Insect pests of stored grain

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It has been estimated that between one quarter and one third of the world grain crop is lost each year during storage. Much of this is due to insect attack. In addition, grain which is not lost is severely reduced in quality by insect damage. Many grain pests preferentially eat out grain embryos, thereby reducing the protein content of feed grain and lowering the percentage of seeds which germinate. Some important stored grain pests include the lesser grain borer, rice weevil and rust red flour beetle.

Overseas customers demand insect-free grain. For this reason, the Australian Department of Agriculture has imposed nil tolerance of insects in export grain. Insect pests also increase costs to grain growers both directly through the expense of control on the farm, and indirectly through the costs incurred by grain handling authorities in controlling weevils in bulk storages.

Grain insect pests may be divided into primary and secondary pests. Primary grain insects have the ability to attack whole, unbroken grains, while secondary pests attack only damaged grain, dust and milled products.

Primary grain pests

Lesser grain borer (Rhyzopertha dominica)

The lesser grain borer is the most serious pest of stored grain in Western Australia. It is a dark brown cylindrical beetle about 3mm long. The head is hidden by the thorax when viewed from above. Females lay up to 500 eggs scattered loosely through the grain. The eggs hatch to produce curved white larvae with brown heads and three pairs of legs. The larvae burrow into slightly damaged grains and eat out the starchy interior. After pupating the adults emerge from the grain, leaving large irregular exit holes. The life cycle takes from 3-6 weeks depending on the temperature. Adults may live up to two months.

Lesser grain borer in grain, reddish brown in colour to 3mm

The adult lesser grain borers chews grain voraciously causing damage which may facilitate infestation by a secondary pest. It is a strong flyer and may rapidly migrate from infested grain to begin new infestations elsewhere.

Granary weevil (Sitophilus granarius)

Granary Weevil, black - brown in colour to 5 mm

When disturbed it sits very still for several minutes. An adult lays up to 450 eggs singly in holes chewed in cereal grains. Each egg hatches into a white, legless larva, which eats the grain from the inside. The larva pupates within the grain and the adult then chews its way out. The exit holes are characteristic signs of weevil damage. The life cycle takes about one month under summer conditions and adults may survive for a further eight months. The granary weevil is a small dark brown-black beetle about 4mm long with a characteristic rostrum (snout) protruding from its head. It has biting mouth parts at the front of the rostrum and two club-like antennae.

Rice weevil (Sitophilus oryzae)

Rice weevil, brown - black in color with a long slender snout. With 4 red brown patches on it's back.

An adult lays up to 450 eggs singly in holes chewed in cereal grains. Each egg hatches into a white, legless larva, which eats the grain from the inside. The larva pupates within the grain and the adult then chews its way out. The exit holes are characteristic signs of weevil damage. The life cycle takes about one month under summer conditions and adults may survive for a further eight months. The rice weevil has four orange-brown areas on the wing cases, and is about 3mm long with a characteristic rostrum (snout) protruding from its head. It has biting mouth parts at the front of the rostrum and two club-like antennae. Unlike the granary weevil, the rice weevil is winged and may occasionally fly.

Angoumois grain moth (Sitotroga cerealella)

The angoumois moth is yellow-brown with darker markings. Its wingspan is 12-20mm. Females lay up to 250 eggs on or near the surface of stored grain. The eggs hatch into a caterpillar which bores into grain kernels remaining inside until mature. It then eats its way out of the grain, leaving characteristic exit pin holes on the grain surface.

Unlike most other moth pests, no surface web is formed. The life cycle may be completed in as little as five weeks.

As well as reducing the weight of grains, Angoumois moth infestations impart an unpleasant smell and taste to the cereal.

Secondary pests of stored grain

Rust-red flour beetle (Tribolium castaneum)

The rust-red flour beetle is frequently found on farms in WA. It is a reddish brown beetle about 3mm long. The final three segments of its antennae are greatly enlarged to form a club shape. Young adults are pale brown in colour becoming darker with age. Females lay up to 1000 eggs loosely scattered throughout infested grain. Cream-coloured larvae with biting mouth parts and three pairs of legs hatch and remain free from the grain, feeding on cereal dust and damaged grains. A generation takes about one month to complete under summer conditions, but longer in cold weather. Adults may live up to a year. The adult is winged and may fly.

