Many species of bugs enjoy the dark, cool areas of wall voids. Some enjoy the moisture within walls, which suits their habitat needs. Depending on the species, a large infestation could mean the bugs have lived there for a long time, such as silverfish that do not build populations quickly. Silverfish are typically grey to pearly grey, although some appear to have an almost brownish colour. House centipedes are brown and are also commonly found in bathrooms. Killing the insects requires diligence due to their habitat being hidden inside a wall and not out in the open.
Clean the bathroom and try to get rid of as much moisture as possible. Many bugs found in the bathroom and bathroom walls are attracted to moisture.
Identify the bug as best you can, although it is sometimes difficult. Identification of the species allows you to more properly treat the problem. Many websites offer pictures of common bugs, and some colleges' entomology departments have sites dedicated to pest identification. You can also take a picture or a sample to some entomology departments and other outlets for identification. To identify the brown bugs, count the legs, see if they have wings or a hard or soft-appearing body and determine the approximate size. These factors help a professional determine the species if you cannot take a picture or get a sample. For example, the house centipede is brown, has many long legs and long antennae.
Light the inside of the wall, if possible. If the insects are nocturnal or prefer the constant darkness of the inside of the wall, the light may force them into the open, making it easier to eliminate them.
Apply a residual insecticide into the wall, cracks in the baseboards, crevices or cracks in the drywall or plaster. Some insects, such as silverfish, are not affected by a spray or insecticide outside the walls because they don't travel there often and their nest or the majority of their population could be hiding in the wall.
Contact a professional company if the residual pesticides don't work. Tell the professional everything you know about the bug including size, legs, wings and other identifying factors.
Although a nuisance at some times, species such as the house centipede are beneficial because they eat smaller insects and rarely pose a biting threat to humans.
Always read the instructions and information for any insecticide used in the home. The chemicals may harm people, pets or material products.