Recently we received a question from someone about what to do about a severe scale infestation on some magnolias in their yard. We thought that it might be helpful to share that information with you.
There are a variety of different scale species and scale affects a variety of ornamental plants such as hollies, boxwoods, crape myrtles, and palms–not just magnolias. Different scale species attack different hosts, but the general guidelines regarding control remain the same.
What is scale?
Scale is an unusual insect that looks very different from most insects. Because of the unique features that give it the name ‘scale’, it can be very hard to treat. Once plants have it, it can be very difficult or impossible to get them ‘clean’ again.
The common name of scale refers to the appearance of the adult female insects (males are rarely seen). In fact they don’t look like insects at all, just a round bump resembling a fish scale on the plant stem. They can be various colors but many times are white. These are adult females that are hunkered down in place and are very hard to kill. They have a waxy coating/shell which gives them the scale appearance and they typically do not move. Nor do not have any visible wings, legs, etc. They don’t look like an insect or even alive.
Don’t let their odd or benign appearance fool you though. They are busily sucking the sap and life out of your plants. As with many pests, a healthy plant grown in an environment that isn’t sprayed with pesticides typically won’t have a harmful level of scale infestation. Predators such as ladybugs and parasitic wasps keep them under control.
Unfortunately, sometimes it can get out of hand and need to be controlled by you. This was the case with this magnolia. Despite being grown in a no spray environment, due to terrible weather circumstances, the tree was very stressed. It was planted in a poorly drained location during a season with too much rain and a lot of flooding. Scale can also be a problem that gets out of hand quickly in a controlled environment such as on houseplants.
So how do you control scale?
The short answer: with difficulty. In fact, if the plant isn’t valuable, disposing of it is probably the best course of action. It is best if you can prevent an infestation rather than trying to treat one. When you purchase new plants it is worth while to check them over thoroughly first. Avoid buying them if they have any signs of scale.
If you have a houseplant with a light infestation, you may be able to control it by carefully washing the plant to remove all of the scale insects. We would also recommend removing and destroying the most infected branches/leaves if possible. If you decide to take this route, be sure to quarantine the plant and watch it carefully for the scale to return. Do not place discarded plants or infected branches in the compost pile.
What if its a large shrub?
If you are unfortunate enough to have a scale breakout too big to be controlled by wiping them off manually, your best chance of treating scale is when the baby scale known as ‘crawlers’ are active. This is because adult scale is very difficult to kill. It does not move about on the plant to come into contact with surface applied insecticides.
The crawlers are the ones feeding and doing the damage to your plants. However, until they start to come out from under their mother’s protective waxy covering, they are also pretty immune to being killed by a non-systemic spray.
The stage at which the crawlers are active is also when the scale on one plant can be transmitted to another plant. The crawlers can walk or be blown on the wind to other plants. Additionally, they may even be carried by ants to other plants in your yard. Ants wish to farm the scale for the honeydew they produce and may transfer scale to new plants to start a new colony.
The timing of crawler activity varies depending on the scale species and your location. It is important to accurately identify what species of scale you have so that you know when crawlers will be active. University of Wisconsin has an informative article about various different types of common scales that can help you determine what kind of scale you have and when crawlers would be active.
The crawlers are very tiny and you will need a magnifying glass to see them. Ideally when it gets close to the time crawlers should be active, you would monitor with a magnifying glass or tape traps. This will allow you to know when to start spraying to achieve good control.
Once you see them start to emerge its time to start spraying.
Once you notice that the crawlers are active, the whole entire plant should be smothered with horticultural oil. They tree/plant needs to be sprayed very, very thoroughly and applications may need to be repeated if you continue to see active crawlers. It is important to make sure every surface on all the branches and leaves are drenched. Do not just spray the outer portions of the plant. All the branches deep inside the shrub need to be thoroughly sprayed as well. The insects need to be coated with the oil to be smothered. They are typically hiding deep within the plant. If you do not ensure that every surface of the plant is drenched, you will miss killing a large portion of the insects.
If you are having problems with scale infestations, dormant/horticultural oil should also be sprayed in the winter. Well timed and very thorough spraying of horticultural oil on the crawlers and a dormant winter spray usually provides very good control of most scale. This treatment method would be the lowest toxicity, most environmentally friendly way to control scale. Keep in mind, you will likely need to treat a large scale infestation for several years to fully control it.
Please note before you spray:
Oils can severely burn plants when applied in hot weather and in direct sun. If you sprays oil in the summer it should be done on a cloudy day when the temperature is below 90 degrees. Alternatively, spray in the evening when it will have time to completely dry before the sun gets on the leaves. It should also not be done below 32 degrees. Take this into consideration when deciding when to spray your dormant spray. It is also always a good idea to test spray a few leaves prior to spraying the entire plant.
If that is not enough to beat your scale problem
The best cure for scale is probably a combination of oils and a systematic insecticide. There are systemic insecticides such as Imidacloprid that are labeled for some species scale. Insecticides such as Imidacloprid are applied as a soil drench.
These pesticides are often restricted to licensed applicators. It is highly advised to speak to local extension agents or an professional pesticide applicator for advice before applying any pesticides. It is essential to seek positive ID of your scale species and recommendations for an appropriate pesticide from a professional before spraying harmful chemicals. Please also remember that these systemic pesticides WILL kill bees and other pollinators that feed from the plants, so do not spray them on plants that are or will be flowering.
Please also keep in mind that foliar sprays of broad spectrum insecticides like seven, Malathion, or permetherin are NOT RECOMMENDED. They are not effective on scale and are highly toxic to beneficial insects and can cause outbreaks of other pests such as spider mites.
Also ridiculous as it sounds
If there are tons of ants on the scale infected tree, control the ants. The ants want the honeydew made by the scale so they will ‘farm’ the scale. They will protect it from things like the parasitic wasps which normally help to keep it in check.
Lastly, if the tree branches or leaves have turned black during this infestation, this is sooty mold on it. It is often also seen with aphid infestations. The sooty mold is growing on all the honeydew from the scale feeding. If the scale problem is fixed, the sooty mold should resolve itself. The black might look alarming, but isn’t really harmful to the plant. It’s just ugly. Don’t go crazy spraying for or worrying about the sooty mold. The black gunk probably won’t fully disappear until the tree has shed and replaced its leaves. Just focus on getting the plant healthy. Removing the scale will resolve the sooty mold problem.
We hope this article helps you to know what to do if scale ever shows up in your yard.
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