Issue 18, October 19, 2015

Magnolia Scale

Magnolia scale is common this year in the northern half of Illinois and occurs sporadically in the rest of the state. Heavy populations and damage was noticeable in early summer when treatment was less effective than now. It attacks star magnolia, Magnolia stellata; cucumbertree magnolia, M. acuminata; saucer magnolia, M. soulangiana; and lily magnolia, M. quinquepeta. It is usually not a serious problem to native magnolias, although it can be numerous on evergreen magnolia, M. grandiflora.

Magnolia scale on magnolia branch.

Magnolia scale females can be very large for scales, about ½ inch in diameter. 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. Tree sap is 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 at this time of year produce living young. 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 high scale numbers. The nymphs then produce white, powdery wax that covers their bodies. 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 at this time controls the crawlers. The same sprays 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, the 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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