Hence, traces of larvae are the most common material found, and these are usually in the form of casings which the larvae moult several times over their growth stage. Frass is the next most common insect trace. Simply put, it is insect excrement in the form of small pellets which are usually dark, but are often the colour of the material being eaten. Adults can also be found. Most of these types of samples filter down to the bottom of storage boxes.
Insects are masters of disguise, though, and it is easy to miss tiny insect matter.
Incidental insect damage is thus another sign to look for. Irregularly shaped holes over 2 mm in diameter are a good indication. Depending on the type of material targeted, these can be a single hole, or a large area that has been entirely eaten away. For leather bindings in particular, graze lines are another sign to watch for. These are lines heading in a single direction which are not completely straight, indicating where the larva has eaten its way across the material.