Come springtime, magnolia trees are a delightful sight, except for those infested with the potentially devastating magnolia scale. The pest, which seems to be an increasing problem in the eastern United States, is typically difficult to manage, but recent research with soil-applied insecticides suggests a solution.
"Magnolia scale seems to be more and more of a problem," said Dan Herms, an Ohio State University entomologist. "Where people have problems, the insect can be quite devastating.
"As far as scale insects go, the magnolia scale is hard to deal with because of its long, drawn-out hatching season in the fall-the normal time for insecticide applications."
A single soil drench application on May 1, however, has proven to be successful, according to research conducted by Herms and colleague Dave Nielsen, both of the university's Ohio Agricultural Research and Development Center (OARDC). Soil drenches that contained imidacloprid, an environmentally friendly systemic insecticide, showed outstanding control of magnolia scale on heavily infested trees.
One of the products tested, Bayer Advanced Tree and Shrub Insect Control, is a great solution for homeowners battling magnolia scale, Herms said, and can be purchased locally where gardening supplies are sold. It is applied as a soil drench to the base of the plant using a watering can. The amount of liquid needed depends on shrub height or tree-trunk diameter. Usage rates, directions and safety precautions are outlined on insecticide labels.
In the past, magnolia scale has been managed with fall insecticide applications that are time consuming and not always effective, Herms said. For instance, insecticidal soap and horticultural oils need to be applied every seven to 10 days throughout the eight- to 10-week period of crawler emergence in the fall. Any applied insecticides need to coat the twigs and branches thoroughly to cover the crawlers, which are hard to detect.
The crawlers are .04 inches in length and are flattened, oval flakes that vary in color from yellow to reddish-brown. Newly hatched nymphs, or "crawlers," are mobile, but only until they find a suitable feeding site, which is usually close to the mother. After picking their spot, the crawlers insert their mouthparts into the plant and spend the rest of their life feeding there.
Crawlers that have not yet chosen a feeding spot and instead are picked up and moved by a bird's feet typically spread infestations from plant to plant, Herms said.
After finding their spot, crawlers then overwinter and begin to molt and grow about the time leaves emerge in spring. As the crawlers mature, they appear as large, oval bumps on twigs and branches. The adult females reproduce, give birth in the fall and die shortly after, leaving behind a hollow, brown shell that stays on the plant for several months. The brown shells can be easily observed toward the end of winter when trees lack foliage.
Another indication of magnolia scale results from the large quantities of sap sucked from the plant as scales feed. The sap provides a low-protein, high-sugar diet, and in order for the scale to obtain adequate amounts of protein, the insect must ingest excessive amounts of sap. Much of this sap is excreted by the scales, which produces a clear, sticky, sugary substance known as honeydew.
This honeydew coats twigs, leaves and anything under infested branches, including cars and patio furniture. If the honeydew is not removed, a more obvious, unattractive black fungus known as sooty mold begins to grow. This is often the first symptom of infestation that people notice, Herms said. Yellow jackets, wasps and ants also are good indicators of infestations as they are often attracted to the sweet honeydew on which they feed.
Magnolia trees that host a large population of scale insects can be drained of energy, resulting in small, yellowing leaves, twig dieback, thinning canopy and even death. Generally, the plants tolerate small infestations fairly well, which allows homeowners time to implement a management program before the problem escalates, Herms said.
Some magnolia trees are more resistant to the scale insect, which is native to the eastern United States. The popular star magnolia, Magnolia stelleta, and saucer magnolia, Magnolia x soulangiana, are very susceptible. Magnolia species native to the United States are much more resistant, such as the cucumbertree magnolia, M. acuminata, southern magnolia, M. grandiflora, sweetbay magnolia, M. virginiana, bigleaf magnolia, M. macrophylla, and umbrella magnolia, M. tripetala.