Scientific Name: Rhyzopertha dominica (Fabricius)

Common Names: English - Lesser grain borer, American wheat weevil, Australian wheat weevil, grain weevil, stored grain eater

Synonyms

Synodendron dominicum Lesne, 1898

Taxonomic Position

Phylum: Arthropoda

Class: Hexapoda (Insecta)

Order: Coleoptera

Family: Bostrichidae

 Host Range

The lesser grain borer feeds mainly on maize, oats, barley, rice, millet, sorghum, wheat, durum wheat, chilli, coriander, turmeric, cassava, beans, ginger, wheat flour, and a variety of dried stored products. The lesser grain borer can also be found mainly in cereal stores, and food and animal feed processing facilities.

Pest Destructive Stage

Primary Pest

Both adult and larva damage the grain by boring/eating.

Damage Symptoms

The pest hollows previously undamaged grains, eating the starch. The holes are characteristic with an even edge. Adults and larvae feed on the entire grain apart from the bran leaving behind empty husks and flour.

Identification Features - 

Egg

• The eggs are ovoid in shape, 0.6 mm in length, 0.2 mm in diameter, laid loosely in grains.
• They are white when first laid, and turn rose to brown before hatching.

Larva

• The larvae are white to cream coloured, with biting mouthparts and three pairs of legs.
• The young larvae are mobile in grain bulks but become immobile and gradually more C-shaped as they complete their development concealed within grain or flour.

Adult

• The adult is 2-3mm long, reddish-brown in colour with a slim cylindrical shape.
• The elytra (wing cases), which cover membranous hind wings, have regular rows of coarse punctures (finer at sides) covered with curved setae (hairs). 
• The front edge of the pronotum (plate-like covering of front segment of the thorax) has a saw-toothed appearance.
• The head is not visible when viewed from above.
• Its antennae end in a distinctive 3-segmented club-shape.

Life Cycle

• Lesser grain borer females lay between 200 and 500 eggs in their lifetime. They are typically white when first laid, turning rose to brown before hatching.
• The larvae, which are white to cream in colour with three pairs of legs and biting mouthparts, undergo four instars.
• The larvae are quite mobile and linear in shape at first but become more immobile and C-shaped as they develop.
• The larvae eat into cereal grains or another suitable hard substance (e.g. wood) where they complete their development.
• Adults, which can live up to 240 days, emerge by chewing through the outer grain layers.

Detection Methods 

A variety of methods have been used to detect insect pests of stored products, including R. dominica. The simplest method is to sieve a 200-1000 g sample of the grain and look for adults. Lesser grain borer can be detected by placing probe pitfall traps in the grain. Insects fall into these traps, as they move through the grain. The traps are left in the grain and inspected periodically.