Ferns may be attacked by a range of pests. In general, garden grown ferns are more resistant to pests than plants grown indoors, or in greenhouses.
Common fern pests include:
Staghorn fern beetle
The best way to control a pest infestation is to act as soon as it first becomes apparent. Affected plants can then be quarantined, and treated separately from others. The most effective way is to remove the affected part by hand, and destroying it. If the infestation has gotten out of hand, more drastic treatment may be necessary.
Ferns tend to be very sensitive to chemicals. It is better to avoid them where possible, as use may cause damage to your plants. Tried and true remedies such as soap and water for scale, while time consuming and messy, is more fern friendly than pest oil (which may kill your fern).
If you must use chemical intervention, Confidor is usually a safe option (watch the concentration!). Dipel is a safe option in controlling caterpillar infestations. Start with a lower concentration, and test on your plants prior to using any chemical.