Controlling Magnolia Scale

If you’ve noticed sticky leaves, black sooty mold, or declining magnolia trees, there’s a good chance you’re dealing with magnolia scale. This common insect pest can be frustrating for homeowners and tree care professionals alike because it is one of the most challenging scale insects to manage successfully.

Magnolia scale is a soft scale insect that feeds on the sap of magnolia trees. Adult females are large, smooth, and often covered with a waxy coating that ranges from tan to brown. As they feed, they produce large amounts of honeydew—a sticky substance that coats leaves, branches, sidewalks, and anything beneath the tree. This honeydew encourages the growth of black sooty mold, which not only looks unsightly but can also reduce the tree’s ability to photosynthesize. Heavy infestations may lead to yellowing leaves, branch dieback, reduced vigor, and increased susceptibility to other pests and diseases.

Why One Treatment Usually Isn’t Enough

Many homeowners hope for a one-time solution, but magnolia scale often requires a long-term management plan. Depending on the severity of the infestation, multiple treatments over several growing seasons are often necessary. Arborists have a few tool in our toolboxes to help get magnolia scale under control.

One of the biggest challenges in treating is the scale’s natural defense. Adult magnolia scales develop a thick, waxy covering that shields them from many contact insecticides. This is why insecticides that are applied as a spray are not effective against adult scale, but they do work against crawlers. Crawlers are newly hatched young that have not yet developed their protective coating, making them much more vulnerable to insecticides. This stage only lasts for a relatively short period, and its timing can vary depending on local weather conditions. It can be very easy to miss spraying during this stage, so insecticide sprays are only considered to be one part of control

Systemic insecticides are more effective against the adult scales. Systemic insecticides are frequently applied as a soil injection. The trees then take up the product from their roots and it travels all the way to their leaves. When the scale start sucking the tree sap, they ingest the insecticide. Not at insecticides are effective against scale, and timing of these treatments is important to avoid affecting pollinators. That is why it is important to treat just after the tree is done blooming in the spring.

The third tool commonly used is horticultural oil applied during the dormant season. At the end of each growing season, scale lay their eggs in the tiny crevices of the tree bark, where they wait until spring to hatch. Horticultural oil, when sprayed thoroughly, smothers these eggs and prevents them from hatching, which is another great method of control.

Occasionally, it can be helpful to prune out a heavily infested branch when appropriate. If you catch a magnolia scale infestation early, and you are only seeing scale on a couple of branches, pruning those out can be helpful to control the population early in an outbreak. However, if scale have already spread throughout the whole tree, pruning can be detrimental to the tree’s health. Once Magnolias start to show obvious signs of stress, like thinner canopies, twig or branch dieback, or reduced growth, avoid pruning your tree. Removing branches means reducing photosynthesis for the tree, which can cause even more stress. Proper watering and mulching of your Magnolias during an infestation can help them in their recovery.

No single treatment is ever 100% effective, which is why we frequently combine treatment options to target the eggs, the newly hatched young, and the adult stages of scale for more thorough control. Even after a successful treatment, nearby untreated magnolias can serve as a source of new infestations. Birds, wind, and crawler movement can all contribute to the spread of magnolia scale. Early detection makes a big difference. Once scale populations have exploded, and your tree is showing obvious signs of stress, treatment becomes more difficult. Watch for early signs of scale like sticky leaves or branches, black sooty build up on sidewalks, decks, or plants underneath the tree, brown or tan bumps on the young branches, or increased activity of ants, wasps, or bees feeding on honeydew. Call your arborist at the first signs of infestation.

Because treatment timing is so important, working with a knowledgeable tree care professional can help ensure the right products are applied at the most effective stage of the insect’s life cycle. With patience and a well-planned management strategy, even heavily infested magnolias can often recover and continue thriving for years to come.

If you are seeing your Magnolia starting to suffer, call Tree Health Management (608-223-9120) to meet with an ISA Certified Arborist and develop a treatment plan that works for you.