If you have a saucer or star magnolia in your landscape, one insect to watch out for is magnolia scale. August through September is a good time of year for control, so it’s worth your time now to scout for potential problems.
What Plant Damage to They Cause?
What would you look for to identify magnolia scale? These insects are hard to spot, so look for the plant symptoms they cause. Heavy infestations weaken plants, cause leaf yellowing or kill entire branches, but the first symptom most gardeners notice is a black moldy coating on their magnolia’s leaves. Insects feed by inserting their needle-like mouth into a stem and sucking up plant sap. They secrete excess plant sugars, which drop to foliage and branches beneath. This creates a sticky, shiny coating which is quickly colonized by a black sooty mold. Look for sticky leaves and branches, or plant parts with a black moldy coating.
The insects feed on plant stems, not on the foliage, so that’s where to look for them. Females do not move once they have found a feeding site on a stem, so insects can build up to the point that stems are completely encrusted with scale. Usually at this point the stem dies. But these insects blend into the plant so well, many gardeners overlook them even after the plant starts to have visible symptoms.
What do They Look Like?
Magnolia scale is one of the largest and most conspicuous scale insects found in the United States. Adult females reach up to ½” diameter at maturity in late July and early August. Each female insect is covered by a soft, irregularly-shaped shell, shaped somewhat like a contact lens, which is shiny and light brown. By mid to late-August, the female’s shell turns white as it is covered by a thin coating of wax. Mature males have a similar shell, although smaller. They pupate under their shell in late July and early August then emerge to resemble tiny flies, which fly to the females for mating.
Females give birth to tiny, dark nymphs in mid to late August. These nymphs are called “crawlers” because at this point in their life they have legs and can move around on the plant to find a feeding site. Once nymphs begin feeding, they create a protective shell and stay in place until the males mature or the females die. But they are susceptible to insecticidal control during the crawler stage. If crawlers are not controlled, they will overwinter on plant stems and complete their lifecycle the following summer.
How do I Control Them?
Since magnolias bloom in spring, one of the best ways to control magnolia scale without harming pollinators is to target crawlers in fall with a contact insecticide. Pollinators will not be present on plants in fall since they are not blooming.
Horticultural oils, also known as summer oils, are a good product to use. Since they are not traditional insecticides, but instead are highly refined oils, they are very safe to use around human, pets, wildlife and other beneficial insects. For good control, it’s important to get thorough oil coverage on plant stems. Oils can be applied from mid-August until freezing temperatures occur in fall and again in early spring before the flower buds begin to swell. Look for products such as Bonide All Seasons Horticultural & Dormant Oil Spray, Ortho Volck Oil Spray, or SunSpray Ultra Fine Year-Round Pesticidal Oil.
But remember – just because a pesticide, like horticultural oil, is considered safe and low toxicity doesn’t mean it can’t damage plants. Use caution when applying oils in late summer; they can burn leaves if conditions are too hot or when applied to drought-stressed plants. Never spray landscape plants with a pesticide if air temperatures will reach 85° F or above during that day. Water plants well a day or two before application and wait until moderate temperatures occur to make your applications. Read and follow all pesticide label directions before use.
More information, including pictures.
- Magnolia Scale, The Morton Arboretum
