Spring beauty is fleeting, and so is the opportunity to stop scale insects that live on the bark of magnolia trees and suck their sap. “With scale insects, timing is everything,” said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle.
For most of the year, adult magnolia scale insects lie still and flat against the tree’s trunk, protected by a hard shell and a waxy coating, impervious to insecticides. But in spring, young, soft-bodied new insects, so small they are barely visible, do not yet have that shield.
More Top Picks Free Dyslexia Screening Tools You Can Use At Home
“If you spray the tree with horticultural oil while the young are still tiny, you can smother them,” she said. This will only work during a period of a few weeks in spring when these “crawlers” are moving about and vulnerable.
The effects of magnolia scale are usually spotted in summer when it’s the wrong time to treat the tree. “What most people notice first is an ugly black mold on a magnolia tree’s branches in June or July,” Yiesla said. The mold is not harmful in itself, but it grows on the sticky, sweet honeydew excreted by the insects. That makes the mold on a branch a strong indication that you will find scale insects somewhere up above.
A large infestation of scale insects can stress or even kill a magnolia tree over time.
The insects appear as half-inch-long bumps lined up along branches. Since adult scale insects never move, they might seem like sitting ducks. But insecticides aren’t effective against the adults because they cannot penetrate the insects’ waxy coating. The insects are only vulnerable at two points in their yearly life cycle. In early spring, spraying with horticultural oil can smother juveniles that have not yet developed their wax shield. “Do not use any oil other than horticultural oil,” Yiesla said. “The wrong product can damage or kill your tree.”
In August, there will be a chance to attack the next generation. At that time, “crawlers” newly hatched from eggs set out across the tree’s bark to find a place to settle down and suck sap. They are vulnerable to a contact insecticide that will poison them.
Before you spray with dormant oil in spring or with an insecticide in August, get expert help to confirm that scale insects are the source of your problem and to make sure you use the right product at the right time. Don’t spray a magnolia unless you have confirmed it is infested with scale. Spraying will not protect a tree from becoming infested with the insects.
“Spraying a tree, or any plant, with the wrong product or at the wrong time will not solve the problem and can be harmful to the tree and to beneficial insects,” Yiesla said.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.