Magnolia and oak trees at risk 

By Mary Maguire-Lerman

Our tropical summer with high humidity levels not only brought us a new disease to wipe out our tasty basil plants, we also saw two insect pests thrive and attack special trees. From Highland Park to St. Anthony Park to Como Park, I see dead or dying magnolia trees and shrubs. And I am upset to see that our bur oaks, abundant in Como Park and St. Anthony Park, are also showing tip dieback. 

What is happening?

About 10 years ago, a new insect pest arrived in Minnesota. Known as magnolia scale, it previously was common on the East Coast. However, climate change has triggered many insect pests to relocate. As I walked Doswell and Carter avenues in St. Anthony Park this fall, I saw numerous trees infected with these scale insects; some trees are nearly dead. 

It was in the early 1980s that star magnolias began showing up in garden centers. Minnesotans were eager to enjoy the fragrant, early blooms (late April-early May) these trees provided. In the 1990s, I wrote an article for the Bugle about the various star magnolias as they were being abundantly planted in our neighborhood.

It will be a battle to save these trees today. Applications of dormant oil need to be applied in November when the leaves fall. This dormant oil will suffocate the scales living on the branches. Then, in early summer, an application of a systemic insecticide can be applied to magnolias. Or, every two weeks during the growing season, you can wipe down the branches with cloths soaked in rubbing alcohol to remove and kill the scales.


Insect diseases are harming magnolia and bur oak trees in
the St. Anthony Park area. Photo by Mary Maguire-Lerman

How does the magnolia scale damage our trees? The insect thrives under a scale covering that protects it as it literally sucks the living daylights out of the magnolia. It feeds on phloem vessels that carry nutrients from the leaves down to the roots. Once damaged, the phloem’s nutrients cannot be translocated. The scale insects appear to start at the bottom of the tree and move upward. They are an orange/gold color. When feeding, they excrete honeydew, which drops onto leaves and then forms a sooty mold—black mold—on the leaf. The honeydew attracts bees and wasps, so if it is not bloom season—be aware: The presence of bees and wasps is a sure sign you have scale. I watched one star magnolia lose the lower 8 feet of its branches this season. It will be dead next year.

What else can you do?

Keep the trees as healthy as possible. Magnolias are shallow-rooted and so are very susceptible to hot, dry weather. Keep the soil beneath the trees well-watered and add a layer of mulch to help retain moisture. Examine your magnolia every two weeks during the growing season for signs of scale. If the scale is just arriving on your tree, the rubbing-alcohol treatment is one approach to follow. Otherwise, contact a licensed, reputable tree service to apply treatments. Keep in mind that the adults of magnolia scale are able to fly, so if these pests are not kept under control, they can easily move from one yard to another.

Meanwhile, two-lined chestnut borer is what is causing bur oaks to dieback, some severely, to the point of death. The adult predators are greenish-black metallic wood beetles with two yellow stripes along their backs. The borer larvae feed beneath the bark and destroy both the phloem and the xylem (water-conducting tissues) in the terminal branches where the newest growth occurs. They work back into the branches slowly, eventually killing the branches or the entire tree, usually between one to five years. The 200-year-old oak in front of the St. Anthony Park Library is severely infected and the library is striving to save it. We will know within a year or two if it will survive. A nearby neighbor’s oak is half-dead and will likely be dead in a year. Don’t let that happen to your oaks.

Attacking the problem

How to prevent the infection: Avoid soil compaction under the tree and keep the tree well-watered, especially during dry periods. Do not apply fertilizer or herbicides to the soil area within 50 feet of a borer-infected tree, as this will further weaken the tree. Have your oak trees inspected every three to five years and prune between September and January, when dieback is observed. Do not save oak trimmings for firewood—you will simply harbor the chestnut borers near your trees. Do not purchase oak firewood—you may be bringing borers into your area. 

I had our oak trees pruned regularly every three to five years to remove any dead wood and to keep the branches from hanging over the house. If you see dieback on a neighbor’s oak tree and they don’t seem to be aware of the issue, inform them. 

Oaks infected with chestnut borer can be treated with Emamectin benzoate by a licensed tree service that has tree-injection experience. Oak tree flowers open in early spring and are wind-pollinated, so this is not an issue for bees. 

Please do your part to help save our remnant oak savanna trees here in St. Paul. For more information on two-lined chestnut borers, visit https://www.dnr.state.mn.us/treecare/forest_health/tlcb/index.html

Mary Maguire-Lerman is a graduate horticulturist and lives in St. Anthony Park. 

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