Rust-red flour beetle, red - brown 3 - 4 mm

Confused flour beetle (Tribolium confusum)

The confused flour beetle closely resembles the rust-red flour beetle in appearance and life history except for the antenna segments which do not have a distinct three-segmented club at the end. It is more often found in flour mills than on farms, as it prefers more finely divided materials.

Confused flour beetle, red - brown in colour  to 4 mm

Saw-toothed grain beetle (Oryzaephilus surinamensis)

The saw-toothed grain beetle is common on WA farms. Adults are dark brown to black with six tooth-like projections on each side of the thorax. They lay up to 500 eggs loosely spread through the infested grain; eggs hatch to produce larvae which feed externally on grain dust and sometimes wheat embryos. The mature larvae pupate within a silken cocoon. A complete generation may take place in as little as three weeks but the adults may live up to nine months. They frequently hide in cracks and crevices of buildings and machinery.

Saw - toothed grain beetle, dark brown in colour to 3.5 mm, with saw like margins on the thorax

Flat grain beetle (Cryptolestes spp.)

Flat grain beetles are small reddish brown insects about 1.5mm long with long antennae and a flattened body. Eggs are laid throughout the stored grain and develop into tiny larvae with characteristic tail horns, biting mouth parts and three pairs of legs. They feed on damaged grain and wheat embryos. Pupation takes place in a cocoon. A complete life cycle takes from 4-5 weeks and adults may survive up to one year.

Warehouse moth (Ephestia spp)

The warehouse moth is a drab grey moth with a 10-12mm wingspan. It usually only infests the surface of stored grain. Moths live for only about two weeks, but during that time lay up to 200 eggs. These are distributed loosely on the grain surface. Larvae hatch out of the eggs and wander over the grain surface leaving a trail of silk which may form a thick mat covering the surface of the infested grain. Mature larvae pupate in a silk cocoon among the grain or on the walls of the building. The life cycle takes at least four weeks.

Warehouse moth larva, yellow - white with brown head capsule to 15mm in length

Indian meal moth (Plodia interpunctella)

The adult Indian meal moth is grey with distinctive brownish-red tips to the forewings. The female lays up to 200 eggs near the grain surface as it slowly passes from grain to grain spinning a silk thread. Severe infestations may form a surface web on the grain heap. Larvae attack the wheat germ, then pupate in a cocoon which may be found in cracks and crevices of buildings. The insects quickly emerge as adult moths. A generation takes as little as four weeks under warm conditions.

Indian meal moth, adult and larva. Adult brown with grey band, larvae off white with brown head capsule

Warehouse beetle (Trogoderma variable)

This beetle was first found in WA in 1989 and that infestation was eradicated. It has now spread to about a dozen locations in WA but has never established to damaging populations.

The warehouse beetle is a pest of stored grain in its own right, but the greater threat is the impact on trade that it could have by masking an incursion of the world's worst pest of stored grain — the khapra beetle. Warehouse beetle and khapra beetle require microscopic examination to distinguish them. Khapra beetle does not occur anywhere in Australia and would have a severe impact on international trade if it became established.

Eggs are usually laid in crevices and under the surface of loose food. They hatch in about a week. Only the larval stage damages grain. It is frequently found in seeds, groceries and used sacks. The larvae are conspicuously hairy. They usually live for about five weeks but may enter a dormant phase (diapause) for more than two years. Larvae may moult up to ten times.

After pupation adults emerge. They are less obvious than the larvae and do no damage to grain. They live for up to five weeks during which females lay up to 80 eggs. Warehouse beetles cannot fly and are spread only in infested commodities and old sacks.

A characteristic of warehouse beetle infestations is the accumulation of cast larval skins. Hairs shed by larvae may cause asthma, skin or gastric problems.

It is impossible to distinguish between T. variabile and several harmless native species without the aid of a microscope. Any hairy larvae found in grain stores should be sent to DPIRD for positive identification.

Warehouse beetle, 3 - 5 mm long, 3 indistinct white bands across it's back. Warehouse beetle larvae, 5-8 mm long, covered in thick reddish - brown hairs.

Insecticide resistance

Insect populations of many species have evolved resistance to insecticides as a result of the widespread use of these chemicals in control. In some cases, insects which have only been exposed to one insecticide develop resistance to other, related compounds.

It takes many years and millions of dollars to develop and test new compounds. Therefore it is important that insecticide resistance is prevented from spreading. This may be achieved by appropriate use of pesticides and by farm hygiene. This consists of careful cleaning of all machinery and buildings used for storing and transporting grain right from the header to the port terminal.

Authors

Rob Emery
David Cousins