An unfortunate reality of houseplants is that, sometimes, they get attacked by pests. Scale insects are some of the oddest-looking bugs that tend to plague indoor plants. This class of plant pests can do a fair amount of damage if left untreated, but because of their strange bodies, many people aren’t sure what they are or how to treat them.
Scale is a catch-all term describing a diverse group of sap-sucking insects that are identifiable by a tough wax covering that often resembles the scale of a fish or reptile, hence their name. Although they can inflict a lot of damage on a houseplant, there are many treatment options to target scale insects for effective control.
Knowing what scale insects are, how they reproduce and spread, and what scale damage looks like on a houseplant can help you identify any scale-related issues quickly, leaving you plenty of time to make thoughtful, thorough treatment applications. In this article, we’ll cover what scale insects look like, how their lifecycle works, and most importantly, which treatments are most effective in controlling them.
What Are Scale Insects?
“Scale” or “scale insects” are umbrella terms for a very large, diverse group of sap-sucking insects closely related to other soft-bodied sap-feeders like aphids and whiteflies. There are roughly 8,000 different species that fall into the scale family. In fact, mealybugs are technically part of this group, but because of their unique attributes, they are not included in this article.
If you are interested in reading specifically about mealybugs, you can get more information about them here.
In general, scale insects are relatively small, ranging from about 1/16” to 3/8”, depending on the variety you encounter. All scale insects are identifiable by a protective wax covering that they develop over their small, soft bodies. These coverings can differ in color or appearance, depending on the species, and may look oval, dome-shaped, oyster or mussel-shaped, or even kind of fluffy. These wax coatings often resemble a single fish or reptile scale, hence the common name of this group.
Female scale insects are also largely immobile. Although they can crawl during the early stages of their lifecycle, most find a spot on a plant to feed and remain stationary as their protective armor grows over them.
Male scale insects are tiny, gnat-like bugs that are very rarely seen. They have no mouthparts to feed, so their only job is finding females to mate with. Keep this in mind when searching for scale. You will likely only ever see and treat for female scale insects on your houseplants.
There is a lot of diversity amongst scale insects, but in relation to houseplants, you are likely dealing with a variety of soft scale. Soft scale varieties are about 1/8” to ¼” long and usually have a lighter wax coating to start, which darkens with age, turning mottled pale brown, yellow, orange, or grey. Many resemble tortoise shells. Female soft scales retain their legs from juvenile stages and, therefore, can move, although they rarely do.
Armored scale is the other category of scale insects, which have tougher protective coatings and tend to be flatter than soft scale. However, most of the varieties in this category prefer to live on outdoor trees and shrubs, and it is not common to find them indoors.
Where Can You Find Scale on a Plant?
As scale insects are sap-feeders, you can usually find them anywhere that gives them access to the plant’s phloem, which transports the sugary liquid throughout the entire plant. Typically, this means you will find them gathered on the undersides of leaves, often close to a vein, or on stems or leaf petioles. However, some varieties will also congregate on the tops of leaves.
If you visually inspect your plant, look for any rounded bumps or knobs sticking off the stems or leaves. They will likely be firmly attached to the plant, but you should be able to easily pry them off with a fingernail or something similar, like a Q-Tip, toothbrush, or tweezers. Any scale you do find are likely female. As was mentioned above, males are so small you’d probably never notice them.
What Does Scale Damage Look Like on a Houseplant?
Scale insects tend to fly under the radar for some time before you notice them. In fact, you may notice your plant suffering from signs of distress long before you realize it has a pest issue at all.
Scales feed on the sap of a plant by piercing into its tissue using long, sharp mouthparts that can actually be six to eight times longer than their entire body! Damage produced from feeding tends to be minimal, but as populations build up, it can become a contributing factor to leaf yellowing or distorted growth.
Severe scale infestations will result in chlorotic leaves, plant wilt, leaf drop, and reduced plant vigor or stunted growth. In very severe cases, your plant is at risk of dying.
Soft scale insects also produce honeydew, which is a sticky excretion that tends to coat plant surfaces where scales are present. Honeydew promotes the growth of sooty mold, a black-colored fungus that can cover leaf surfaces and interrupt photosynthesis, causing yellowing. In addition, sticky honeydew can prevent new growth from unfurling correctly, causing tears or deformities on tender plant parts.
Life Cycle of Scale Insects
Scale insects have three distinct stages, all of which happen on plant foliage. Female scales will lay eggs underneath their protective coating. Depending on the species, a female can lay anywhere from 50 to 2000 eggs or live young beneath their shell. Within one to three weeks, those eggs are ready to hatch.
As eggs hatch (or the adult female births live young), “crawlers” are produced. Crawlers informally refer to the first juvenile stage of scale insects when they use their legs to crawl to a new spot on the plant where they’ll begin to feed. Depending on the species, a scale might morph through up to three different nymph stages, with only a few species retaining their ability to move after the “crawler” stage.
Once the juveniles find a good feeding spot, they’ll insert their mouthparts into the plant tissue and begin to develop their waxy protective coating, protecting them from predators, environmental conditions, and potential insecticidal treatments as they feed.
