When small crusty bumps or patches appear on the stems and leaves of plants, the symptom is often mistaken for some kind of fungus or other plant infection. In fact, these seemingly inanimate deformities often signify an infestation of scale insects. Scale insects are rather tiny insects that protect themselves inside a waxy coating as they feed on the juices of plant stems, leaves, or fruit. The waxy protective coating is what forms the crusty patches or bumps you see.
There are more than 8,000 different species of scale insects, all of which are members of the insect superfamily Coccoidea. One of the more troublesome species is magnolia scale (Neolecanium cornuparvum), which is one of the largest of the scale insects at nearly 1/2 inch in length. Magnolia scale is often evident from the bumpy areas covering magnolia twigs, due to a sticky residue that is produced by the insects, or from the general decline of the tree.
What Causes Magnolia Scale?
Magnolia trees that experience stress are more likely to suffer from scale infestations. A tree that doesn't get enough moisture is especially susceptible, as are those suffering from other diseases. For reasons that aren't clear, trees that receive excess fertilizer in an effort to improve their health seem more likely to get serious cases of scale.
Lifecycle of Magnolia Scale Insects
Magnolia scale insects spend the winter as dark nymphs clinging to small twigs on magnolia bushes and shrubs. They begin to feed and grow in the spring. The insects mate in late spring, then the males die and the females continue to enlarge through July. The females give birth to live young (crawlers) in August through September, which then crawl about to seek a spot to feed on plant stems and spend the winter. It is during this juvenile crawler phase that magnolia scale insects are most vulnerable to pesticides and other control strategies.
Damage Caused by Magnolia Scale
Magnolia scale will be evident from the bumpy patches on a magnolia's twigs, but a minor infestation usually does not seriously affect the plant's overall health. A major infestation, however, can drain a tree of enough sap to seriously stress it, causing the leaves to yellow and the twigs to die back. Excess sap drained by the insects is excreted in a honeydew substance that can foster the growth of sooty mold, which will, in turn, draw ants, wasps, and other insects to the tree. The sooty mold itself is not harmful to the tree, though it does serve as an indication that scale insects are present. Star magnolia, saucer magnolia, and the many hybrids are the types of magnolia trees that often see the most severe damage from scale.
5 Ways to Get Rid of Magnolia Scale
Pick Insects by Hand
There are two types of scale insects—armored scale and soft scale—and magnolia scale falls into the soft scale group. It's possible to pick off the insects by hand when you spot them. The bumps you see on twigs are actually nymph-phase insects, which can be picked off fairly easily in the spring. You can also pick off crawlers when they emerge in late summer and early fall.
Physical removal of insects is most practical with smaller shrub-sized plants, where you can reach most branches from the ground or using a stepladder. It becomes harder, if not impossible, with large mature trees.
Spray With Horticultural Soaps or Oils
A variety of horticultural soaps and oils, such as neem oil, will work to smother insects at the crawler phase in late summer, but they do not work on the adult insects present at other times throughout the year. However, you will need multiple applications to fully control the infestation, so it does take a bit of a commitment. There are also dormant oils that can be used to coat the overwintering nymphs in early spring (before March) before they begin to move around.
Remove Affected Branches
If the infestation is limited to a few twigs or the ends of branches, these stems can be pruned away and destroyed to halt your scale problem. This solution works best if you catch the infestation at a fairly early stage.
Spray With Pesticide
Chemical pesticides can be used to kill scale insects at the crawler stage. However, once the scale insects move past the juvenile stage, their hardening exoskeleton makes them quite resistant to contact with pesticides. Thus, late summer is the best time to spray for scale—September is a traditional time to spray for magnolia scale. Apply the first spray treatment in early September and follow up with a second treatment 10 to 14 days later.
Scale insects killed by pesticides do not generally fall off the tree, but the dead insects can be brushed off by hand or sometimes with hard blasts of water.
Large-scale chemical spraying of a mature magnolia tree is no easy matter and there is the potential for the chemical to drift about on air currents. While a homeowner equipped with an extension ladder and sprayer is capable of such spraying while wearing a respirator and protective gear, spraying a large tree is usually done by a tree-care professional. Because chemical pesticides will usually also kill beneficial insects such as ladybugs and honeybees, many university experts recommend horticultural oils as a better method of controlling scale.
Use a Systemic Pesticide
Systemic insecticides, such as those containing imidacloprid, can also work to control magnolia scale control. Drenching the root zone in early May gives the best results for this method.
How to Prevent Magnolia Scale
The best preventive measure for scale insects is to keep the tree in good health—especially by keeping it well watered and mulched to protect soil moisture. Prune away diseased or damaged limbs as soon as you notice them.
Avoid over-fertilizing these trees, as it seems to encourage scale insects to attack.
Do scale insects have natural predators?
Ladybugs and lacewings are natural predators of scale insects at the beginning of their larval "crawler" stage. Homeowners can research ways to enhance their garden space to invite in these natural predators to help keep pest pressure down from season to season.
Can magnolia scale spread to other trees?
This particular scale insect is specific to magnolia and tulip trees, and it will not spread to other species.
What chemical pesticides kill magnolia scale?
Bifenthrin, carbaryl, cyfluthrin, malathion, and permethrin are among the spray pesticides known to work on scale insects.