Stored grain insect pest biology

Beetles and moths, the most common stored-product insect pests, have the following four stages:

  1. Egg
  2. Larva
  3. Pupa
  4. Adult

Egg

Eggs may be laid either in the crevices of kernels or in dust and refuse within bins. Some species, such as granary weevils, lay their eggs inside kernels.

Larva

The larva is the only stage during which the insect grows. It consumes several times its own weight in food, and, as the larval skin cannot stretch, it periodically molts allowing it to increase in size. Cast-off skins found in grains and oilseeds and their products indicate that insects are, or were, present.

Pupa

The pupa, which forms after the last larval molt, does not feed. In some species, the pupa is enclosed in a cell, or cocoon, constructed by the larva. During the pupal stage, the insect undergoes extreme internal and external changes that lead to the development of the adult.

Adult

Adults of stored-product insects are between 0.1 and 1.2 centimetres long. They have three pairs of legs, and their bodies are divided into three parts:

  • The head includes the mouthparts and sense organs.
  • The thorax bears the legs and wings.
  • The abdomen contains the reproductive organs.

Adults move in spaces between kernels and can penetrate deeply into a pile of grains or oilseeds, with the exception of moths and spider beetles. Some stored-product insects can fly and are widely distributed. Beetles have poorly developed wings and some species are unable to fly, although the rusty grain beetle and the red flour beetle can fly well.

Description of insect orders

Coleoptera / Beetles

The order Coleoptera is the term that references beetles and is the largest order of insects. About 30,000 species of beetles are found in North America and close to 7,000 in Canada. Beetles can vary in length from less than a millimeter to up to 75 mm, with the stored product beetles being usually less than 1 centimeter in length. Beetles are also characterized by the hardened covering over their abdomen which is actually a pair of wings. Beetles have 2 pair of wings. The front pair are hardened and leathery or thickened and brittle. These wings, which meet in a straight line down the middle of the back are called elytra. The hind wings underneath the elytra are membranous, and are used for flying. When the beetles are not flying, the hind wings are folded under the elytra. Beetles have chewing mouthparts with well developed mandibles. The weevils (which are also beetles) mouthparts are actually elongated and drawn out into a snout. Also, beetles develop by complete metamorphosis. That is, they have 4 developmental stages: egg, larvae, pupae and adult.

Beetle larvae are very different looking compared to adults. Most stored product pest beetle larvae are elongate and pale white or yellow in colour. They have 3 body parts: head, thorax and abdomen. They have a distinct head capsule, mandibles, eyes and antennae. Most species have three distinct pairs of jointed legs attached to the thorax (weevil larvae are essentially legless). Beetle larvae do not have false legs on the abdominal segments (as do caterpillars or Lepidoptera larvae). Many species of stored product beetle larvae considered as pests also have tail or fork like appendages found on the last abdominal segment, called urogomphi.

Beetles infesting stored grain not only directly damage grain but can cause heating (development of hot spots) and moisture accumulation in the grain bulk due to the insect feeding and respiration. Hot spots may cause a decrease in grain quality due to a reduction in germination and/or increases in fungal and bacterial levels. Generally, larval feeding causes more damage than adult feeding.

Life cycle of beetle or weevil: egg, larva, pupa and adult.

Lepidoptera / Moths

The order Lepidoptera is a large insect order and includes species of moths and butterflies. There are 11,000 species in North America with 4,700 in Canada. The stored product moths considered as pests are less than 3 cm in length with wingspans less than 4 cm. They have two pairs of wings which are covered in scales. Most stored product pest moths are dull in colour, usually consisting of grey and brown. Adult moths have mouthparts modified for sucking. Like the beetles, moths develop by complete metamorphosis. Most stored product moth larvae are elongate, and whitish in color. The larvae have a distinct head capsule with developed mandibles for chewing and eyes and antennae. They have three pairs of 5 segmented legs attached to the thorax, while the abdominal segments usually have a pair of prolegs. Usually there are 7 pairs of prolegs. The thoracic legs are used for mobility while the prolegs are fleshy with tiny hooks on the bottom and are used for griping and balance. Many stored product moth larvae produce silk from their mouthparts and spin a mat of webbing on the surface of stored grain. While adults do not damage stored grains, they seek stored grains to find mates and suitable egg laying sites.

Life cycle of moth: egg, larva, pupa and adult.

Psocoptera / Psocids (booklice)

Booklice and mites have egg, nymph and adult stages. There is no larval stage.

The Psocoptera are a small order of insects. There are about 340 species in North America and 72 species in Canada. These insects are very small, usually being less than 6 mm long. They are soft bodied and are brown, grey, yellow or whitish in colour. There are winged, short winged and wingless species and when present, the wings are membraneous and only functional when the insect is an adult. The antennae are long. They have chewing mouthparts with mandibles. Psocids are omnivorous and feed on molds, grain dust and dead insect parts. They develop through incomplete metamorphosis. That is, egg, nymphal and adult stages. They spoil grain by reaching incredibly large numbers (usually in the absence of predators and parasites) and contaminating it with excrement. Given they are so small, they can often be mistaken for mites.

Life cycle of Psocoptera: egg, nymph, winged adult, short winged adult and wingless adult

Acari: Mites

Mites belong to the Class Arachnida, Order Acarina. There are 1,900 species found in Canada. They are very small and large populations found on foodstuffs can often be mistaken as dust. The body is oval in shape with very little distinction between the two body regions. They develop through incomplete metamorphosis. In the nymphal stage they have 3 or 4 pairs of legs while adults have 4 pairs of legs. Usually they are bright white in colour.

Life cycle of grain mite: egg, nymph and adult.