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Friday Garden Goods: Treating Magnolia Scale

Scale is a common pest that appears like a white, fuzzy mold on the branches of trees and shrubs. Despite its appearance, scale is armored insect and is one of the most successful plant eating insects. It is most commonly found on magnolia trees. When magnolia scale is detected in the early stages, this is an easily corrected problem. Larger infestations may require repeat treatments with the correct insecticide, applied at the right time.

Female scale insects look like small oval, legless bumps. The males are minute yellow-winged insects (a few millimeters in size). On many plants, scales are the most serious pests, and most plants are susceptible to scale insects. Scales cause damage by sucking the sap from the plants. Heavily infested plants appear unhealthy and produce little new growth. If the scales are not controlled, leaves will drop prematurely, sometimes killing entire branches and sometimes entire trees.


Female scale insects produce eggs, which hatch soon after laying, while others are live-bearers that lay crawlers. Within a few hours male and female crawlers move off in search of new sites to colonize. They usually settle within a few centimeters of the parent but can travel up to 100 meters. They can be dispersed by the hands and clothing. The crawlers settle on a new site within a day or two and begin feeding and start to grow a protective cover. Once in place, they do not move again. You can identify scale insects by looking on the undersides of leaves and around leaf joints. Scale-damaged plants look withered and sickly and may have sticky sap or a black fungus, known as sooty mold on the leaves and stems.

Scale tends to colonize most heavily on the underside of leaves and close to the trunk. On Magnolia they can be found often on the trunk, as it is very soft. The scales have a hard protective shell which reduces the effectiveness of many surface contact sprays. Surface sprays, such as white oil, need to be applied so as to cover the entire plant, especially under the leaves.

The traditional remedy is a systemic insecticide or horticultural oil. They work by bonidesystemicinsectsmothering and suffocating the insect and by killing those in the crawler stage on contact. Dry, systemic insecticides such as Bonide Insect Control remain effective for about 6 weeks while horticultural oils remain effective for 1-2 weeks. Horticultural oils main disadvantage is that it will damage flower buds and may also damage sensitive leaves. Repeat spraying may also be necessary. It is critical treat the infected plants when the insects are moving, outside their hard shells. This occurs in early spring and late fall.  We recommend using Bonide Systemic Insect Control to treat scale.

Scale can be difficult to kill because their waxy hard shell coating prevents the white oil from penetrating during some of the growing season. Break the breeding cycle and eradicate scale by spraying every 2-3 weeks to kill the emerging larval scale insects before they form their protective coating. The adult scale dies once it has bred and spawned. It is the young scale you need to target for 2-3 weeks and after, if there are no signs of more young, keep spraying for at least another 3 week cycle to ensure you have got them all. One surviving scale insect will re-infest the plant.

Neem Oil is another option which kills by suffocation but scale insects will not drop off when they die. Their hard shell stays attached to the stems even when dead. Whichever solution you use, a week after a thorough spraying take a sponge and a weak detergent solution to wipe off the dead scale and any remaining chemical.

Organic alternatives are available. You can physically wipe scale from the plants leaves. Remove scale insects with a twig or your fingernail. They will scrape off of plant tissue easily. The dish soap is toxic to the scale and will break down the armor coated shell. The vegetable oil helps the soap to stick and smother the scale. Soft scales excrete large amounts of honeydew, which is rich in nitrogen compounds and sugars. The honeydew excretion promotes the growth of a fungus known as sooty mold. This black/grey colored fungus coats the topside of leaves, interfering with photosynthesis and makes the plants unattractive. Armored scales do not excrete honeydew.

Black sooty mold is often a result of the scale as it is the secretion of honeydew from the scale that causes the sooty mold. The scale secretes honeydew as it feeds on the sap and that honeydew residue falls and is spread around the foliage of the plant. Once the scale insects are dead, the mold will dry and eventually wash off.

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