Ferns are suseptable to attack by several pests, and it is unfortunately true that certain insecticides are as apt to kill your fern as they are to kill the pests.
Before usong a pesticide read the lable carefully.
Do not use more pesticide than is recommended or the plants may suffer.
Avoid spraying when the temperature is above 85 degrees F. Some pesticides are likely to damage plants in hot conditions.
In many cases it will be found that pest control is possible without the use of exotic chemicals. For instance, aphids, which often attack young fronds, can be washed of with a weak solution of soap.
Malathion, or lindane if you don't want to mess around.
Denatured alcohol is an effective agent for control of two particularly troublesome pests, scale insects and mealy bugs. But the cure itself is a bit bothersome if you've more than a few poted plants. Scale insects are likely to occur along the veins on the underside of the fronds. Brush alcohol on them and wipe off the insects... For a larger problem or oudoor ferns, Malathion. Mealy bugs are pale pink or yellow insects covered with a white mealy substance. They congregate in sheltered corners of the plant and suck the sap. They are sometimes found on new fern growth, they can be rid the same as scale (and or Sevin), if you're into the alcohol thing please apply carefully on young fronds, because too much of it may damage the fiddle heads or younger fronds.
Most other pests of ferns are larva stages of insects, and in certain types of years can do quite a lot of damage (use malathion). The last pest that must be mentioned is the eelworm, or nematode. This creature can enter a fern through the fern's roots, and the first sign of infestation is distortion of the fronds. There is no known way to combat nematodes ( and have a live plant). Fortunately, they seldom infest ferns.
A particuliar type of weather can encourage insect damage... Here is where I go back to my preaching podium...
Plants are a source of food for many types of insects and other small creatures. The damage severity usually depends on the weather conditions of the current and previos seasons. A wet summer and fall results in the destruction of many Japanese Beetle larvae in the soil, leaving a smaller population in the following year...
The variation in the range of pests found in different localities is probably due to the climate, but is also influenced by the available shelter from the weather, the type of soil and the kind of vegatation growing near the garden.
Thus no single garden is likely to harbor all of the pests mentioned here.