Ficus trees are relatively resistant to most common plant pests, but a notable exception is scale. When an apparently healthy ficus tree suddenly loses vigor, yellows and drops its leaves, closer inspection might reveal clusters of brown, to gray, shell-like insects, coated in a sticky substance. Adult scale insects make a living sucking juices out of plant tissue with specialized mouth parts. Understanding these insects' life cycles can help keep ficus trees healthy.
Scale Life Cycle
Scale insects begin life as tiny mobile nymphs called "crawlers." The nymphs seek out soft plant tissue, especially on the undersides of leaves, along the veins, or on tender parts of plant stalks or new buds, where they will lose their legs and attach themselves. As the nymphs mature, they secrete a waxy coating that hardens to become a shell. They continue to grow, sucking juices from the plant with needle-like mouth appendages. The adult scale insects excrete a gummy fluid, poetically called "honeydew."
Scale insects are not conspicuous because of their small size and habit of colonizing leaf undersides and other less visible plant parts. On ficus trees, they tend to colonize mid veins on both sides of the leaf, green wood and even young trunks. The adults are one-sixteenth to one-eighth of an inch in diameter. Their shells are round or oval and can be brown, gray, tan or white, depending on the species. They usually occur in colonies, and are sometimes attended by ants, which farm them for their honeydew.
Adult scale weakens plants by extracting sugars and starches used for plant growth. Large infestations can seriously weaken plants and cause leaf drop and other signs of decline. Ficus trees tend to show yellow leaves as a first sign of scale stress.
The honeydew they secrete is a growth medium for fungus and other plant pathogens, which can infect the already weakened plant through the open wounds left by the scale. Sometimes the honeydew makes ficus leaves visibly shiny. These secondary infections are usually to blame for the plant's collapse.
Vigilance is the first line of defense when it comes to scale. Look carefully at the undersides of leaves on new ficus before you buy them. If your ficus has been outside for the summer, check carefully for scale before you bring it back inside. Scale can reproduce rapidly in indoor environments. If your plants become infested with scale, you can scrape off the adult insects with a paper towel moistened with rubbing alcohol. For hard-to-reach places use a cotton swab. For larger infestations, consider using horticultural oil or insecticidal soap with pyrethrins. Both of these have low toxicity to humans and pets. Insecticides are very effective on the tiny nymphs, but the adults are more resistant because of their waxy coating. Add a few drops of dish soap to the insecticide to help penetrate the insect's shell.
The nearly invisible scale nymphs can easily move from your ficus tree to other houseplants, or even outdoor plants in close proximity. If your ficus has adult scale, isolate it from other plants until you have the infestation under control. Inspect the leaves of all your houseplants regularly for scale and other pests.