Magnolia scale

Asked July 8, 2019, 7:09 PM EDT

We have a large Magnolia tree that has fuzzy white patches all over the branches. I have read your August 2015 article about Magnolia scale (Neolecanium cornuparvum, Thro) and it sounds just like our tree. I first noticed that the hostas all around the tree had the black sooty mold which led me to seeing the insects. 1. In your article you spoke about an insecticide and a basal soil drench as well as spraying the tree. You also mentioned oil. Can you give me a detailed step-by-step (timing included) guide on these methods? 2. I'm preparing to treat my hostas with Neem oil, should I spray the lower branches of the tree at the same time? 3. Is this a job better left to professionals? I noted the suggestion to prune branches with the infestation but our tree is covered and it is probably over 25 ft. high. Also, whatever we do I want to protect the hostas. Thank you for your time and look forward to your response. Vicki Iseler, Hanover Township, Jackson County

Jackson County Michigan trees and shrubs insect issues magnolia scale

1 Response

I presume that the article that you are referring to is the 2015 MSU Extension article by Diane Brown and Dave Smitley that can be found at this url: https://www.canr.msu.edu/news/check_magnolias_for_magnolia_scale_now
This is an excellent article, however, it is a few years old, so please be sure to verify that the pest control products it mentions can still be used legally for magnolia scale control in Michigan.
In response to your three questions:
1. Unfortunately, there is no one-size-fits-all step by step approach that I can provide for you in this forum. It is up to you to decide the best course of action. If you want to reduce harm to pollinators and other beneficial organisms, the very best thing to do would be to prune off the most heavily infested branches and burn them or have them hauled away with your yard waste. The next least toxic option would be to use the neem oil in the fall to target the crawler stage of the scales. Using neem in the summer will have no effect on the female scales because they are protected under their waxy coverings. Alternatively, horticultural oil, rather than neem, could also be used in the fall. Either neem oil, or horticultural oil could be used in the fall or spring to target the crawlers and nymphs, respectively. Whichever product you choose, be sure to read the label and follow the label directions to the letter, in order to maximize the effectiveness of the product and to minimize the risk of harm to yourself, your family, pets, and beneficial insects.
2. Applying neem to the magnolia will do nothing at this time to affect the scale insects, because the adults are not susceptible to neem (see above).
3. A certified arborist would certainly be the most qualified person to perform the required pruning of a large magnolia such as this. They would probably also have access to a wider range of scale control products and would be more knowledgeable about their effectiveness and application timing. The International Society of Arboriculture (ISA) has an online search tool that can help you find a certified arborist near you: https://www.treesaregood.org/findanarborist

Finally, regarding your concern for the hostas: the sooty mold fungus that grows on the honeydew from the scales, is not particularly harmful to the plants that it grows on. The fungus is feeding on the honeydew, not on the leaf tissue of the hostas. There is some indirect impact from the blocking of sunlight that occurs as the sooty mold covers the leaf, but this can be relieved simply by washing the leaves with plain water or slightly soapy water to remove the honeydew.