This summer has been exceptional for magnolia scale. Master Gardeners at the help desks have constantly been addressing this situation with homeowners for the last 5+ weeks. A typical life cycle of a scale is an overwintering female producing eggs under her protective scale covering. The eggs will hatch into very tiny crawlers that may resemble an aphid. These crawlers then move farther out on the branch to tender twigs and the leaves where they use their piercing sucking mouth-parts to tap into vascular system to get their liquid diet.
Most scales are a bother earlier in the summer and Magnolia scale later in August. This year, calls started coming in mid -June, well ahead of normal. Calls were not always about the scale in the beginning, rather "What is all this black stuff all over everything?"
As the young scale crawlers begin to feed on the plant juices, much of it passes through their bodies and onto surfaces below. That sticky exudate is a great growth media for sooty mold, the black material covering foliage, lawn ornaments, the grill, sidewalks and driveways, not to mention the car parked under the canopy on the driveway.
Magnolia scale has very likely been living quietly and peacefully on the Magnolia tree for a long time, going unnoticed or creating minimal sooty mold below. In order to have such a large population this year, conditions last year were perfect, allowing large numbers of female scales to overwinter to produce all the crawlers this year.
While the sooty mold is not desirable on plants below and other belongings, it is worse for the Magnolia as it can completely cover the upper sides of the foliage, greatly reducing the plants ability to photosynthesize. Additionally, the crawlers are removing the liquids from the plant as well. The magnolia will not overwinter as well as it should, may not produce the blooms for next year and even suffer dieback from winter cold weather. With these heavy infestation, losing interior foliage and limbs is a strong possibility.
With the abundance of crawler scales there should also be a strong increase in the number of natural predators feeding on the crawlers. The Twice Spotted lady beetle and other lady beetle larvae stages eat well. If predators are present, control options are limited to insecticidal soap. If management is going to include pesticides, the label will need to list both the insect to be sprayed. Like other scale insects, dormant oil sprays are also effective in controlling overwintering female adult scales.
To help the magnolias with the loss of sap, water now and keep watering until it is time to remove the hose from the house for the winter.
About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.