30 year plus white star magnolia w/scale BIG TIME
Confirmed the ailment by bringing a sample branch to a local nursery but they were vague with amounts of imidacloprid I should use. Tree is at least 13 feet tall, there is some "black soot" on some leaves. Someone suggested I power wash the tree to knock off the scale & then treat. Am told this is the best time of year to treat it. How much imidacloprid powder do I mix w/water & how often do I use it? My common milkweed plants are right next to Magnolia--is this why that sticky "honeydew" is on milkweed & flies are stuck to those milkweed leaves? The infestation of scale is ALL OVER all the branches, even high up--should we just whack down the tree? Do we then treat the dirt it has been growing in all these years before we plant something else? Thank you.
Certified GKH Gardening Expert
No need to cut down the tree to control the Magnolia scale, (or it could be tulip tree scale on magnolia). In either case its a large, soft-bodied scale insect that puts out lots of honey dew and is quite likely the cause of the sticky sap on your other plants and surfaces. It requires chemical insecticide control, I don't think power washing will do it and may harm the tree.
If you have the resource and budget, the fastest avenue would be to use a tree service that provides pest control.
To do it yourself with materials from the local nursery, Imidacloprid only treatment now is not appropriate for an active advanced infestation. It is very slow to take up into the foliar canopy, at least a month. And even then it is not the best material to get control of an active advanced infestation. It's better for prevention in a long term program.
A do it yourself option is to start a repeat spray program to target the immature stage of the insect life cycle, called "crawlers." They are vulnerable to contact sprays but the adults are not, because the adults are protected with the turtle like coverings. There should be a late summer crawler stage now and into early September. If you spray with summer horticultural oil, add insecticidal soap or a pyrethroid insecticide to the spray mix. You will kill the crawlers and limit the next generation of adults developing to feed on the tree. It's not fast and won't knock out the adults now, but it will eventually work. Spray every two weeks through mid September. A good backpack sprayer should reach the top of your 13 foot tree. Start again in early spring, or use the soil drench method.