There are several species of scale insects that can affect orchids, but they can be classified as hard scales (pictured on the left, above) and soft scales (boisduval scale on the right).
“Hard” or brown scale insects resemble tiny turtles . . . but they’re even lazier. Like turtles, scale have hard shells-1/16 to 1/8 inch in diameter, usually brown or green in color. Unlike turtles, scale don’t move around after they mature; instead, they affix themselves to a plant leaf or stem and feed off its juices.
You can easily spot wart-like adults lined up along a leaf vein, or attached barnacle-fashion to stems as shown at left. Baby scale are a different matter. Shell-less and invisible to the naked eye, they crawl about seeking fresh foliage to get their teeth into.
Boisduval scale – the photo at the right – can sometimes be mistaken for mealybugs, but are far more difficult to get rid of.
Scale can sometimes be mistaken for mealy bugs. Look closely, though, and you’ll see a shell instead of the mealy bug’s waxy white covering. Many scale shells are simply round or oval domes. Others look like miniature oyster shells. Scratch one with your fingernail or the blade of a knife and it’ll pop free.
Also like mealy bugs, most scale excrete droplets of a sweetish, sticky “honeydew” that gives foliage a shiny look. This gooey stuff attracts black mold, and sometimes ants as well.
When babies-called crawlers-leave home they have legs and antennae to help them in their wanderings. Since they can travel for as long as two days without feeding, they’re hardy enough to drop off one plant and seek out another.
Once a crawler finds a likely homestead, it sinks its mouth into the leaf or stem, then tucks its legs under its body and settles down. Eventually the legs and antennae disappear and a shell begins to form. This continues to enlarge, providing protection for new generations of scale born underneath. Females produce three to seven broods in a year’s time.
High humidity levels make it difficult for scale insects to make headway. In dry winter months you might want to consider running a humidifier near susceptible plants.
For light infestations, dislodge scale with a soft toothbrush or wet cloth-or dab with denatured alcohol, just as you would for mealy bugs. Once you’ve popped off a shell, make sure you also get the eggs underneath. Wash with soapy water and quarantine the plant for several weeks. You may have to repeat the treatment several times. Discard heavily infested plants or they’ll infect healthy ones.