T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network


Weevil (Grain) Sitophilus granarius

Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Coleoptera
Family: Curculionidae
Subfamily: Dryophthorinae
Tribe: Litosomini
Genus: Sitophilus
Species: S. granarius
Binomial name: Sitophilus granarius
Common names: Grain Weevil, Granary weevil, Wheat Weevil

Sitophilus granarius is found throughout the temperate regions of the world and in cool upland areas of the tropics. It is a pest of stored grain and a variety of stored products as both the adults and larvae feed on the grain kernels and damaging them. The hosts for Sitophilus granarius includes the following: maize, barley, wheat, groundnut, oats, chickpea, sunflower, rice, millets, rye, sorghum, triticale, broad bean and a variety of dried stored products. It causes significant damage to stored grains and drastically decreases crop yields.

Sitophilus granarius are tiny, flightless weevils that are 2-4 mm in length. Depending on the grain kernels, the size of the weevil varies.They have the characteristic rostrum (snout or beak) and elbowed antennae of the family Curculionidae. They are shiny and are 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. The larvae of are white, fleshy and legless.

The lifespan 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 gelatinous secretion waxy plug, which the female secretes. Female weevils can tell if a grain kernel has had an egg laid in it by another weevil. They avoid laying another egg in this grain. 
The eggs incubate for about 4-14 days before hatching, depending on temperature and humidity. 
One larva develops in each infested kernel. The 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. The larvae are legless, humpbacked, and are white with a tan head. In the pupal stage, they have snouts like the adults.

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. In warm conditions, the life cycle can be completed within 4-6 weeks, but this can up to 21 weeks in the winter. One pair of weevils may produce up to 6,000 offspring per year.
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.  


Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/