The problem with sewing costumes is that, if they’re worn by a human, you’re eventually going to have to wash them. That means that your costume has to be wash-proofed. We’ve all heard about the merits of prewashing fabrics, but a lot less time has been devoted to care and feeding of hems and seams to prevent the Attack of the Thread Monsters!
The Thread Monster (Seamus fraydicus) is generally found nesting in untreated seams. Infestation begins during the washing process, when the agitation of water begins to loosen the cut fibers of the fabric. Thread Monster larva need heat to fully mature, and usually require several trips through the dryer before reaching adult size. However, as they grow they extend their territory and seek loose threads elsewhere on the seam they inhabit. The resulting nest can become sizable. The fibers become firmly matted together, often causing the seam to buckle and bind. The normal solution to infestation is to rip out these fibrous nests when they become large enough to cause problems, resulting in damage to the seam allowances of the garment. In extreme cases, a nest in a prime location (the bottom of an armscye, for example) will gather threads from several adjoining seams, and removal may damage the garment itself. Thread Monsters inside of lined garments have a particularly protected home, and are nearly impossible to remove.
Fortunately, it is possible to protect your seam allowances from infestation by treating them to prevent fraying. Without loose threads, Thread Monster larvae have no place to mature and nest. I personally prefer French Seams, as they are completely immune to Thread Monsters. At the very least, running a zig-zag stitch along your seam allowances will deter Thread Monsters.
All silliness aside, it’s incredibly important to take the time to make seams and hems that will stand up to repeated washings without binding up into little thready messes. This is especially important when working with cotton, in my experience (although the sample pic is taken from a piece of linen blend that was prewashed – that happened in one wash/dry, and the resulting ball of threads is surprisingly well formed.) I’m nearly fanatical about french seams in unlined garments, and about serging the internal seam allowances inside anything with a lining. I’ve just had too many hastily sewn skirts turn themselves into some sort of 1980’s-tribute-‘bethan-poof-skirt in the wash. It saddens me greatly. Don’t let it happen to you!