Tropicals ferns have a reputation for being sensitive to many pesticides. There are systemic insecticide granules (usually with the active ingredient imidacloprid) labeled for indoor plant use which suppress pests, but scale insects aren't always well-controlled by that ingredient. (Both soft scale, like Brown Soft Scale and Hemispherical Scale, and armored scale, like the aptly-named Fern Scale, can infest tropical ferns. Systemic insecticides work on some of them better than others.) You can learn more about the most common types of scale found indoors on our Scale Insects on Indoor Plants page.

Using a systemic insecticide preventatively at home is generally not recommended, though commercial nurseries may. Scale insects need to crawl onto their host from an infested plant (females cannot fly, and juveniles don't crawl very far), so if no other plant near where you will be growing the fern has scale, there is little risk of it developing a problem. Granted, a plant might have a small, undetected population of scale already on it when it's purchased, but in such cases nurseries growing the plants often use insecticide preventatively already. Soft scale, which in this case look like small, glossy brown lumps on the stems or leaves, produce a sticky, glossy residue (basically sugar-water) called honeydew as they feed. Honeydew can be a good indicator of a scale presence if the scale themselves are hard to find. Fern Scale do not produce honeydew, but their white body coverings makes them easier to spot. For any plant where scale become abundant, it's often not practical to use insecticide to treat it because it's not often successful. If you've discarded a scale-infested plant, wipe-down with a damp cloth or paper towel the area where it was sitting, so any crawlers (the tiny mobile juveniles) don't find another plant in the meantime. Without food, they are not likely to live long.

Bear in mind that systemic insecticide should not be used if the treated plant could be chewed on by a pet or child, since the chemical is present throughout the plant's tissues. Typically, a soil-applied dose of the imidacloprid granules lasts about 8 weeks per dose (it might take a week or longer to take effect after being applied), but you'll need to read the product label instructions for specifics. The timing and size of pesticide dosages depends greatly on the formulation used and the size of the pot or plant.

Miri