These are some 100 species of beetles, moths and dustlice that have become specialised to survive in a comparatively dry environment. Usually, the moisture content in the product is sufficient for stored product insect survival and reproduction. Additional water is produced chemically by respiration, thus breaking down carbohydrates by producing water, energy and carbon dioxide. This is the main reason why heavily infested goods tend to become moist and hot. The Figure shows that within a month, 20 granary weevils in 200 grams of triticale can increase the moisture content to levels where microbial development commences and leads to a rapid deterioration of grain.

Only in extremely arid climates (relative humidity < 35 %) may grain kernels become so hard (e.g. wheat moisture content < 9 %) that not even stored product insects can attack. The latter are also adapted to detect suitable food and oviposition sites by smell, a point that will be discussed later.