Answer: Scale insects can cause problems on all kinds of plants. They're sucking insects and the excess plant fluids they consume ends up as a sticky residue on the upper sides of leaves. Scales are difficult to eradicate because they're protected from predators and chemical sprays by their hard outer shells. The best defense is to pry them off, one at a time. (Ferns would be harmed by the usual horticultural oil application which suffocates them.) If your fern is really under attack, you may want to cut back the affected fronds and allow new fronds to grow this spring.
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