How to Treat Scale on Magnolia Trees
Magnolia scales are scale insects of various colors, from light to dark. Magnolia trees and bushes are susceptible to this scale, not to be confused with black scale, which refers to a very specific species (Saissetia oleae) that attacks citrus and olive trees. Nevertheless, both are unarmored scales and the treatment for them is the same. Magnolia scale insects are mobile only during the crawler, or nymph, stage. They insert their mouthparts into the plant and don't move thereafter, feeding out of the same hole in the tree the rest of their lives.
Look for young crawlers on the undersides of magnolia leaves starting in August when eggs are laid. At 1/25 of an inch, the nymphs are very hard to see without a magnifying lens. Sometimes you can find an infestation by tapping a leaf or twig over a sheet of white paper.
Use a sprayer to hose down crawlers on trees on which you found infestations. Use an insecticidal soap or horticultural oil, or both. Make coverage very thorough, because crawlers can hide under dead adults' exoskeletons and inside bark crevices.
Repeat the application several times during September and October, due to the protracted period of crawler emergence.
Target dormant scale surviving through the winter by spraying the tree with horticultural oil in late winter. The lack of leaves helps you to be thorough in your coverage. Make sure the temperature is above freezing for the oil to be effective.
Drench the soil at the start of May with an imidacloprid-based insecticide, such as Merit 75 WP or Bayer Advanced Tree and Shrub Insect Control. Pour the insecticide into the soil within a foot of visible roots or the trunk.
Soak a toothbrush in a mild detergent solution and scrub the waxy shells of the adults off the branches. The detergent should make them slippery and fall off. If you want to be environment-friendly, use a 30-to-1 dilution of Simple Green nontoxic all-purpose cleaner. The shells will start appearing in late spring or summer and become quite large, easy targets.
- When you select a magnolia to plant, it helps to know that certain kinds are rarely infested by magnolia scale: sweetbay magnolia, bigleaf magnolia and umbrella magnolia.
- Check the tree for any evident ant population. Ants eat scales' predators for the honeydew that the scales secrete. Attack any nearby anthills by using boiling water with boric acid dissolved to saturation. Or soak pipe tobacco in water overnight and pour the strained liquid into the anthill.
- Consider creating an ant barrier to the tree. Form a collar of inverted duct tape around the trunk, or use Tanglefoot, also an insect adhesive. Avoid using diatomaceous earth, because it could kill the scales' predators as well.
- Multiple years of heavy scale infestation can kill a magnolia tree from regular depletion of its sap. Getting rid of the problem is therefore a serious matter.
- Simple Green nontoxic all-purpose cleaner
- Insecticidal soap
- Horticultural oil
- Merit 75 WP or Bayer Advanced Tree and Shrub Insect Control