Insects and vermin are naturally attracted to paper because paper is made of cellulose, starch and protein, materials that provide sources of nourishment.[36] The most common pests are roaches, silverfish, and various types of beetles.[38] Book lice feed on mold spores found on paper and cardboard, and although they do not cause visible damage, their decomposition and excretions can stain paper and may also nourish other pests, continuing the cycle of damage.[2] Freezing collection items can mitigate pests.[38] However, some materials should not be frozen, such as books made with leather, because the cold temperatures may cause the fat to rise to the surface of the leather resulting in a white or yellow area called a bloom.[38] The use of insecticides directly on collection materials is not generally recommended.