Asked July 02, 2018, 4:59 PM EDT
Hennepin County Minnesota
Hello and thank you for contacting AaE for help.
You have what appears to be magnolia scale which has been taking hold in Minnesota over the past few
years. Magnolia scale is caused by an insect that feeds on the foliage, twigs and branches then matures in late
July/early August. Crawlers hatch in late August or early September.
Crawlers settle on the young twigs where they overwinter.
Their waste product is called honeydew. It lands on anything below them and because it is sweet, it attracts all kinds of other insects which is what you're seeing.
It is very difficult to treat but left untreated. One of the difficulties is that different options have been used then abandoned as those options have been banned as too dangerous to the environment. Additionally, timing and the method used is very important. At this time, spray treatment is not recommended because there are other plants around and the insecticide can harm or kill other non-targeted insects. Horticultural oils applied in autumn can smother overwintering nymphs.
Your best bet is to contact a certified professional arborist of which there are many good ones in the metro area to assess the problem and provide the correct treatment. Some treatments can only be applied by trained arborists.
It won't be an easy fix and may actually take several years to clear up. Even if you do get it cleared up, other magnolias may reinfect your tree so at that point you may want to consider a yearly prevention plan.