Scale insects attacking a star magnolia

Email the author | Follow on Twitter
on March 19, 2007 at 11:28 AM, updated March 19, 2007 at 11:30 AM

By George Weigel/The Patriot-News

Q: We have a star magnolia tree that we don't want to lose due to its location - front yard, it provides a screen for our bedroom window, and it is really pretty in the spring. Last year I noticed insects swarming around it, but I ignored them. Later I found swollen "things" that when squeezed, bled considerably. I did a lot of squeezing. I await this year with dread. Do you have any idea what's the problem?

A: Sounds like a classic case of magnolia scale - the biggest of all scale insects. These pests latch onto selected spots on the twigs and branches and proceed to build a shell around themselves for protection. These are the "swollen things" you saw last year. They can be nearly the size of a pea and do squish nicely when you pinch them. That's a good organic control, by the way, but you could be out there squeezing all day in a bad infestation. (Hope you didn't just eat lunch.)

Scale fake people out because they seem to be more of a disease or tumor than a bug. We're used to seeing bugs crawl around. Baby scale (known as "crawlers") do crawl around while they're searching for a home to feed. That's the best time to attack them - when they're young and unprotected.

In our climate, sprays are most effective in late August through early September. That's the time when females have laid eggs, and the eggs are hatching into a new crop of crawlers. Horticultural oil, neem oil and insecticidal soap are three effective and non-chemical choices to fight magnolia scale when you time it right. I'd suggest two and maybe three treatments 7 to 10 days apart. Also helpful is a dormant oil spray in late winter before the buds have begun to open. It might be getting a little late for that now. Check your tree to see how far along bud development is. You don't want to spray oil on flower buds that are already opening. A chemical alternative is imidacloprid (Merit), a systemic insecticide that you mix up and pour around the base of the tree in early spring. One treatment gives you season-long protection throughout the tree.

Keep an eye on this pest because you probably won't eliminate it for good with a single-year treatment. On the other hand, scale can run in cycles, so just because you had a bad outbreak one year doesn't mean you're doomed year in and year out without regular spraying. Left untreated, though, scale can kill a magnolia. It's one of those insects that do more than just cosmetic damage.