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Thursday, Aug. 27, 2009 02:58 pm

Get on the scales

Sticky stuff? Black magnolia leaves? Here’s what to do.

Magnolia scale.
JOHN A. WEIDHASS, VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY
One of the worst things to tell a gardener is that their tree is heavily infested with a destructive insect. The U of I Extension office has received numerous calls about magnolia trees. A caller usually describes the problem as “sticky stuff all over the magnolia leaves and the leaves are turning black.” The answer: an explosion of magnolia scale is among us.

Magnolia scale could be described as cottony white bumps on a twig. During the summer, the half-inch smooth and shiny, tan-brown, oval shape female develops a white mealy wax coating.

Magnolia scale attacks species of magnolia including star magnolia (Magnolia stellata), saucer magnolia (M. soulangiana), cucumbertree magnolia (M. acuminate) and lily magnolia (M. quinaquepeta) trees.

Magnolia scale usually begins on the underside of new twigs. The first noticeable sign of an infestation is a sticky sap or black mold on the leaves. Magnolia scale feed by sucking sap out of the plant. As they feed, the scale produces a sticky, sweet waste called honeydew. The honeydew drips onto leaves, stems and the occasional car, making them sticky. The sticky liquid provides an ideal substance for sooty mold fungus to develop. Ants, bees, wasps and flies are attracted to the sweet sap and feed on the honeydew.

Magnolia scale can be controlled in the nymph stage of its life cycle. Tiny nymphs called crawlers begin to emerge in August. This is the only mobile life stage. The crawlers will look for a suitable feeding site, usually a one- to two-year-old twig. It will spend its entire life at this spot. The dark colored nymphs overwinter in clusters.

Double-stick tape can be used as a tool to observe the crawling stage. Wrap a piece of double-sided tape on each side of a scale colony. The nymph will stick to the tape, letting you know that treatment can begin.

Chemical control measures can be taken when crawlers hatch or during the overwinter stage. Unfortunately, chemical control of magnolia scale is challenging and may need to be repeated over several seasons. From the time that they emerge from the female until late April, the nymphs are vulnerable to insecticide sprays.

Insecticidal spray of acephate (Orthene), insecticidal soap, or summer-oil spray can control crawlers. (Read and follow all label directions.) It may be necessary to apply two applications, 7 to 10 days apart, of these contact insecticides. Be sure to get coverage over all parts of the twig, especially the underside where most crawlers will be found. The insecticide must contact the scale directly. If you observe lady beetles, which feed on magnolia scale, you might want to avoid using insecticides when the beetles are present.

Dormant oil can be applied in late winter or early spring to kill overwintering nymphs. Apply dormant oil when the temperature is above freezing at time of the application. Also apply before budbreak.

Another control option is a single soil drench of imidacloprid, sold as Bayer Advanced Tree and Shrub Insect Control. This can be done in late April. The amount to apply depends on the size of the plant. Read and follow label directions.

Another control option is to remove and destroy heavily infested branches. This option works best if only a few branches are heavily infested.

If you have a magnolia tree and don’t have signs of scale, count yourself lucky. But, be sure to inspect the tree each year for signs of this pest. Inspection of the tree can be done anytime of year. In the winter, look for exoskeletons of the previous season’s insects.

Initial damage caused by magnolia scale is weakened plants with slow plant growth, reduced and smaller foliage, and flower production. Severe infestations can result in branch dieback. Repeated severe infestations can cause plant death.

Magnolia scale is a devastating pest that is a challenge, but with good monitoring and management it’s possible to get control of this pest. For more information visit PennState, Magnolia Scale Factsheet, http://woodypests.cas.psu.edu/factsheets/InsectFactSheets/html/Magnolia_Scale.html.

Contact Jennifer Fishburn at fishburn@illinois.edu.
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