The life cycle of Coleoptera and Lepidoptera (the main insect pests of stored grain) comprises four stages: Egg, Larva, Pupa, Adult.
Eggs
Females lay their eggs either in cracks in the grain's protective envelope, or in residues and dirt accumulated in storage silos. However, for some species, such as the grain beetle, oviposition takes place directly inside the grain itself.
Larva
The larva is the only growth stage for these insects. During this period, it feeds voraciously, ingesting several times its own weight in food. However, its rigid outer shell forces it to moult regularly to continue its development. The presence of these molts or "exuvia" among grains, oilseeds and their derivatives is a telltale sign of current or past infestation by these pests.
Nymph
After the last larval moult, the insect enters the pupal stage, where it stops feeding. In some species, the pupa is protected by a silken cocoon woven by the larva. During this pupation phase, the insect undergoes major internal and external transformations, leading to its final stage: the winged adult.
Adult
In insect pests of stored grain, adults measure between 0.1 and 1.2 cm in length. Their body, articulated by three pairs of legs, is divided into three distinct parts: the head, which carries the mouthparts and sensory organs; the thorax, to which the legs and wings are attached; and the abdomen, which contains the reproductive organs. These adults move through the interstices between grains and can penetrate deep into the mass, except in the case of Lepidoptera and Ptinidae. Some of these insects are capable of flight, and are widely distributed. Beetles, on the other hand, have poorly developed wings, and some species are unable to fly, although the red flour beetle and the red flour beetle are good fliers.