Scientific Name: Sitophilus granarius (Linnaeus, 1875)

Common Names: Granary weevil, grain weevil, maize weevil.

Synonyms

Calandra granaria, Curculio contractus, Curculio granarius 

Taxonomic Position

Phylum: Arthropoda

Class: Hexapoda (Insecta)

Order: Coleoptera

Family: Dryophthoridae

Geographical Distribution

Asia - Afghanistan, India-Andaman and Nicobar Islands, Iran, Iraq, Israel, Japan, Kazakhstan, Malaysia, Saudi Arabia, Sri Lanka, Syria, Thailand, Turkey, Yemen

Africa - Algeria, Cameroon, Egypt, Morocco, South Africa, Swaziland

North America -Canada-Nova Scotia, Mexico, USA

South America -Argentina, Chile, Falkland Islands

Europe- Austria, Belgium, Bosnia-Hercegovina, Croatia, Czech Republic, Denmark, France, Germany, Germany, Greece, Hungary, Ireland, Italy, Poland, Romania, Russian Federation, Serbia, Slovenia, Spain, Sweden, UK, Ukraine

Oceania - Australia

Host Range

Maize, barley, wheat, groundnut, oats, chickpea, sunflower, rice, millets, rye, sorghum, triticale, broad bean and a variety of dried stored products.

Pest Destructive Stage

Primary pest

Adult and larva feed both can feed on commodity. Larvae live concealed within grain.  Adult can’t fly i.e. flightless.

Damage Symptoms

The pest causes hollowing of whole previously undamaged grains. In severe infestations only the grain hull is left along with powdery white frass (fine powdery refuse or fragile perforated wood produced by the activity of boring insects). The large roughly rectangular exit holes with ragged edges are characteristic. Grains which float in water often indicate larval damage.

 Identification features

Adult

• Adult granary weevils can vary considerably in size; between 2.5-5.0 mm in length, although 3 to 4 mm is usual.
• They have the characteristic rostrum (snout or beak) and elbowed antennae of the family Curculionidae (weevils).
• They are shiny and reddish-brown to black in colour and pitted with numerous punctures.
• The punctures on the thorax are in an irregular pattern while those on the elytra (wing cases) are in lines.
• The body has a sparse covering of short, yellow hairs.
• The antennae have eight segments and are often carried in an extended position when the insect is walking.

Larva

• The larvae of maize weevils are white, fleshy and legless.

 Similar Species

• The granary weevil (Sitophilus granarius) can be separated from the maize weevil (Sitophilus zeamais) by the absence of wings beneath the elytra (present in S. zeamais) and by having oval, rather than circular, punctures on the prothorax. The larvae of the two species are not easy to separate.

Life Cycle

• The life span of the granary beetle is 7-8 months on average. Females usually lay around 150 eggs, and up to 300 eggs, throughout their lives.
• Eggs are laid individually in cavities that the female drills in the grain kernels. Cavities are sealed by a waxy plug, which the female secretes.
• Eggs incubate for about 4-14 days before hatching, depending on temperature and humidity. One larva develops in each infested kernel. Feeding larvae excavate a tunnel and may keep feeding until only the hull remains.
• There are four larval instars all of which occur in the grain. Pupation also occurs inside the grain.
• The newly emerged adult chews its way out of the grain, leaving a characteristic large and roughly rectangular exit hole with ragged edges.
• Having left the kernel the female releases a sex pheromone to attract males for mating.
• Adults can survive for a month or more without food in cooler conditions.
• This species is flightless but can walk fairly long distances and can be dispersed further afield in infested grain.

Detection methods

• Sitophilus spp. can be trapped woth a range of commercially available pitfall and probe traps placed at the surface or inserted into grain bulks.
• Efficacy of traps can be enhanced by the addition of baits such as cracked grain and grain oils and synthetic aggregation pheromones.
• Disturbance of the grain causes adult Sitophilus spp. to migrate upwards and become visible on the surface.
• Egg plug staining method can be used for early detection of weevil infestation.
• X-ray radiography technique can be used to detect hidden larvae developing within the grains.