i grow a number of different adiantum species. do you know what kind
you have and where it is from? from my experience, mealybugs colonize
stressed plants, so the best method is to remove all the stressors. do
not let the soil completely dry out, but don't let it sit in water
either. maidenhairs like moist soil, not wet soil. they also like
some oxygen around their roots and plenty of good air circulation.
they do not like any fertilizer, but they do like pieces of wood in
various stages of decomposition. i had my first successes w/
propagating these plants in wooden crates, which is where i keep most
of mine now. some of them need to dry out during the summer, but stay
moist during the winter. they also are usually found in the coolest
place in your region, almost always out of direct sun, but sometimes
requiring very bright shade. putting it outdoors is probably very good
advice. but then again, i don't know where you are located, so if you
live in anchorage, it might take care of the mealybugs, but a tropical
maidenhair probably wouldn't be too happy there either. i've had very
good luck w/ these ferns and for the most part, find them to be very
hardy.

best of luck.