Scale insects are unique and look quite different from other insects. In their juvenile growth stage, they are referred to as "crawlers". As crawlers, they are highly mobile, six-legged, have no protective cover, and are usually smaller than a pinhead. However, at maturity, scale insects are immobile and have no visible legs or antennae.
Scale insects can be divided into two groups: armored scales and soft scales. Armored scales secrete a protective cover over their bodies and usually overwinter as eggs beneath the female cover. Soft scales are usually larger, lack the protective cover, but protect themselves with waxy secretions. Soft scale produce a sticky substance called honeydew; armored scale do not.
Magnolia scale is our largest soft scale insect, reaching ½ inch in length. This scale spends the winter on small twigs as tiny, dark-colored nymphs. In the spring, the scales begin to feed, mature, and change color. The males, which turn white, are smaller than the females, about 1/8 inch in length, and emerge as tiny, gnat-like insects. The males mate with the females and then die. The females turn white to brownish-purple in color and continue to enlarge through July.
Magnolia scale eggs hatch internally and the crawlers emerge from the mother insect. Crawler emergence occurs late summer into early fall. Insect life cycles are dictated by heat so emergence of crawlers will vary from year to year. On average crawler emergence occurs from late August through the end of September. This would also be the best time to treat with insecticides (see chemical management below). The crawlers move around until they find a suitable feeding site, usually on branches, where they settle down and remain through the winter. The adult female dies after reproducing, but may remain attached to the stem for many weeks, making the population seem larger than it really is.
Scale insects have sucking mouth parts and feed on sap from the tree. They can remove large quantities of sap and can stress the host tree. Trees can usually tolerate small populations of scale. The extensive feeding by a larger population will stress the trees and often leads to yellowing of leaves and twig dieback. Over time, an untreated population of magnolia scale may lead to decline of the tree.
Excess sap is excreted by the insects as honeydew. Honeydew is sticky and will coat plant parts and often drip onto surfaces under the tree. A black fungus called sooty mold will grow on the honeydew, but does little actual damage to the plant. The sticky honeydew and black sooty mold are often noticed before the insects are seen. The honeydew may also draw other insects like ants and wasps to the tree.
Magnolia scale will attack magnolia trees and tulip tree. Star magnolia, saucer magnolia and many magnolia hybrids are most commonly and severely affected.
Trees that are stressed are more prone to attack by scale. Keep trees in good health through proper watering, mulching, and pruning of dead or infested branches. Avoid excessive fertilization as this make lead to a build up scale populations.
It may be possible to manage small populations of magnolia scale by hand. If the insect is limited to a small branch or two, pruning out those branches may be enough. Small populations can also be carefully wiped off of branches.
Scale insects are very vulnerable in the crawler stage when the young are looking for a place to feed. Adult scales are usually protected from chemicals because of their protective coating. Registered sprays applied before the crawlers are present will have little effect on population control. Timing of application is critical. Chemical sprays would be used at the time of crawler emergence (on average from late August through the end of September).
Insecticidal soaps and horticultural oils can be effective against the crawler stage but usually have no effect on the adult scale. These products require thorough coverage and will also need multiple applications since they have no residual activity. Check the label of the product you use for timing of additional sprays.
Dormant oil applied to overwintering nymphs in early spring can also be effective. This treatment should be applied before buds open in the spring.
Other insecticide sprays are available to treat the crawlers. Contact the Plant Clinic at The Morton Arboretum for current recommendations (630-719-2424 or firstname.lastname@example.org). This information can also be found in “Pest Management for the Home Landscape” published by University of Illinois Extension.
Systemic insecticides that contain imidacloprid or dinotefuran can be drenched into the soil several weeks before the crawlers become active. These products will kill the crawlers but take time to move through the tree, so they must be applied early.
Special notes regarding chemical management of magnolia scale:
Insecticidal soaps and horticultural oils are good choices as they have a limited impact on beneficial insects (like lady bugs) that may attack magnolia scale.
Do not expect adult scale to fall off after an insecticide treatment. The treatments target the crawlers, not the adults. Adults will die naturally after reproducing, but will remain attached to the tree for several weeks. Dead adults will be dry and can often be easily knocked off by hand. Live adults will be firmly attached and will produce a liquid when smashed by hand.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum. Use pesticides safely and wisely; read and follow label directions.