My magnolia had a problem with magnolia scale last year. Will it come back, and should I take some action to protect my tree? The leaves were covered in a black mold late last summer.
— Frank Butler, Forest Park
Many gardeners do not notice a scale problem until summer, when they see leaves turning black with sooty mold that grows on the clear, sticky fluid secreted by the scale insects as they feed. Look for scale now on the branches of your tree to confirm that it is there before planning any treatment. Magnolia scale is common in northern Illinois and is one of the largest of the soft-scale insects that attack trees by sucking plant fluids and sap. It is most noticeable in late spring and summer when you can see the large, yellowish to brownish-colored lumps that are attached to the branches. They can be up to 1/2-inch long with an oval or round shape and readily visible on close inspection. When there are heavy infestations, the entire branch may be covered with scale and appear purple and rough from the high scale numbers. Depending on the time of year, you may also see nymphs (immature form of the scale) that produce a white, powdery wax that covers their bodies, causing twigs to appear whitish in high infestations.
A large population of magnolia scale will reduce the overall vigor of your tree and lead to the death of individual branches and eventually the tree if left unchecked. As with other insect pests, it is important to choose the least toxic approach at the most susceptible stage of their life cycle during which the treatment will be most effective. The mature scale lays eggs over the summer that then hatch into a new generation of crawlers in late August and September depending on temperature. The scale overwinters in the crawler stage in clusters on one- to two-year-old twigs and will begin to move again when temperatures warm up in spring. From the time that the eggs hatch in summer until they molt in late April or early May, they are vulnerable to horticultural oil treatments and insecticide sprays.
The adult magnolia scales’ waxy coating protects them from directly applied insecticides that would also kill the many natural predators of the immobile adult scales. If your tree is small, you may be able to use your fingers to squish and kill the scale now. Treating with a horticultural oil in early spring (dormant season oil) can be very effective in smothering overwintering nymphs and active and recently settled crawlers and will have less impact on the environment. It must be applied when the temperatures will be above freezing for at least 24 hours and before the leaf or flower buds start to open. Be sure to read the instructions on any product labels before applying. Once you have eradicated the scale, you can stop treatments and then monitor your tree going forward. There is no need to apply any controls if the scales are not present in future years.
You can apply an insecticidal spray of acephate (Orthene), insecticidal soap or a summer-oil spray in September to control the crawlers. It is important to get good coverage, particularly on the twig undersides, where the crawlers will be most numerous. As these are contact products, insects not hit directly with the insecticide, soap or oil spray will probably survive, so thorough coverage is essential. If you observe lady beetles, which feed on magnolia scale, you might want to avoid using insecticides. When the beetles are present, it is best not to spray.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.