I have a very large 20-30 ft tall beautiful yellow Elizabeth magnolia which has magnolia scale and is dripping the honeydew sap as well as losing some leaves. I understand from reading up on it that the most effective treatment is spraying with a horticultural oil in August/September. I don’t want to lose the tree – is there anything I can do now to help and is there anything I can do to protect the plants under the tree as they are being coated with the sap? I’m currently trying to wash them down with water every morning.
Thank you for contacting Toronto Master Gardeners.
Magnolia scale has become a problem here in Ontario. The Ministry of Agriculture, Food and Rural Affairs for Ontario is tracking this pest and says the following to help diagnose the problem:
“Magnolia scales are one of the largest scales in North America. Its primary hosts are Magnolia acuminata (cucumber tree), M. soulangiana (saucer), and M. stellata (star). The mature female is about 12.5 mm long, oval convex, smooth, dark brown and covered with a waxy bloom. It overwinters as a nymph on one and two-year-old shoots and the crawlers emerge in late August and September. There is only one generation per year. Honeydew and sooty mould on the branches and leaves indicate presence of magnolia scale.” http://omafra.gov.on.ca/english/crops/gardbk/gh-ch6-2ins.htm
Late August and September is the time when scale eggs hatch and the nymphs begin crawling. At that time, you can treat the tree with dormant or horticultural oil. This is available at most nurseries. Using a sprayer and following the directions, you can do this yourself. The oil smothers the nymphs, but has no effect on the adults which are covered in a waxy coating. Caution, the oil can burn the leaves.
ONNursery crops blog (https://onnurserycrops.com/2016/08/03/magnolia-scale-the-plague-of-the-landscape/ ) advises that a fall dormant oil spray (with emphasis on contacting undersides of twigs) can significantly reduce the nymph population in October. In fact, many horticulturalists say that fall dormant oil applications are the best way to manage this pest issue.
In early spring, before the leaves bud, horticultural oil can also be used effectively. By following the same directions, and being aware that the day and night temperatures must be above freezing, and no rain or high winds are in the forecast, spraying the overwintering nymphs will smother them. As there are no leaves yet, the risk of burn is minimal.
Another method which is less messy can be done now (June-July). Use a strong blast of water from your hose to knock the magnolia scale off. At this point in their life cycle, they don’t have legs, and can’t crawl back into the tree. Take care to aim the spry to hit the bark, and avoid blowing off the leaves and flower buds. You may want to cover the area below the tree with plastic sheet to collect the scale and protect the plants below.
Another, albeit tedious, solution is to wear gloves, and perhaps a raincoat, and pick off the scale off the leaves.
Finally, if the infestation is large or the tree is big enough that you cannot do the job yourself, it may be the time to call in a professional arborist.
You can find a certified arborist on the Landscape Ontario site: https://landscapeontario.com/ or on the International Society of Arboriculture (ISA) site https://www.treesaregood.org/findanarborist/findanarborist
As regards protecting the plants under the tree from being coated with the sap, the only thing I can think of is to erect some kind of physical shield over the plants. A sheet of clear plastic suspended above the plants would catch the drips and allow light through. Note that this could increase the heat load on the plants below.
Good luck with treating the scale on your magnolia.