Sticky Leaves? Look Up!

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Have you noticed a sticky, shiny substance on the leaves of some of your houseplants? If so, look up, on the same plant or its neighbor. For the sticky stuff is probably honeydew, a substance produced by various sucking insects, including aphids, scale insects and mealybugs. Since the insect ingests enormous quantities of sugar-rich sap, more than it can digest, it must necessarily reject the excess sugary liquid and droplets of this honeydew fall onto leaves below.

Aphids. Photo: auxincc.wordpress.com

This problem can show up on houseplants any time during the year, but is most common in late winter and spring, as many insects, in diapause (near dormancy) in winter, rewaken and begin to proliferate massively with the return of longer days. Soon, there are not only a few drops of honeydew, but sticky liquid everywhere!

The insects responsible for this damage are found on the stems and leaves above, and can be quite unobtrusive, but now that you know where to look for them, you’ll find them without problem. Their honeydew has given them away!

What to Do?

What to do about such an infestation? The most logical thing is to (horror of horrors!) toss the plant and then closely monitor its neighbors for possible signs of future infestation.

You can also treat the plant, but…

Washing and rinsing a plant often rids it of insects temporarily, but 9 times out of 10, the pest will return. Ill.: Claire Tourigny, from the book Les 1500 trucs du jardinier paresseux.

Aphids, small plump insects, often green, are “relatively” easy to keep under control. If you wash the plant at the sink with soapy water and a sponge, cloth or soft brush, making sure you reach all its parts, and then rinse it with a strong stream of water, you will get rid of most of them, but if a single aphid remains, the infestation will start all over. 

Bummer!

Note that you need to use soap (insecticidal soap, black soap, Ivory soap, etc.) to treat insects, not dishwashing liquid. Most modern dishwashing liquids no longer contain soap and thus are not very effective against insects; plus they can damage soft plant parts.

Scale insects (left), mealybugs (right): both are prodigious producers of honeydew … and terribly hard to control. Photo: Gilles San Martin, Flickr & http://www.growweedeasy.com

As for mealybugs and scale insects (actually, close relatives), the same treatment, that is a thorough washing with soapy water followed by a strong jet rinse, is also possible, but even less effective. They often hide in places near but not on the plant, so you never get them all. Thus, they reappear, often months later and the infestation starts anew. And in the meantime, mealybugs and scale may have had time to spread to your other plants. 

Even the radical idea of cutting the infested plant to the ground, cleaning the stump with soapy water, then allowing it to grow back is rarely effective. I’ve done this more than once and at first it always seems to be working, then suddenly the scales or mealybugs are back. I just tossed a plant I’d cut back no fewer than three times, yet was never able to truly rid it of scale insects.


There you go! Examine your houseplants occasionally and if you discover sticky, shiny leaves, you’ll know what to do!

Scale Insects on Houseplants: A Nasty Surprise!

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Scale insects are quite visible on a green leaf, but often blend in with bark when they’re on a branch or trunk.

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!

Well Camouflaged

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.

Personal experience

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Cute, n’est-ce pas? But this little cycad caused me years of trouble!

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

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Brown soft scale (Coccus hesperidium).

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.

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Mealybugs are close relatives of scale insects, but unlike the latter, are mobile.

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!

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Outdoors, scale insects can be spread by ants.

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

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Always, always, always put new plants in quarantine.

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!

Outdoors

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There are literally hundreds of species of scale insects that can infest outdoor plants.

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!

The 30 Year Mealybug Infestation: a Horror Story

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These new houseplants may come from a reputable nursery, but that doesn’t guarantee they’re not infested with mealybugs or scale.

Where do the unwanted insects found on our houseplants come from? Some come indoors all on their own (spider mites are so small they can pass right through a window screen) or by clinging to our clothes when we work in the garden. Some are blown indoors by the wind when we open a door. But others arrive by hitchhiking on the new plants and cuttings we bring into our homes.

That’s the case with mealybugs and their equally sneaky cousins, scale insects. Female mealybugs and scale insects can’t fly, nor can those species that infest our houseplants survive in our outdoor gardens, at least not in cold climates. They only live on other houseplants. So the one way for mealies and scale insects to reach your houseplants – assuming you agree that it has been proven that spontaneous generation just isn’t possible – is through an infested plant or cutting.

