Issue 13, July 31, 2009

Magnolia Scale

Magnolia scale is common this year in northern Illinois. We have received numerous requests for information, so this article is being provided to help answer client questions even though it is still too early to achieve effective control. Magnolia scale is common in northern Illinois south through Kankakee County and occurs sporadically in the rest of the state. It attacks star magnolia, Magnolia stellata; cucumbertree magnolia, M. acuminate; saucer magnolia, M. soulangiana; and lily magnolia, M. quinquepeta.

Magnolia scale females can be very large for scales, about 1/2 inch in diameter, but are usually smaller. They range from yellowish to brownish, from oval to a roundish blob. Magnolia scale produces large amounts of honeydew, resulting in shiny, sticky leaves, as well as sticky sidewalks and cars underneath infested trees. Black sooty mold grows on the honeydew, resulting in black branches, foliage, and sidewalks. Tree sap is very low in nitrogen, so soft scales consume great quantities of it, separate out much of the water and nitrogen, and excrete most of the remainder as the concentrated sap, or light syrup, called honeydew.

Mature females produce living young in late September to early October. These first-stage nymphs, or crawlers, are oval and gray, with a reddish brown ridge running down the back. Each crawler has two white, waxy spots, one on each side. Crawlers mass on the undersides of 1- and 2-year-old twigs for the winter. From the time that they emerge from the female until they molt to the second nymphal instar in late April or early May, they are vulnerable to insecticide sprays. In early June, they molt again to the third-instar nymphal stage and are deep purple.

Heavily infested twigs and branches appear purple and rough from the high scale numbers. The nymphs then produce white, powdery wax that covers their bodies, causing twigs to appear whitish in high infestations. As they mature to adults, the white wax wears away, being heaviest on the edges of the scale. There is one generation per year.

An insecticidal spray of acephate (Orthene), insecticidal soap, or summer spray oil in late September into October controls the crawlers. Injection of acephate (Lepitect) should also control them at that time. The same treatments at bud break in the spring are also effective. With the insecticidal soap and summer spray oil, be sure to get good coverage, particularly on the twig undersides, where the crawlers will be most numerous. As these are contact insecticides, insects not hit directly with the soap or oil spray will probably survive; so thorough coverage is essential.--Phil Nixon

Phil Nixon

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