Houseplants can become infested with a variety of insects as well as mites but one of the most serious pests are scale. To put it bluntly, scale really suck! They actually do suck sap from plants to collect the sugars in it as food. They have a tiny stylet that they insert into the plant, almost like a mosquito sucking blood. The digestive system of a scale insect is not that efficient so a lot of the sugar in the sap gets excreted as waste. This sticky waste is called honeydew. It may accumulate on the plant’s leaves and stem, on the pot, windowsill or even on the floor.
So why is this potentially such a serious insect pest? It may attack just about any kind of plant. It multiplies rapidly and spreads easily. It often multiplies on plants undetected until the infestation is severe. The only life stage of the insect that is really susceptible to treatment is the crawler or first stage that hatches from the eggs. The crawlers are so tiny they are barely visible to the unaided eye. After that each young insect develops a waxy body covering, making them nearly impervious to most treatments that kill by contact. You may read about using an old tooth brush or cotton swab to get the final adult stage insects that look like brown bumps off the leaves or stems, but often those are dead females, the younger stages are much harder to see so many of those will remain and continue to develop and multiply.
The best control of scale is prevention. Be VERY careful of bringing new plants into your collection unless you examine them carefully for scale and other insects or disease. If you see scale, do not even bring the plant home. If you don’t find any, that’s good, but you should still quarantine them away from your other plants for several weeks so you can check them over again before moving them near other plants. If you handled plants with scale, do not handle other healthy plants until you wash up and maybe changed your clothes. You might be bringing scale crawlers along with you and could potentially infest healthy plants.
Carefully scout your plant collection and look for the adult scale. Often most people don’t know they have a problem until they see the honeydew. While it is usually clear, sticky and shiny, it may also look black because sooty mold likes to grow on it. Be sure to check those plants in the back of your collection that might not get very close scrutiny on a regular basis. If you find an infested plant, carefully remove it from its spot in your collection if it is near any other plants, maybe within 3’. If it is badly infested, place it in a garbage bag and get it out of the house. I have found that about 4 hours outside when the temperature is below 0 is quite effective in killing the scale. And even if it doesn’t kill the scale, the plant will look so bad after it thaws out you won’t want it anymore anyway.
If you want to try treating it, an insecticidal soap, ultrafine horticultural oil, Neem oil or similar product may kill the earliest stages of the scales as it clogs up their capacity to breath and may break down their waxy coating. If you can find a systemic insecticide that is labeled for houseplants, that might knock them back, but likely only for a while. Initially many people think they can cure a scale problem. But often they soon find out that they have spread to other plants. In the end, instead of just throwing out one plant they may have to get rid of a lot more plants to finally get it under control.