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!

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.