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Mealybugs are brought in on infested plants or cuttings.

I also regret to inform you, in case you didn’t already know it, that mealybugs and scale insects can be found in the very best, most reputable plant nurseries. Such places make a valiant effort to keep them under control, but since they’re constantly bringing in new plants and can’t inspect each one, errors happen. And it takes only one stray mealie or scale insect to produce thousands more.

My Sad Story

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A simple dieffenbachia led to 30 years of infestation.

In my case, my mealybug infestation started with a simple dieffenbachia I “saved” from the compost pile out behind my local botanical garden (hmm… I wonder what it was doing in said pile!). It was the source of an infestation that lasted 30 years!

I made the error of not putting it quarantine, proudly setting it among my other houseplants. When I finally noticed it was infested (mealybugs can be very surreptitious), several of my others plants were too.

Well, I treated those plants, sometimes again and again, using various products. I tried soaps, oils, rubbing alcohol, putting the plants outside in summer, etc., each time apparently with some success… only to discover later that the mealies were back, usually on the same plant I was so sure I had thoroughly treated, but also elsewhere. I keep up this game of treating infested plants on a per case basis for 30 years (I’m a slow learner), even, out of frustration, treating every single plant on more than one occasion and that was quite a job. It never worked: the bugs always came back.

Scale insects on the stem of Cornus sanguinea

Scale insects

At some point in time (probably various points in time), scale insects also joined the fray. I actually found several species of them, some strictly on certain plants (orchids, bromeliads), others more generalized. I applied much the same treatment as for mealybugs with much the same results.

In the back of my mind, I knew the right solution: you don’t treat mealybugs or scale, you throw way the infected plants. But with a collection of some 600 plants, some very precious to me, I was just unwilling to face that fact.

5 years ago, though, I decided that enough was enough and I did it. I tossed out all the infested plants, even ones that I knew I would never be able to replace. That wasn’t enough: I have soon found others that were infested. I tossed them too. Then tossed even more. But at least the numbers kept diminishing. For 2 years, I still kept occasionally finding infested plants, but I stuck to my guns and threw them out as well. By then, my houseplant collection had dropped by nearly half: nearly 300 plants ended up in the garbage. But eventually my perseverance paid off: I haven’t seen a single mealybug or scale insect in 3 years now. If I do, though, you can bet the plant is it found on will go straight into the garbage.

What I Should Have Done

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You can use a plastic bag to isolate your plants.

What I should have done from the start, and now do religiously, is to always isolate (quarantine) any new plant or cutting I bring home, whether I purchased it, received it as a gift or got it through a plant exchange. You can easily isolate new plants by putting them in a room separate from your other plants. Or at least far away from them in the same room. I put them in clear plastic bags, one plant per bag. You get to choose your method of quarantine.

I love the word quarantine: it means 40 days. And 40 days is just about right. If your new plant has hidden mealybugs or scale insects, they’ll have come out of hiding after 40 days and will be visible if you know what to look for (white cottony growths in the case of mealybugs, bumps on the stems and leaves and sticky foliage in the case of scale insects). And if you do see any, may I suggest the best thing to do is to simply to discard the plant.

I know you won’t follow my advice (if anyone had told me 30 years ago the best treatment was to toss a plant, I would have disregarded it too), so if you insist on keeping an infested plant, please at least keep it in isolation and treat it again and again until you no longer see the pest. Personally, I would require of a plant previously known to be infested with mealybugs or scale to show a full year of negative inspections before letting it loose among my other houseplants. But today I simply can’t imagine any plant that would be worth the effort.

That Was Then…

I just got back yesterday from a houseplant exchange organized by a online houseplant group. There were beautiful plants and cuttings, great conversations, wonderful exchanges of information, doughnuts, and much, much more: it was certainly well worth attending. And all the plants I brought back are now in individual transparent bags, the cuttings after I potted them up. Not one will be allowed free access to my home until it has proven itself free of bugs.

My collection is now increasing again… but I still occasionally find, inside my quarantine bags, plants with insect problems. Maybe it’s just a case of bad luck, but every orchid I’ve bought lately has turned out to be infested with mealybugs, so if orchids are your thing, be especially careful.

I learned at my expense that you don’t trifle with mealybugs or scale insects. I hope the lesson from my sad story will save you years of wasted efforts.