Stored food insects

Page last updated: Friday, 11 May 2018 - 2:00pm

Dried fruit and milled cereal products have been subject to insect attack ever since humans began to store food. The control of insect pests in stored products is difficult because these pests generally live inside the food product they attack.

To combat these pests one needs to understand their life cycle and habits.

This article describes some common storage insects and their control and also warns of the potential threat of the khapra beetle, an exotic storage pest, which is not present in Australia.

Stored food insects

Grain weevils

True weevils can be distinguished from other grain insects by their long snouts which contain their mouth parts.

The female weevil chews a small hole into the grain or other solid food material, deposits an egg, then seals the hole with a gelatinous substance. The larvae hatch and feed within the foodstuff until they pupate, eventually emerging as adults. This segment of the life cycle takes four to six weeks and the adults can live up to eight months, laying 300–400 eggs in this time.

Western Australia has two common grain weevils, the rice weevil (Sitophilus oryzae) and the granary weevil (Sitophilus granarius). Rice weevil adults can fly. They are 2.5–3.5mm long and reddish-brown with four paler brown spots on the wing covers. Granary weevils are flightless, 2.5–4.0mm long and shiny dark brown to black.

Flour beetles

Flour beetles lack the typical weevil snout, are 3–4mm long and reddish-brown. Unlike the weevils they are secondary pests, which means they are unable to attack sound kernels.

They generally infest products such as flour, oatmeal and bran, but occasionally can be found in dried fruits, spices and chocolate. Each female lays up to 400 eggs loosely among food materials. Larvae hatch and feed on fragments of food along with the adults. The life cycle takes four to eight weeks and adults may live as long as 12 months.

The two most common species in Western Australia are the rust red flour beetle (Tribolium castaneum) and the confused flour beetle (Tribolium confusum). Apart from some minor differences, these two species look very similar. However, unlike the rust red flour beetle, the confused flour beetle does not fly.

Brown flour beetle on wheat grains
Brown flour beetle (Tribolium castaneum).

Cigarette beetle

The cigarette beetle (Lasioderma serricorne), as its name suggests, is primarily a pest of stored tobacco. However, it occasionally breeds in milled cereal, stored grain and spices, even the hot ones like curry powder and cayenne pepper.

The beetles are oval, reddish-yellow and 2–3mm long and live for two to four weeks. In a side view they look humped, with a hood-like thorax with the head set beneath it. Each female may lay up to 100 eggs. The entire life cycle takes one to four months.

The emerging adults often chew out through plastic, paper and cardboard packaging, leaving small circular holes.

Brown cigarette beetle
Cigarette Beetle (Lasioderma serricorne)

Sawtoothed grain beetles

The sawtoothed grain beetle (Oryzaephilus surinamensis), is named because of the saw tooth-like projections around its thorax and is a cosmopolitan pest of stored grain and grain products.

Adult beetles may live for more than three years, in which time each female lays up to 300 eggs. Development from egg to adult takes three to four weeks. Large numbers can develop unnoticed in undisturbed household foodstuffs.

Warehouse beetle

The warehouse beetle (Trogoderma variabile), is a major pest of stored grain. Large quantities of cast larval skins can accumulate in and around infested material and cause allergenic reactions to workers. It looks similar to the khapra beetle, the world’s worst pest of stored grain, which is not in Australia.

The adult beetle is 2–3mm long, oval, brown and the wing cases have an irregular pale marking. They are strong fliers. The larvae are very hairy and infestations are often first noticed by clusters of cast larval skins.

Contact information

Pest and Disease Information Service (PaDIS)
+61 (0)8 9368 3080