Why are the leaves on my magnolia turning black and attracting wasps?

Although magnolias are usually free of any major insect or disease issues, they are often attacked by an insect called magnolia scale. Magnolia scale appears as rather large, white, waxy or brown bumps on the twigs and leaves of magnolia trees. Star magnolias (Magnolia stellata) and saucer magnolias (Magnolia soulangeana) are especially susceptible. The insects are immobile for most of their life cycle and often avoid detection when their numbers are low.  Magnolia scale feed on plant sap with piercing-sucking mouthparts and excrete a sweet, sticky fluid called honeydew. Unsightly black fungus called sooty mold often grows on the honeydew, making the leaves look dirty and reducing photosynthesis. Honeydew also attracts sugar loving insects such as ants and wasps. Heavy scale infestations can reduce plant vigor, stunt growth, and cause twig and branch dieback.

Instead of laying eggs, female scale insects give birth to live young called “crawlers”, which are the only mobile life stage. As the crawlers molt into adults they stop moving and develop shell-like, waxy, outer coverings that conceal and protect their bodies. The crawler stage is most susceptible to treatment.

If magnolia scale is limited to a few branches, it’s possible to prune out infested twigs. Otherwise, a number of registered pesticides are labelled for use on magnolia scale. Multiple treatments may be necessary to fully eliminate heavy infestations. Wait to spray until the vulnerable crawlers are active in late August through early October. An April application of dormant horticultural oil can also reduce the number of overwintering nymphs. In order to avoid damage to the plant, apply horticultural oil before the buds break. Before using any insecticides make sure you read and follow the label carefully.