Q: Help! I went to wipe off what looked like black dust on my Umbrella Tree leaf and got sticky goop all over my finger. What is it? How do I get rid of it? —T. S. Hatfield
A: Hang in there, T.S.! The sticky residue you encountered is called ‘honeydew’ (though it has nothing to do with the melon). It can be quite surprising when you inadvertently touch it as you did.
Honeydew results from the eating activity of a common indoor plant pest called scale. It is a combination of undigested sugar and water that passed through the insect’s digestive system. If the plant were outside, the honeydew would serve as food for ants, flies, and other scavenger insects. The black dust is a fungus called sooty mold that grows on the honeydew. The mold often results from excess humidity around the plant. Get rid of the scale and you get rid of the honeydew and the sooty mold.
Scale is a tiny insect with piercing sucking mouth parts (sounds lovely, doesn’t it?). Most are only between 1/8-inch to ¼-inch in size and typically only move when they leave their mother’s scale and crawl to find a suitable feeding spot where they usually just stay put.
The type of scale that produces honeydew is the soft scale. It makes a soft, thin, cottony, waxy layer over itself which cannot be separated from the insect. This is different from armored scale, which has a hard tan or brown shield-like covering of shed skin and wax over the insect that is separate from the insect. Both are often located on the underside of leaves and where the leaf meets the stem. Armored scale do not make honeydew.
A small scale infestation will not severely damage a plant. If left untended, though, the infestation may grow and weaken your plant leaving it susceptible to infection and disease. If you have a large infestation, throw out the plant — that is hard to say as a gardener.
Treating scale takes some persistence, as the shield and soft covering are effective, impenetrable deterrents. Since houseplants are inside your home, it is best to use natural methods of treatment instead of chemical insecticides. This is especially true in wintertime when people typically have their windows shut for long periods of time.
Insecticidal soap and a toothpick or strong fingernail are generally your best treatment options. The only time scale are truly vulnerable is at the nymph stage before they have created their protective coating. Since timing that stage is difficult, spray the infected plant with insecticidal soap every 7 — 10 days until they are gone. Make sure any commercial product label says it is safe for houseplants and use it according to manufacturers’ instructions. You can also scrape off the scale with a toothpick, fingernail, tweezers, or dab each scale with an isopropyl alcohol-soaked cotton swab. Remove any infected leaves or stems.
Scale and other houseplant insect pests can seem to appear from nowhere. When you buy a new houseplant, be sure to check its leaves, stems, and soil for signs of pests before bringing it home. Even then, it is easy for an egg or young insect to go undetected. Check your plants weekly to ensure they stay healthy and beautiful.
I hope you found this helpful, T.S. — thanks for asking a local Master Gardener.
Have a gardening dilemma? Please send questions, along with your name/initials and community, to the Western Massachusetts Master Gardener Association at AskAMasterGardener@wmmga.org. One question will be selected and answered per week.
wmmga.org