(From January/February 2021, Northern Gardener) By Don Engebretson
If you have magnolias in your landscape, there are two things I know about you: You adore these fabulous spring-blooming trees and would be heartbroken if you lost them. That’s why it’s important you learn about magnolia scale (Neolecanium cornuparvum) and how to detect and treat an outbreak of this devastating insect.
What Is It?
Adult magnolia scale females are pinkish-orange to shiny brown, oval, convex and up to ½ inch in diameter. Attaching themselves to newer (one- to two-year-old) branches (not leaves), at a glance they can be mistaken for buds. Males impregnate females in spring, then die. Females lay eggs in June. Midsummer, they begin to secrete a whitish waxy substance that covers their entire body. Eggs hatch in late August, producing tiny, gnatlike insects. This is the “crawler” stage when insects move about the landscape, increasing spread within an infested tree and spreading to new trees. Crawlers may appear as late as October.
Insects enter the nymph stage and overwinter on branches. They mature in spring and the one-year cycle begins anew. This is a “soft” scale insect (versus hard); the whitish, elliptical shell of the mature insect is not the insect itself but a waxy coating developed as a protective measure.
Adult females and nymphs insert their sharp, sucking mouthparts into tender bark to tap plant sap. Loss of sap as the infestation grows causes loss of energy resources. The stress causes leaf yellowing and loss, branch dieback and canopy thinning. If left untreated, magnolia scale can greatly harm plants in one to two seasons and injure them to the point of no return in three.
Magnolia scale has been on the radar of university and industry experts across North America for the past 20 years. As with most harmful insects, infestation is cyclical, worse some years than others. Unusually warm summers with abundant moisture are ideal conditions for increased magnolia scale activity. All magnolia varieties sold in the northern zones are susceptible.
A second-year infestation will produce visible honeydew and sooty mold in summer. Honeydew—black, sticky, oil-like—is the excess sugars in the sap excreted by the scale insects. It will cover and drip from leaves and be visible dripping on mulch, walkways, vehicles or anything beneath the plant. Sugar-loving insects—bees, wasps, flies and ants—will often be present in high numbers.
Sooty mold is an aptly named, dark brown to black fungal growth caused by honeydew that discolors all or a portion of leaves. Since the presence of honeydew and sooty mold are most often what alerts homeowners to a magnolia scale infestation, I sense reeducation on the part of homeowners is called for.
How many times have I said it in the pages of this magazine—to be a gardener means closely observing all your plants several times per week. Buy a good quality magnifying glass, as essential a tool to a gardener as a hand pruner or spade. Be it insect infestation or fungal disease, early detection is key.
Early detection starts at the nursery. Bring your magnifying glass with you when purchasing a magnolia tree. Nurseries in general do a good job of monitoring their plants for pests, but they also miss them. Be certain you are purchasing a pest-free plant.
Closely observe your magnolia(s) before leaves bud in early spring. Females from the previous year are dead, their white, waxy coating is gone and they appear dark tan. Nymphs that have overwintered will be visible through the magnifying glass.
Now monitor weekly. As temperatures warm, overwintering nymphs mature, females lay eggs and begin creating their white convex shells (visible to the naked eye). Remember, initial infestation will occur in late summer, so with the magnifying glass be on the lookout for pinhead-size, black to reddish insects in the crawler stage August through October.
If you catch the infestation in the initial, late summer crawler stage, blast the insects off with a sharp stream of water from the hose. Repeat as needed. Another option is to spray the plant with an all-purpose organic chemical or synthetic chemical insecticide. Neem oil is a good organic spray to use, malathion an effective synthetic.
If adult scale is detected—the white shells—insecticides aren’t very effective. Try scraping the insects off branches with a gloved hand or Popsicle stick. Adults cannot climb back on the tree. If it’s only a few branches, another option is to prune these branches off.
The worst-case scenario is one I faced in a client’s yard four years ago. Here’s what it looked like and here’s what I did:
Late August, I’m in a client’s gorgeous backyard for the first time since the previous summer and walking past the 8-foot ‘Ricki’ magnolia I planted five years before. I see black honeydew dripping from leaves. A full one-quarter of the tree is infected, heavy white shell growth along one- and two-year-old stems, crawlers hopping around here and there. This is a borderline heavy infestation that originated one year ago.
The first thing I did was spray the plant with an insecticide to kill the crawlers. Then I went to a local nursery and purchased the chemical imidacloprid, sold under a variety of brand names. I mixed it according to label directions as a soil drench, pouring 2 gallons of the water/chemical solution in a 3-foot diameter circle around the plant. The insecticide is drawn into the plant via the roots. In late September, I returned and sprayed the tree with petroleum-based organic horticultural oil to ward off any late arriving crawlers.
In early May the following year, I did a second imidacloprid soil drench. The tree bloomed sporadically and was slow out of the gate that season. The next year—two years after the fall treatment—‘Ricki’ was the picture of health, as it is to this day.
The eagle-eyed homeowner now makes sure of it.