Sticky Leaves? Look Up!

Honeydew, the sticky substance left behind by insects such as aphids and mealybugs, can quickly become a real headache for your houseplants. In this guide, first published on March 12, 2019 and ranked among the most viewed articles at #14, Larry Hodgson helps you understand how to effectively manage this nuisance.

This year marks the 10th anniversary of the Laidback Gardener blog, and for the occasion we’ve compiled a list of the 30 most popular articles of all time. Some must-reads are a natural on the list, while others may surprise you. These choices reveal what has piqued our curiosity and fueled our passions over the years.

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.

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…

Aphids

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 and Mealybugs

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. 

70% isopropyl alcohol can be effective against scale insects and mealybugs. Mix 1 part alcohol to 7 parts water and spray generously on all surfaces of the infested plant: stems, leaves (top and bottom), and leaf axils.

However, it is extremely difficult to completely eradicate a mealybug infestation, even with rubbing alcohol. Place any infected plant in isolation for at least six months, as a single surviving female is enough to lay eggs and restart the infestation.

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.

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