If you've spotted ants and/or moldy deposits on your magnolia tree, there's a good chance you'll also see magnolia scale insects—if you look closely. Scale insects are commonly found on magnolias, where they suck moisture from the branches. These bugs also secrete a substance that can promote mold growth and attract ants.
Which Came First: The Mold or the Scale?
Mold and ants may be the effect, rather than the cause, of magnolia tree problems. The real causes often are the magnolia scale bugs. The scale insects secrete a liquid called honeydew that invites sooty mold. And because this honeydew—true to its name—is a very sweet substance, ants like to feed on it.
What Is Sooty Mold?
"Sooty mold" sounds disgusting, and it has an appearance to match. It appears as a sickly, black film covering the bark and/or leaves of a plant. Known scientifically as Capnodium, sooty mold is a kind of fungus. Sooty mold itself usually does not cause a direct problem for your plant but rather an indirect one: namely, it inhibits the photosynthesis in a plant.
Photosynthesis is the essential process through which the leaves of a plant convert nutrients into carbohydrates by harnessing the energy derived from the rays of the sun. With a layer of sooty mold fungus covering the leaves of a plant, the sunlight cannot get through properly. The result is that the necessary photosynthesis can't take place, and the health of the plant suffers accordingly.
What Are Scale Insects?
Scale insects are commonly the culprit behind this chain of events on magnolia trees. They are commonly found on one- to two-year-old branches and can look like tiny, dark nymphs in the winter, waxy, white bumps in midsummer, and yellowish-tan bumps in late summer. The males of the species actually turn into gnat-like flying insects, while the females are the "bumps on a log" that stay on the branches through the summer and produce the honeydew.
At the end of summer, the females give birth to the nymphs. The nymphs are called crawlers in the early stages of life, and they move around for a short period before they hunker down and remain dormant while overwintering.
Controlling Scale Insects
To stop the food source for the sooty mold and the ants (and other sweet-loving insects) you have to control the scale bugs. You can do this effectively if you apply a treatment at the right time of year. Scale insects are the most vulnerable (and the treatments the most effective) when they are in the crawler stage, the period between birth and when they go dormant for overwintering. Typically, the best time to spray them is late-August and into September.
Spray the small, reddish bugs with neem oil or similar organic treatment, using the recommended concentration. You can also spray with oil again later in the fall, before freezing temperatures hit, as well as early in spring before the tree's buds begin to grow.
Remember that it is the sucking insects, rather than the sooty mold fungus or the ants, that you should be fighting. The fungus and the ants will go away once you have eliminated the sucking insects.