Harmful insects

Magnolia Under Attack

Magnolia soft scale (Neolecanium cornuparvum). Photo: http://www.ontree.ca

Question: My magnolia has white bumps on its bark and a lot of insects like ants and wasps seem to be visiting it (looking for food?). Is this a problem? What should I do?

Marlena Nappi

Answer: The visiting insects are not a problem as such, it’s what they’re coming for that is worrisome. They’re probably looking for honeydew, a sugary liquid given off by sucking insects, and that means the white bumps on your magnolia’s bark are likely scale insects.

Magnolias are grown for their spectacular bloom. Here Magnolia × loebneri ‘Ruth’ . Photo: http://www.newplantsandflowers.com

Magnolias (Magnolia spp.) are usually quite resistant to insect pests, but magnolia scale, also called magnolia soft scale (Neolecanium cornuparvum), is an exception. This insect, once limited to North American magnolia species*, especially cucumber tree (M. acuminata) and southern magnolia (M. grandiflora)—other American magnolia species, like M. virginianaM. macrophylla and M. tripetala, are rarely colonized—was therefore also restricted to their natural territory, that is, eastern United States as far west as eastern Texas in the south. And it was considered a minor pest, doing little damage. 

*It has been claimed to affect American tulip trees (Liriodendron tulipifera), which are in the magnolia family, but some entomologists believe this is a different scale insect.

However, magnolia scale has since switched its allegiance to imported magnolias, like star magnolia (M. stellata), lily magnolia (M. liliflora), and various hybrid magnolias, like the highly popular saucer magnolia (M. × soulangeana) and the very hardy Loebner magnolia (M. × loebneri). It seriously weakens these species and can even kill them. They seem to have no resistance to magnolia scale.

With magnolias becoming more and more popular as garden plants, magnolia scale has spread beyond its original range, and is now common well into southern Canada and is making serious inroads into the US Midwest. The original spread was apparently due to infected nursery plants, but is now self-sustaining. 

One Big Bug!

Magnolia scale can show up in impressive numbers. Photo: Rebecca Bolestra, nwdistrict.ifas.ufl.edu

Magnolia scale is huge for a scale insect: ½ inch (12.5 mm). It looks like a tan or whitish to pinkish bump on trunks and branches where it accumulates in great numbers. 

The honeydew it secretes often leads to sooty mold, a black powdery-looking fungus and leaves and branches. It’s often the sooty mold that alerts the tree owner to the presence of a problem. Either that, or the presence of ants and wasps feeding on the honeydew. 

Also, leaves yellow and drop off and smaller branches die. Growth is stunted and flowering is reduced. Heavy infestations kill the host tree.

Life Cycle

Eggs hatch out of sight under the female scale’s protective covering in mid-August to mid-September. At this point, the female is at her full size and often takes on a pinkish tinge. The nymphs emerge in waves over about a month at the end of summer and early in the fall, usually late August through September, although that will vary from year to year depending on the weather.

In late summer, removing a female scale insect often reveals the crawlers hiding underneath. Photo: onnurserycrops.com

The brown-colored six-legged nymphs, called crawlers at this stage, are the only mobile stage and soon leave their mother, wandering to other parts of the magnolia over a 48-hour period. They are hard to see, being smaller than a pinhead. They are wingless and unable to fly, but at this stage, are often carried to other magnolias by insects, birds or pruning tools. Once they settle down, they pierce the thin bark and start drinking sap, then lose their legs. Bt now, they are oval, with a waxy coating, and 3/64 to 5/64 inches (1 to 2 mm) long. 

The females die after releasing the nymphs, but their shell can remain fixed to the bark for weeks, making it hard to determine whether any given treatment was effective.

After winter dormancy, the nymphs molt and increase in size, forming large rounded domes on the bark. The females become covered in white powder. The males, only 1/8 inch (3.5 mm) long, form a flattened dome and turn translucent white.

Around the beginning of July, males metamorphose into gnatlike pink to yellow winged insects. After mating with the females, they die, and females lay their eggs.

Then the cycle starts all over.

Treatment

Always carefully inspect magnolias before purchase. Photo: http://www.wilsonnurseries.com

Since magnolia scale mostly spreads through the transportation of contaminated nursery plants, carefully inspect all magnolias plants before purchase. If you do this and there are no other magnolias in your neighborhood, your magnolia will likely forever remain scale free. 

Brush infected shrubs with a soft brush dipped in soapy water, removing all the scale insects, dead and alive. If the branches are too high, sometimes you can knock them off with a good spray of water. They’re easiest to remove in midsummer, when they turn pinkish just before releasing the nymphs. Prune off any severely infected branches, disinfecting the pruning shear between each cut with 70% rubbing alcohol (isopropyl alcohol). 

Horticultural oil is often used to control magnolia scale. Photo: http://www.produitssuperieur.com

Early in spring, before the leaves open, spray your magnolia with horticultural oil (dormant oil). Look for a period when daytime temperatures will be cool but above freezing for 3 to 4 days in a row (54 to 60 °F/12 to 15 °C would be ideal) with no risk of rain for 24 hours. Make sure to coat all parts of the magnolia evenly.

You can also treat young nymphs with insecticidal soap, therefore in late summer and early fall as they emerge or have just settled down. Spray weekly for 6 weeks to ensure reaching all of them.

In the future, avoid overfertilizing magnolias, especially with high-nitrogen fertilizer (lawn fertilizer), as this leads to tender shoots scale insects can more readily infest. 

Finally, once you have the initial infestation under control, inspect your magnolias annually, especially if there are other magnolias in the neighborhood.

_____________

Magnolia scale: surprisingly large, surprisingly common and not easy to control. Keep your eyes open, as it’s far easier to nip an early infestation in the bud than to bring a full-fledged one under control.

Larry Hodgson is one of Canada’s best-known garden communicators. After studies at the University of Toronto and Laval University where he obtained his B.A. in modern languages in 1978, he succeeded in combining his language skills with his passion for gardening in a novel career as a garden writer and lecturer. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He is a regular contributor to and horticultural consultant for Fleurs, Plantes, Jardins garden magazine and has written for many other garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening, Rebecca’s Garden and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 50 other titles in English and French. He can be seen in Quebec on French-language television and was notably a regular collaborator for 7 years on the TV shows Fleurs et Jardins and Salut Bonjour Weekend. He is the President of the Garden Writers Association Foundation and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. An avid proponent of garden tourism, he has lead garden tours throughout Canada and to the gardens of over 30 countries over the last 30 years. He presently resides in Quebec City, Quebec.

3 comments on “Magnolia Under Attack

  1. I saw this up close and personal this summer while working on a county project. Their tree was so covered and the leaves turned black that they are removing it rather than treating.

  2. Various scale are common on deciduous magnolias here, and sometimes become a serious problem, but a particular species of scale is killing Southern (evergreen) magnolias in the Los Angeles region! We have never seen anything like it.

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