Common name: Magnolia scale
Scientific name: Neolecanium cornuparvum
Hosts: Magnolia scale feeds on magnolia trees. Star magnolia, saucer magnolia, and many magnolia hybrids are most commonly and severely affected.
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 very small (<1/32 inch). At maturity, scale insects are immobile, have a covering over their body and are large enough to see without a magnifying glass.
Scale insects fall into two categories, soft scales and armored scales. Magnolia scale is a soft scale. The soft scales are usually larger than armored scales. They lack a hard, protective cover, instead covering 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. When populations are high, they can cause decline in the health of the host. In addition, the honeydew and the black sooty mold that grows on it, produce a mess on and under the tree.
All scale insects have sucking mouthparts and feed on sap from the tree. Soft scales, like Magnolia scale, feed directly in the tissues that move sap (phloem). They can remove large quantities of sap, stressing the host tree. Trees can usually tolerate small populations of scale. The extensive feeding by a larger population often leads to yellowing of leaves and dieback of twigs. Over time, an untreated population of magnolia scale may lead to the decline of the tree.
Soft scale excrete their waste 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 spends the winter on small twigs as tiny, half-grown nymphs, called crawlers. In the early spring, the crawlers become active, moving about and resuming feeding. As the season progresses, the scale begin to 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, then develop a white, waxy coating by early to mid-summer. They continue to enlarge through July.
Magnolia scales give birth to live young and the crawlers emerge from under the mother insect. Crawler emergence occurs late summer into early fall. Insect life cycles are dictated by heat so the 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 insecticide sprays (see chemical management below). The new 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.
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 may lead to a build-up of 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.
There are some natural predators, like certain species of lady beetle, that attack magnolia scale, but they generally do not offer enough control of this pest.
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 waxy coating. Registered sprays applied before the crawlers are present will have little effect on population control. Timing of the application is critical. Chemical sprays should be used at the time of crawler emergence, on average from late August through the end of September.
When sprayed on the tree, 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 must come in contact with the crawlers to kill them. These products may also need multiple applications since they have no residual activity. Check the label of the product you use for the 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. Insecticidal soaps and horticultural oils are good choices as they have a limited impact on beneficial insects, like ladybugs, that may attack magnolia scale.
Other insecticide sprays are available to both home gardeners and professionals, for the treatment of crawlers. Systemic insecticides can be injected into the tree by a certified arborist 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 enough to take effect at the appropriate time.
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.
For more information, contact The Morton Arboretum Plant Clinic (630-719-2424 or email@example.com).