I'd always try manual removal first. Using a clean damp cloth, literally wipe the insects from the plant's leaves.
If manual removal does not work or the infestation is too heavy, I use white oil to control insects on plants such yours (Asplenium sp.).
White oil is a very light petroleum based product that is mixed with water for spray application to the top and underside of all leaves on your plant. The oil literally coats the insects in a film, which in turn starves them of oxygen. If you use this method, add a tiny squirt of dish washing detergent (not dishwasher detergent). The detergent will act as a surfactant, which will allow the spray to spread easily over all surfaces.
Its worth noting that insect infestation is often a sign of poor plant health.
In addition to removing the infestation, I'd recommend you investigate a couple of options to improve your plant's health.
- Larger pot;
- Pot saucer with high sides or pot with water reservoir (keep soil moist);
- Fortnightly to monthly application of a liquid tonic to improve soil biology;
- Monthly or two-monthly application of a liquid feed to supply nutrients.