Ew, scale insects! Whether flat against leaves or fruit, or lumpy bumps on branches or stems, this widespread superfamily of insects is well over 8000 species strong. Many of them are agricultural pests, while others prey on trees or other plant life.
But they’re all sap-sucking invaders who can spread a wide variety of plant diseases, and nobody wants to discover them on their plants! Today, we’ll go over a variety of these insects, and I’ll tell you how to get them out and keep them out of your green spaces.
Listen to this post on the Epic Gardening Podcast
|Common Name(s)||Scale insects, scale pests, scales, mealybugs, etc.|
|Scientific Name(s)||Over 8000 species with unique names|
|Family||Coccoidea superfamily, multiple families of insects beneath that|
|Plants Affected||Most food crops, ornamental plants, trees, and grasses|
|Common Remedies||Horticultural oils, neem oil, AzaMax and other azdirachtin products, etc.|
Types Of Scale Insects
There are about 8,000 types of these plant pests. All of these fall into the superfamily Coccoidea, but are subdivided from there into smaller family groups. Let’s go over some of the most prominent types of scale bugs and talk a bit about their similarities and differences.
The coccidae are a family that often secrete a waxy coating. This white waxy covering can protect them against many forms of insecticide, although any oily insecticide will likely stay on long enough to have effect.
One of the best-known in this family is Coccus viridis, also known as coffee scale or green scale. Coffee scale is a major agricultural pest in coffee crops.
Other species of wax scales include tree dwellers like the cottony maple scale (Pulvinaria innumerabilis) or the calico scale (Eulecanium cerasorum). These scales flatten themselves against tree branches to feed.
While most people don’t realize that mealybugs are a form of scale, they are. Unlike most scale, they have legs, but they seldom if ever move once they’ve located a good feeding spot. Mealybugs are common greenhouse scale pests.
Widespread throughout agricultural areas, they also attack many commercial crops.
Over 2650 species of armored scale pests exist. Needless to say, these scales have an armor-like coating which they use to protect themselves from predators or insecticidal sprays.
One of the most stubborn examples of an armored scale is the San Jose scale, Quadraspidiotus perniciosus. This agricultural pest is widespread throughout the United States. While it was documented in the late 1800’s in San Jose, it originates from China.
In 1914, the San Jose scale was the first documented scale pest to develop resistance to insecticides.
Another major agricultural pest is the California Red Scale, Aonidiella aurantii. While its primary target is citrus trees, it also feeds on olives and other fruit, and can be found on some vegetables such as pumpkin.
Ground pearls are strange-looking large scale insects. Many of them appear to be cottony or soft. Others look almost berry-like in their shape and coloration, such as the Armenian cochineal. They are larger than most other forms of scale, mostly because of their cottony exterior.
The photo above shows a variety of this scale with some ants on it, harvesting the honeydew which the scales provide.
One excellent example of the Eriococcidae is the wooly beech scale, Cryptococcus fagisuga. These scales look more like lumps or bumps on twigs and branches, and can be mistaken for buds on the branch. Sapsuckers and other related scales are at risk from birds and beneficial predators like ladybugs.
Now that you have an inkling of the diversity of this persnickety pest, let’s go over their life cycles. We’ll also cover where they’re most likely to be found, and some information about their preferred plants.
Most scales do not move once they’re adults, so adult females will lay eggs beneath her protective external coating. Over the space of 1-3 weeks, these scale eggs hatch into a form that’s called a crawler.
These immature scales, called crawlers, will move away from their parent. Sometimes hatched crawlers will be caught and blown to other plants by the wind, and at other times they will simply move to a different portion of the same plant.
Once they’ve found a suitable place, the remainder of the scale’s life will be lived attached in that spot. Most species of scale lose their larval legs as they mature, and they feed on the plant’s sap.
Some forms produce honeydew, a sticky secretion that draws ants or fungal diseases to the plant.
Some varieties of male scale insects have wings, but they aren’t strong fliers. Instead, the wings are used to help guide the male if it’s caught and pulled from its plant in the wind.
However, the specifics of gender greatly vary amongst scale insects. Many are hermaphroditic, and those which do have gender may reproduce asexually as well as with fertilization.
The majority of adult scales are immobile and permanently attached to their desired host plant. However, the variety of plants is extremely wide.
There are scales like euonymus scales which prefer a specific type of evergreen tree but will as easily attack holly, ivy, and other evergreen plants, and those will be found around those plants.
Other scales, like the cochineal shown above, will form colonies on large cactus plants. Widespread on prickly pear plants in the southwestern United States, these scales look like whitish deposits across the surface of the cactus pad.
Some forms of scale are extremely flat and hard to identify. If you see what appears to be a whitish coating on the underside of a plant leaf, you may have discovered a form of scale.
There is some correlation between the type of scale and its favored plant, but it’s not universal.
Still, armored scales tend to be more prevalent on harder plants such as trees or thick-branched foliage. Soft scales are usually on stems or leaves of ferns or other softer plant material.
