The question that leads us to diagnose Magnolia Scale is, “What is this black sticky stuff all over my Magnolia?” While diagnosis is easy during this stage, treatment must wait until a different lifecycle stage. It’s still possible to save your Magnolia, so don’t panic.
Magnolia Scale (Neolecanium cornuparvum) is an insect pest that’s frequently mistaken for a fungal disease. It’s one of the largest scale insects found in the United States, with adults forming an outer shell up to ½” in size. They attack only Magnolia. Infestations commonly go several years and reach significant proportions before being identified. Knowing what to look for and keeping a watchful eye on your Magnolia will help prevent this scale infestation from weakening or killing your plant.
The adults attach themselves to the undersides of Magnolia stems and use their sucking mouthparts to feed on the sap of the plant. Adults are protected under a waxy outer shell that makes them impervious to most pesticide treatments in this life stage. Treatment, to be effective, must happen when the insects are in their immature, or “crawler” stage.
Magnolia Scale Life Cycle Stages & Treatments
You can usually identify Magnolia Scale in midsummer when the adults are at their largest and most conspicuous. Scale appears as raised clusters of shell-like brown discs, often covered in a white waxy coating.
1. Honeydew Stage: mid-Summer
Most common stage for identification, but not treatment.
Under these shell-like coverings, the adults are feeding on the plant’s sap. Excess sap is excreted as a sweet sticky substance called honeydew that accumulates on leaves, stems, and may even drop to objects under the tree. This honeydew then develops a black sooty mold and attracts large numbers of ants, wasps, and other insects.
It’s usually at this stage that gardeners come seeking help, asking the tell-tale question “What is this black sticky stuff all over my Magnolia?”. While diagnosis is easy at this stage, treatment must wait until the next life stage.
Excess sap dropping to walkways below.
2. Crawler Stage: late-Summer
Best Treatment Stage: Insecticidal Soaps
In late summer, usually mid-August to late September in Wisconsin, young “Crawlers” are born. Adults will die after the crawlers are born but their shells will remain on the stems. Dead scale shells can be scraped off the stems easily, whereas living scale shells will be firmly attached to the stems. Crawlers are extremely small, soft-bodied, six-legged insects that can be seen moving around on the stems and leaves if you look closely.
Insects are especially susceptible to insecticide during this immature, youngest stage. Generously spray Insecticidal Soap on all stems and leaves, including the undersides.
“An early autumn application of even a mild, organic insecticide such as Insecticidal Soap will be highly effective in killing scale in the crawler stage.”
3. Nymph Stage: late-Autumn
Treatment Option: Horticultural Oils in Spring
In late autumn the nymph stage begins. In this stage, the scale insects are small, dark brown in color, and oblong in shape. They may be mistaken for raised areas on the bark. Magnolia scale overwinters in this nymph stage.
A thorough application of horticultural oil in early spring will significantly knock down nymph populations. Horticultural oil is an environmentally safe treatment that kills the nymphs simply by smothering them. Treatment must be made when temperatures are above freezing and before the buds open on the plant. Make sure all stems and twigs are coated in oil.
Healthy, vigorous plants are less susceptible to scale insects so keeping your Magnolia healthy through proper watering, fertilizing, mulching, and pruning will also greatly reduce its vulnerability to damage.
Shells remaining on the stems when the adults die.
More Magnolia Scale Resources
UW-Madison Wisconsin Horticulture UW Extension article on Magnolia Scale.