Hard, shell-like bumps are insect infestation on Magnolia trees

Published Sep 2, 2018 at 9:00am

Cornell Cooperative Extension’s Horticulture Hot Line has received several calls recently about small hard shell-like bumps seen on magnolia tree branches. This is actually an insect infestation called Magnolia scale.

The Magnolia scale starts off as a soft bodied insect, as it ages its exoskeleton hardens giving it a disc shell scale appearance. 

Magnolia scale has one generation per year. They overwinter as nymphs (crawlers) on 1- to 2-year-old tree branches.

The insect will reach maturity during the month of August. The nymphs (crawlers) mature in early June.

The adult females will give birth to living young. The young crawlers will remain under the parent scale covering for a short time.  

The adult females are shiny brown, elliptical and convex in shape.And about ½ inch in diameter. The crawlers are yellow to a reddish-brown in color and are elliptical in shape.

As they mature, they will secrete a white mealy wax on the outside of their bodies. The white mealy wax tends to disappear by late August.  

As the crawlers feed, they suck sap from the tree and excrete a large amount of drippy, sticky honeydew.

The honeydew most likely will attract the sooty mold fungi by providing a medium for these spores to grow. The sooty mold may cover leaves and branches displaying a black, sooty appearance.

The sooty mold could stain lawn furniture that is placed near the infested tree.

One may also notice the presence of wasps and ants on the tree since the excreted honeydew contains sugars which may attract them.

A large infestation of Magnolia scale may cause slow tree growth, weaken branches and cause branch dieback that could eventually lead to tree mortality. Mother Nature can help to control the scales by visiting parasites and predators from nature.

For a light infestation one can remove the scale by hand. This is best done in the month of July before crawlers emerge. Use a scrubbing brush and rub gently to remove them. Make sure to wear rubber gloves when removing the scale. Place in a bag and place in the garbage to dispose of. 

For a large scale infestation, most scales can be controlled by insecticidal sprays applied at the correct timing.

The insecticide “horticultural oil”, dormant spray is recommended to be applied before tree bud break in the spring. And in October foliar sprays of other insecticides may be applied 7 to 10 days apart for severe infestations. The crawler stage is the most susceptible to insecticides in the fall. Be sure to read label carefully before applying the insecticide and horticulture oil.  

For more information please visit our website at cceoneida.com or call our horticulture hot line at 315-736-3394 or visit the agency’s website — cceoneida.com