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 three to six weeks depending on the temperature. Adults may live up to two months.
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)
Rice weevil (Sitophilus oryzae)
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.