What Plants Do They Like To Eat?
All scale insects feed on the sap or plant juices of their host plant.
However, the range of plants affected is extremely wide. Some species prefer fruiting trees like orange, olive, or lemon. But there are species which attack a wide variety of bromeliads, flowering plants like roses, or even fruit and vegetable plants such as brassicas or beans.
Scale bugs can be found indoors or outdoors. Finding scale inside generally means that you brought an infested plant indoors. It’s easy to spread them to other houseplants, so check your plants carefully!
A plant which is suffering from scale infestation may show signs of premature leaf drop or yellowed leaves. Whitish or yellowish patches on leaves, stems, or branches is a common sign. If not treated, your plants are at risk of numerous plant diseases or branch death.
How To Get Rid Of Scale Insects
Now that you know what the scale pest is capable of, let’s go over some ways of getting rid of them. They can be tricky to combat, because it depends on which type of scales you have.
Soft scales secrete large amounts of honeydew, which can cause the growth of sooty mold. These scales tend to be harder to spot on plants, and can be hard to treat.
Armored scales do not secrete honeydew, so mold growth is far less likely around them. They are a bit more noticeable, but their exterior tends to protect them from some insecticidal measures.
Mealybugs tend to be the easiest to control, as they have no waxy coating nor hard protective covering to protect them. However, they do reproduce quickly and can quickly become a problem.
Let’s go over some options for each type of scale now!
Remember: The best approach makes use of integrated pest management, where you attack scale in multiple ways.
Smother the scales. Scale insects may be resistant to some pesticides (read the pesticide label to be sure), but they can’t live if they can’t breathe. Regular applications of a horticultural oil such as Bonide All-Seasons Horticultural and Dormant Oil will help.
Cover all plant surfaces with the oil in an even layer, and all life phases of scales will suffocate.
Neem oil is a godsend. Not only does the oil coat plants, but it naturally contains azdirachtin, which will slowly poison most soft scales and mealybugs. You can use it on armored scales as well, but it will work like a horticultural oil in that usage.
As a last resort, a much stronger azdirachtin product may help tip the scales in your favor (so to speak). Try using a product like AzaMax, which is derived from neem oil naturally, inside greenhouses or indoor growing areas. Even better, it works against spider mites, aphids, leafhoppers, and other pests too!
Wash your plants. This sounds a bit strange, but sometimes you can control scale by washing the leaves or stems/branches with no more than water and your hands.
If they’re particularly stubborn, use an insecticidal soap such as Safer Soap to try to loosen the scale the day before, and then try again. This is particularly useful in smaller infestations.
Invite in beneficial insects that eat the crawler stage. Both ladybugs and lacewings are natural enemies and find the scale larvae quite tasty. Some species of birds will peck off adult armored scales, but they’re less likely to stick around than beneficial insects are.
If you see a few scale pests, dab them with alcohol. Use a cotton swab and coat them thoroughly. Alcohol will dehydrate the pests and cause them to come off the plant.
If you do this whenever you first notice them, and follow up with soap or horticultural oil, you likely will not have major outbreaks.
Catch the problem early. Be attentive to your plants and you won’t have to go to war with scale pests.
Prune any infested branches and destroy them. If you find a single branch of a tree or bush that is infested with scale, carefully prune it away and get rid of it before they can spread. Spray down the plant thoroughly with oil sprays finish the job.
Finally, ants farm soft scale insects. Honeydew, the sticky, sweet secretion that soft scales exude, is a favorite food of some species of ants. Those ants will protect the source of their honeydew.
You can find a fascinating description of the process through the Ask A Biologist page of Arizona State University at this website.
To reduce the farming of scales, aphids, or other honeydew-producing pests by ants, you can use Tanglefoot Tangle-Trap around the trunks of trees or the base of rigid plants. The sticky surface will catch ants and prevent them from getting into the tree to protect other pests.
You can also place diatomaceous earth around the base of softer-stemmed plants and dust it on all the plant surfaces. A little ring of diatomaceous earth will deter ants from coming near. The dusting over the tops and bottoms of leaves and along the stems will deter scales from taking up residence.
Frequently Asked Questions
Q: Can scale insects fly?
A: … sort of. While some male scale pests do have wings, they are vestigial at best. They’re not very strong fliers. In calm conditions (such as inside a greenhouse), you might see a few wobbly flying pests around your plants, but they’re just as likely to be whiteflies.
The females and hermaphroditic scales don’t fly, and spreading of the pest is generally done when they’re in their larval crawler stage.
Q: If I scrape off scale, will it get back on my plant?
A: What you scrape off will not climb back up, because once it’s attached to a leaf, the scale pest is fixed in place for the rest of its lifespan. However, tiny little crawlers may still be on the plant.
If you opt to scrape scale from your leaves, stems, branches or trunks, be sure to follow up with a thorough spraying of horticultural oil.
The Green Thumbs Behind This Article: