Scale insects are some of the most common houseplant pests, and unfortunately, also among the most destructive. They cause damage by sucking the sap from a plant, which can weaken it and cause unsightly yellow spots. Scale insects can also leave a sticky residue on leaves, which is commonly referred to as "honeydew." This substance can cause secondary problems for the affected plant, such as sooty mold.
There are thousands of species of both soft-bodied and armored scale insects. They can be up to 6mm in size and come in a variety of colors, but are most commonly white, black or brown. Scale insects are present year-round, but tend to be more abundant during periods of warm weather. Their damage often takes the form of yellow spots on the plant, which can grow progressively larger as the scale insect feeds on the plant's juices.
Life Cycle Upon Hatching
After scale eggs hatch, the "crawlers" only have mobility for a short period of time. While they are able to crawl, they look for an appropriate plant host. When one is found, the scale insect will attach itself to a leaf or stem using its strong mouth before shedding its legs. This act will render the scale insect immobile for the rest of its life; thus, it will remain in the same spot and feed off its host.
Scale insects cause the most damage by sucking the nitrogen-rich sap from plants. However, they also secrete a substance, known as "honeydew," which leaves a sticky, glue-like residue on the plant. Honeydew is a common cause of sooty black mold. This black fungus is wind-borne, and can inadvertently be spread to other plants nearby. Sooty mold can interfere with photosynthesis and also leaves an unsightly charcoal-colored coating on the leaves of an affected plant.
Examine all of your plants regularly for signs of scale insects. If any are found, they can be removed by hand or by using a slightly damp cotton swab. Commercially available insecticides and soaps to combat scale insects are available at garden centers and plant nurseries, and can also help to eradicate scale insects. If you notice a large infestation on a plant, it is usually best to discard it, to avoid contaminating nearby plants. It is also important to be mindful about over-watering and using excessive fertilizer on your indoor plants, as doing so creates an ideal growth environment for scale insects.
The best defense against scale insects is to carefully and methodically examine plants before purchasing them and bringing them into your home. And whenever a new plant is introduced, it is important to keep the plant isolated from your other plants for a few weeks to ensure that there are no scale insects or other pests hiding in your new addition.