The tiny males will fly around looking for females to mate with before they die. Once a female lays her eggs, she will die, as well. Soft scale insects typically only complete one lifecycle per year, but a plant can contain multiple insects at different stages, requiring you to be smart about how you pick and apply treatments.
How Do Houseplants Get Scale?
Because scale insects seem to be fairly immobile, you may be wondering how your houseplant actually became infested. Although it may seem like a total fluke, there are a couple of basic ways scale insects find new plants.
First, and likely most commonly, is that scale insects are introduced to houseplants by us inadvertently bringing them indoors. We can do this by bringing juvenile crawlers in on our clothing after we’ve spent some time outside in the garden. Crawlers are so tiny that we probably have no idea we even worked with a plant that had a scale issue.
Alternatively, we can bring scale indoors through the introduction of new houseplants. Growers spend a lot of time and money making sure their crops are pest free, but it’s definitely plausible that a few stray scales might survive any treatments and are now producing offspring on a newly purchased plant.
This is why isolating any new plants by themselves for a few days before incorporating them into your collection is always a good idea. Crawlers might be able to find a new host plant if put in close enough proximity, especially if any foliage overlaps. Also, if you like to bring houseplants outdoors during warmer months, realize scale insects are more prevalent outside. Make sure you take some time to monitor these plants for possible pest infestations, including scale.
Treatment Options to Control for Scale Insects
Once you’ve identified that your houseplant is suffering a scale infestation, you’ll want to act quickly to knock back the population to a manageable size. Scale insects can be persistent pests, mainly because they have built-in armor that protects them during most stages of their lifecycle. Because of this, you need to be strategic about treatment methods and deal with adult scales and juvenile crawlers. This means you will likely have to enlist multiple treatment methods and be ready to repeat them.
First Steps for Scale Infestations
The first step you should take once you discover your houseplant has scales is to isolate it from the rest of your plants. Even though scale insects are fairly sedentary, you never know if active crawlers are present, so if your plant is arranged with others nearby, you risk spreading the scale to them.
Place your plant somewhere far away from other houseplants in a spot where it’ll be fairly easy to apply treatments. A spare bathroom or a utility sink in a basement or laundry room works well.
Hitting the leaves with a strong stream of water is a good start to removing scale insects from your plant and can knock the population down to a more manageable size. We like to take our plants outdoors and hit them with the hose. That way, we don’t have to be too careful about where any pests land after we blast them from the plant.
Obviously, if you are dealing with a delicate houseplant, skip this step. The idea is to remove scales, not to cause any additional physical damage.
As you assess your plant, pay attention to the distribution of scale. If only a handful of stems seem to be infested, or a majority of the population is gathered under a few leaves, an easy solution to help combat the spread is to prune your plant back.
This, of course, depends greatly on the type of plant you have and how much you would need to cut back. Pruning is a very effective way to reduce the population up front, making subsequent treatments easier to administer and, hopefully, more successful. It’s best to have a trash bag handy to discard any clippings immediately.
Consider throwing out your plant
Unfortunately, there are instances of scale infestation that are already so progressed that treating and nursing your plant back to health may require more time, energy, and supplies than you’re willing to give. If you’d rather not fight the good fight, ditch your plant. Yes…in certain plant nerd circles, that suggestion is blasphemy! However, I’m of the mind that if it’s easy enough to replace, save yourself the trouble and buy a new plant.
If you think it’s time to throw out your plant, place the entire thing in a trash bag and carry it to the bin. If you want to save the pot it’s in, remove it outdoors, far away from any other houseplants. Be prepared to give the pot a proper scrub down before reusing it. Also, be mindful of potentially carrying any scale crawlers back inside on your shirt sleeves.
Treatments for adult scale
A two-pronged treatment plan is preferred because scale insects tend to be under a protective coating for most of their lives. We’ll get to juvenile treatment options below, but an effective method to deal with adult females will go a long way in controlling the population in the long run.
When it comes down to it, physically removing individual scale insects from the plant is usually the most effective way to treat for adults. These adult females produce the wax coating that essentially adheres them to the plant and provides protection for the multitude of eggs that make up the next generation.
Although somewhat tedious, pulling off the adults from the plant will knock back the population while exposing any eggs left behind. To remove scale, you can either pry them off with a fingernail (kind of gross, but effective), or you can use a damp cotton swab, cotton ball, tweezers, or soft-bristled toothbrush.
Another good option for treating adults is to apply soapy water to individual scales using a Q-Tip or cotton swab. The soap will work to dissolve the protective cover, making the insect vulnerable to desiccation or suffocation. Soapy water applied to a toothbrush or wipe will also help physically remove the adults. It also has the added benefit of washing away honeydew from the plant, avoiding the spread of sooty mold.
If you want to take things to the next level, you can utilize 70% isopropyl alcohol as a spot treatment for adult scale. A Q-Tip dipped in rubbing alcohol and then applied to the insect usually dissolves the protective coating and kills the bug on contact. If death isn’t immediate, the scale will be susceptible to desiccation as the alcohol dissolves its shell.
