Magnolia scale (Neolecan cornuparvum) are one of the largest soft scale insects in North America measuring 1/2″. They feed on the sap of magnolias weakening them. Their feeding creates a sticky mess that attracts wasps, ants and the like. The sticky leaves grow black sooty mold on them, and the beauty of the tree is greatly marred.
Identification
If your Magnolia tree has Magnolia Scale (Neolecan cornuparvum) the first thing you will likely notice is a sticky substance on the leaves and surfaces under and around the tree. This substance is called honeydew and is caused by a soft bodied scale insect, excreting the excess juices they have been sucking from the tree. The honeydew then turns to a black sooty mold (as shown in the first photo). The honeydew also tends to attract quite a few wasps and ants. The insect itself can be found on the smaller branches and twigs of the tree. The females are sedentary (stay is one place), white to brownish purple and grow to up to 1/2″ in length at maturity (Morton Arboretum (n.d.)). They secrete a waxy substance over themselves that serves to protect them. Mature males are smaller than the females reaching 1/8 ” in length and white in colour, they are active and kind of resemble tiny flies.
Life Cycle
The Magnolia scale overwinter on the twigs as dark coloured nymphs about the size of a pin head. In the spring they begin feeding, mature and change colour. The males mate with the female, then die. The female is immobile and continues feed and grow until late summer/early autumn at which point they begin to turn a yellow-tan color and lose their coating of wax (Brown D. & Smitley D. 2015). The eggs then hatch within her, killing her. The crawlers leave the mother and move around in search of a good feeding spot where they settle down for the winter (Morton Arboretum (n.d.)).
Treatment
Insecticidal soap or horticultural oil is effective only for the crawler stage. Thus, the best time to spray your Magnolia tree is in late August through September. Apply a thorough coating and respray as directed on the package.
Dormant oil spray in the early spring, before buds open, can also be effective. It smoothers the overwintering nymphs.
Prune out effected branches
Hand pick them or spray a little rubbing alcohol directly on the insect and rub them off the branches with a small clean rag, as pictured below. Alternately a cotton swab dipped in rubbing alcohol also works. These last two strategies are suitable when there is a small infestation, or the weather is to hot to spray the crawlers in autumn.
BotaniGard ES a biological insecticide containing Beauveria bassiana strain GHA, an entomopathogenic fungus (available for use in Ontario but not for home gardeners).
Photo credits: all photos have been taken by the author.
References:
Beaulieu D. rev./2020, How to Control Magnolia Scale, The Spruce, Retrieved on Feb. 10, 2021 from: https://www.thespruce.com/what-do-i-do-for-magnolia-scale-control-2132828
Brown D. & Smitley D. 2015, MSU Extension, Department of Entomology, Magnolia scale is having a good year, Retrieved on Feb. 10, 2021 from: https://www.canr.msu.edu/news/check_magnolias_for_magnolia_scale_now
MortonArboretum (n.d.); ‘Magnolia scale (Neolecanium cornuparvum)’, https://www.mortonarb.org/trees-plants/tree-and-plant-advice/help-pests/magnolia-scale-neolecanium-cornuparvum
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Horticulture For Home Gardeners 