You find a strange growth on one of your houseplants, a sort of small brown bump. In fact, not just one brown bump, but several. Even many? Unfortunately, there’s a very good chance that what you’re seeing is an insect, a very sneaky one called a scale insect or just “scale”. If so, the faster you react, the faster than you can regain control!
Detection of scale insects is difficult because they are so well camouflaged. The mature insect is covered with a shield-shaped shell that can be rounded or almost flat; brown, gray, green or transparent; or oval, comma-shaped or resembling an oyster shell. They often seem to be part of the plant’s bark or you may take them to be drops of dried sap. They can be especially confusing on ferns as the latter produce sporangia (spore-producing organs) that some scale insects are able to imitate!
Not sure if that small bump is part of the plant or not? Give it a flick with your finger. If it stays put, it’s part of the plant; if it comes off, it’s a scale insect.
Infestation usually begins out of sight on less visible parts of the plant (under leaves, at leaf axils, etc.) before spreading to cover much of the plant.
Often the first symptom you’ll notice is a clear, sticky substance that drips onto lower leaves and nearby objects. This is called honeydew and is excreted by scale insects. It is even more obvious when the honeydew becomes covered with sooty mold, looking much like black powder, and that can happen over time.
The infested plant usually continues to grow normally at first, but possibly with less vigor, then as the scale population increases, the leaves gradually turn yellow. It not treated, the plant will likely die.
A few years back, I found an adorable little cycad (Cycas revoluta) at a very reasonable price and I brought home without thinking too much about it. It seemed healthy and I placed it among my other houseplants. A few months later, I noticed some fronds turning yellow, then saw that the lower leaves were abnormally shiny and that there was a sticky substance in the saucer below. Upon closer inspection, I discovered it was full of scale insects: literally hundreds of them! And other plants in the area were too. After repeated treatments, I was able to save most of my plants, but I had to admit defeat and tossed the cycad and five other plants into the garbage: despite treatment after treatment, the scale insects kept coming back.
And now I have to inspect all my houseplants regularly because even 3 years later, I still occasionally find a plant I need to treat or toss. All because I brought home one plant without isolating it!
A Truly Weird Insect
Scale insects don’t look like insects. They have no antennae, wings or visible legs and live permanently attached to their host plant. The species most commonly seen on indoor plants is brown soft scale (Coccus hesperidium) which produces a brown to tan domed shell from 2.5 to 4 mm in length… and it’s one of the larger ones! This scale insect has a wide host range and can attack almost any houseplant. Other scale insects are specific to a single group of plants. Some are only found on orchids, others only on bromeliads, others only on ferns, etc.
A quick word about mealybugs, another houseplant insect. Scale and mealybugs are closely related, but the mealybugs are white and appear covered with cotton. Even more obviously, they have legs and move when disturbed, while a scale insect will not move when you touch it.
For more information on mealybugs, read Just Toss Plants Infested with Mealybugs.
How Can a Legless, Wingless Insect Get Around?
You must be wondering how an immobile, legless, wingless insect is capable of spreading from plant to plant. The secret is that scale insects are mobile… in their youth. Nymphs, called “crawlers”, are born from eggs hidden under their mother’s shell. Crawlers are so tiny they are rarely seen. In most species, they wander about for just two or three days, then settle down for the rest of their life on a new stem or leaf. Nymphs readily crawl from one plant to another and thus start a new colony. And there are several generations per year. One scale insect that escapes your treatment can result in to 20,000 scale insects in a single year!
In nature, as well as by wandering on their own, nymphs are often carried from plant to plant by the wind, by birds or by ants. In fact, some ant species even raise scale insects in order to harvest their sweet honeydew and carefully move them from one plant to another.
In our homes, though, we can’t blame wind, birds or ants. Scale insects inevitably arrive on infested plants we bring home. Then crawlers move on to new plants or the plant owner inadvertently carries them from plant to plant when watering or pruning or when he moves an infested plant.
If scale insect nymphs crawl, the males fly. They too are tiny and rarely seen. With no mouthparts, they live only a few days and their only role is to fecundate females. But they’re pretty much redundant, as the females of most species can produce eggs without fecundation through a process called parthenogenesis.
Controlling Scale Insects
It’s important to understand that, unless you live in the Tropics, the scale insects that infest your houseplants did not come from outdoors. The species of scale insects that infest houseplants are of tropical origin and can’t survive outdoors in temperate climates, at least not over the winter. Nor do scale insects spontaneously generate (although they may seem to!). The source of just about every infestation is always a plant that was already infested, brought home from somewhere else, often a nursery, but maybe too as a cutting offered by a friend.
Don’t do like I did: inspect every new houseplant before you buy it and then put it in isolation for a good month once you do bring it home. Did you know the word quarantine means 40 days? And that’s just about right as an isolation period for new houseplants.
If you have no separate room in which you can put your new purchase, just put it 6 feet (2 m) away from other plants or isolate it inside a transparent plastic bag.
If you do find scale insects, isolate the infested plant immediately if you already haven’t done so… and ask yourself seriously if it wouldn’t be better for you to simply discard it. Sometimes you can nip the infestation in the bud by cutting the plant right to the ground… but if so, do clean your pruning shears before using them on any other plant! Of course, not all plants will tolerate such a heavy pruning, so you’ll need other methods.
Insecticide sprays are not very useful. Adult scale insects are essentially immune to insecticides due to their protective shell. Moreover, even if the insecticide treatment did kill them, how would you know? The shell of a dead scale insect can remain on the plant for years, making you think the plant is still infested. The best treatment is to go over the whole plant with an old toothbrush dipped in an insecticidal soap solution to remove any shells. Yes, it’s tedious, but if it works, it may well be worthwhile! Then rinse the plant thoroughly to knock off any crawlers that may be present… and watch it for a few weeks. If new shells appear, repeat the treatment… or toss the plant!
There are also many species of scale insects that attack outdoor plants, mostly woody plants like trees, shrubs and evergreens, but they are not the same ones that live on houseplants (again, if you live in a temperate climate).
The initial treatment for scale insects outdoors remains essentially the same: clean the trunk and branches with a brush dipped in a solution of insecticidal soap. Then spray the plant all over with insecticidal soap or horticultural oil. Also, a dormant oil treatment can be very helpful in reducing the population if you apply it at just the right time, and that is almost always just before the buds open in early spring, as that is when crawlers are active or when they have just settled down and therefore before their protective shell is fully formed. Even so, as with indoor plants, the shells of dead scale insects can remain attached to the bark for ages, so you still have go over the infested plant with a brush to be absolutely certain whatever treatment you used was effective.
Yes, scale insects are sneaky and hard to control, but if you’re persistent, you can succeed. Still, do remember they are much easier to prevent than cure… and prevention starts wit isolation!