolia scales remove and destroy heavily-infested branches. Treat with a contact insecticide (horticultural oil, insecticidal soap, other ornamental synthetic insecticide) in August to early September. Dormant horticultural oil can also be used in early spring.