The bugs that infest flour, known as flour beetles, usually enter the product from other contaminated food found in the store or the home. Flour also can be contaminated during production.
If the flour has only a small contamination when it is brought home, the pests may not be easily visible. Adult beetles continue eating the flour and laying eggs, eventually creating a large infestation. Flour beetles target opened containers, but they also can penetrate plastic wrap and thin cardboard. In addition to flour, the beetles target bran, cereal, chocolate, dried fruits and nuts. Susceptible foods can be stored in the freezer or refrigerator.