When it comes to scale insects, the thing about sprays and application treatments is that getting good coverage is really tough. Scales are often protected under their waxy shells, so getting liquids, powders, or oils in direct contact with the soft-bodied insects underneath is quite difficult.
That being said, horticultural oils have been known to do an okay job against adult scales. While they can help keep a population in check, you often won’t achieve full eradication with oils alone, but of all the different spray options, they seem to be the most helpful.
If you plan to spot-treat or smother the adult scale insects rather than physically removing them, just know that, even if they die, they won’t fall off the plant on their own. Their coatings help fuse them to the plant, so they’ll stay put long after they perish.
Pick a scale from the plant to check if your treatments are effective. If the shell is dry and crumbly, there’s a good chance that the scale was already dead. If it is juicy, or if it leaves a wet streak when smudged on a piece of paper, the scale was still alive.
Periodically check this to ensure any treatments you repeat are continuing to kill off any remaining adults. If you find the adults are surviving treatment applications, you may need to switch tactics.
Treatments for juvenile scale
Physical removal and spot treatments are the best ways to combat adult scale, but gaining total control over an infestation is almost impossible without targeting juvenile scale insects. Remember that the eggs and crawlers are often protected underneath an adult female’s protective coating, so most common pest treatments offer less coverage. To efficiently kill off crawlers, you need to wait until they leave the protection of the adult in search of a new spot to feed. This all comes down to timing.
To hit the timing of a treatment application right, you need to closely monitor your houseplant for signs that crawlers are out and about. The best way to do this is to closely inspect the leaves and stems of your plant with a magnifying glass every day to see if you can spot the juveniles moving around. Alternatively, you can place double-sided tape along stems or leaves and periodically check if your “traps” caught any crawlers.
It can also be helpful to actually identify the species of scale you are dealing with, as it will inform you on how often to expect a crawler stage. Remember that soft scale insects usually only complete one lifecycle per year, so you can wait patiently until crawlers appear before treating.
If you happen to be dealing with armored scale species, they can complete several lifecycles in a short period of time, so beginning and repeating treatments is a better option for them.
Many college horticultural extension websites have articles identifying scale species. Find one from your area to help you identify the local species you might be dealing with. Once you’ve determined that crawlers are active on your houseplant, you can deploy a spray treatment targeting vulnerable juveniles.
One good option that is effective against crawlers is insecticidal soap. These are typically non-toxic formulas of potassium salts of fatty acids that are pre-mixed as a ready-to-use spray. The soap dissolves the soft-bodied juveniles’ cuticles, similar to what soapy water or alcohol can do.
Good coverage is essential when it comes to controlling juveniles, so make sure you are soaking all parts of the plant, paying extra attention to the undersides of leaves, and getting deep into dense foliage. Plan on repeating this treatment about once a week to ensure all juveniles have been hit before their protective coatings develop.
Neem oil/horticultural oils
We briefly mentioned horticultural oils being used to control adult scale, but they are much more successful in killing off juvenile crawlers. Without the protective coating, juvenile-scale insects are very susceptible to horticultural oils.
Neem oil and other horticultural oils work to coat the crawlers so that respiratory parts are gummed up, causing suffocation. Again, this treatment is only as good as how you apply it, so ensure you get good coverage. You may also need to repeat applications as many times as necessary to fully eradicate any juveniles.
If you feel like other treatments for juvenile scale might not be strong enough, consider the use of chemical insecticides.
Many different formulas can control for soft-bodied insects, including scale, some of which are very low impact and perfect for use indoors. Look for formulas that contain pyrethrins, which are plant-derived insecticide compounds. They are very effective and a great substitute for harsher chemicals that are often unnecessary.
Just like with the soaps and oils, it will probably be necessary to repeat treatments every week until control is reached. However, read the label of whatever formula you use to be sure how often you can safely apply the product.
Long Term Monitoring for Scale
Being diligent about repeating treatments that target both adults and juveniles is the key to gaining the upper hand against a scale infestation. Once you’ve seemingly eradicated the population, it’s time to start monitoring your plant for signs of a relapse. Because scales tend to be well-protected and good at hiding away, just a few untreated individuals can start a population resurgence.
To avoid this, watch your plant closely for several weeks after treatments cease. Be sure to inspect all parts of the plant closely for any sign of scale. Because you are most likely dealing with a soft scale species, it may be several weeks before any eggs that were spared hatch and produce juvenile crawlers.
Keep your plant isolated during this time, as well. The last thing you want is for a new scale population to not only reinfest your plant but to spread to others in your home. When you are certain the scales have been fully eradicated, and your plant’s health has bounced back, you can move it back to its former spot in your home.
Scale insects are strange and difficult to get and keep under control once they have a strong foothold on your houseplant. Hopefully, after reading this article, you better understand how scale populations grow and what you can do to save your plant from succumbing to the damage they create.
It really comes down to being thorough and consistent with whatever treatment options to use. They may be stubborn pests, but with a little diligence, you should be able to keep your houseplants scale-free and thriving.