Sitophilus granarius (grain weevil)

Sitophilus granarius (Linnaeus) [= Calandra granaria (Linnaeus)] is a typical member of the family Curculionidae. Two other species in this genus, S. oryzae and S. zeamais, are also common grain storage pests.Adults can be identified using the keys of Haines (1991) and Mound (1989) and both adults and larvae using keys of Gorham (1991).

Eggs, Larvae and PupaeThe developmental stages of S. granarius are all found within tunnels and chambers bored in the grain and are thus not normally seen. The larvae are white and apodous. There are four larval instars. The general appearance of the larva and pupa is similar to that of S. zeamais and S. oryzae.AdultAdults of Sitophilus granarius 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 and elbowed antennae of the family Curculionidae. They are uniformly polished chestnut-brown or reddish-brown to shiny black in colour. The body is sparsely covered with short, stout yellow hairs. The head is prolonged into a slender snout. The dorsal surface of the rostrum is more closely and strongly punctured in males than in females. The antennae have eight segments and are often carried in an extended position when the insect is walking. The prothorax has distinctly oval punctures. Adults do not have wings, and therefore cannot fly.Identification keys have been provided by Haines (1991), Mound (1989) and Gorham (1991).

S. granarius is distributed throughout the temperate regions of the world. In tropical countries it is rare, being limited to cool upland areas. The FAO global survey of insecticide susceptibility recorded it from the UK, France, Italy, Spain, Denmark, Sweden, Poland, Algeria, Iraq, Canada, USA, Chile, Argentina, Swaziland, South Africa, Australia, Russia and Thailand (Champ and Dyte, 1976). It is also known from Yemen (Haines, 1981).

A record of S. granarius in Japan (Morimoto, 1978) published in previous versions of the Compendium has been removed as it refers to interceptions of the pest in Japan. In a more recent publication, Morimoto et al. (2007) state that although S. granarius was detected for the first time on imported rice in 1923, it has not established in Japan. 

Many countries, including those that have exclusively hot climates, have S. granarius as a named quarantine pest. This is presumably because it is not widely recognized that it will only establish itself in temperate climes. Nevertheless this species can cause serious damage to grain stocks under hot conditions even though it will eventually die out.Many countries, including those that have exclusively hot climates, have this species as a named quarantine pest. This is presumably because it is not widely recognized that it will only establish itself in temperate climes. Nevertheless this species can cause serious damage to grain stocks under hot conditions even though it will eventually die out.

Adult S. granarius cannot disperse by flight, although they are very active walkers. They are transported within grain as eggs, larvae or pupae. They can readily spread in grain residues.

S. granarius is a frequent pest of wheat and barley. It can attack other cereals such as maize, sorghum and rice, but it does not compete well with the other two Sitophilus species on these grains.

The developmental stages of S. granarius are not normally seen as they occur inside intact grains. Adult emergence holes with irregular edges are apparent some weeks after initial attack. Adults can be found wandering over the surface of grain especially if the grain has been disturbed.

S. granarius can be separated from S. zeamais and S. oryzae by the absence of wings and by the presence of oval, rather than circular, punctures on the prothorax. The larvae cannot be easily separated by superficial features; only characters accessible to a microscope are useful (Gorham, 1991).

The biology and behaviour of S. granarius is similar to the tropical species S. oryzae and S. zeamais (see data on these species), except that it cannot fly. A detailed review of all three species has been prepared by Longstaff (1981). Adults live for 7 to 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 bores in the grain kernels. Cavities are sealed by a waxy egg 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. Pupation occurs inside the grain. The newly emerged adult chews its way out of the grain, leaving a characteristic exit hole. In warm summer conditions the life cycle can be completed within 4 to 6 weeks, but can take as long as 17 to 21 weeks in the winter. Adults can survive for a month or more without food in cooler conditions. Optimum conditions for development are similar to other tropical species of Sitophilus, about 30°C and 70% RH (Richards, 1947), but in tropical areas it is apparently not able to compete with S. oryzae and S. zeamais. It seems that its distribution is limited more by its commodity associations with cool climate crops (see Host Range) than by its direct response to temperature. However, it can develop at temperatures down to 11°C, and is therefore successful in temperate regions that are too cool for other Sitophilus species (Howe and Hole, 1968). Being flightless, S. granarius cannot usually infest crops in the field before harvest.

Anisopteromalus calandrae, Cephalonomia tarsalis, Lariophagus distinguendus, and Theocolax elegans [Chaetospila elegans] are all known to parasitize the larvae of S. granarius. Steidle (1998) reviewed the literature on L. distinguendus.

S. granarius is a serious pest of stored cereal grains in cool climates, whether in temperate or tropical latitudes. It can also cause serious damage under hot conditions before populations die out. Larval stages feed inside the grain on the kernels, leaving only the hulls. Severe infestations can reduce stored grain to a mass of hulls and frass. It is a severe pest of wheat and barley grains. It also infests rye, rice, maize and other cereal grains. It sometimes infests sunflower seeds, dried beans, chickpeas, groundnuts, acorns, chestnuts, pasta products, ornamental dried corn and birdseed.Feeding damage by S. granarius can make grains vulnerable to attack by other pests, such as the weevil Caulophilus oryzae, which are unable to penetrate intact grains. Pajmon (2000) described its economic impact in Slovenia. Trematerra et al. (1999) described infestations in traditional grain storage warehouses in Italy.

Granary weevil infestations in stored cereals are generally difficult to detect, particularly in the initial stages, since the life cycle mainly takes place (from egg to pupa) inside the kernel. Pitfall traps placed on the grain surface and probe traps inserted into grain bulks have been used successfully to detect adult Sitophilus granarius (e.g. Buchelos and Athanassiou, 1999; Wakefield and Cogan, 1999). Larval stages in the grain may be detected using hidden infestation detection techniques. These can involve squashing the grain against indicator papers, testing for changes in specific gravity, or using X-ray machines (Haff and Slaughter, 1999). A novel method using microphones to detect larval movement in grains at temperatures above 15°C was described by Weinard (1998). Rotundo et al. (2000) described a serological method for detecting the immature stages of S. granarius in kernels, while Ridgway and Chambers (1996) described a detection method involving near-infrared reflectance spectroscopy.

Chemical Control

Grain may be protected by the admixture of insecticides. S. granarius has a low susceptibility to synthetic pyrethroids but is readily killed by organophosphorus compounds such as fenitrothion and pirimiphos-methyl. Grain stocks may be fumigated with phosphine to eliminate existing infestation. However, fumigation treatments provide no protection against reinfestation. Fumigation of S. granarius pupae with phosphine at 20°C resulted in a LT95 of 3.9 days (at 0.5 g/m²) and 100% mortality after 10 days (Goto et al., 1996). Carbon dioxide fumigation, in controlled-atmosphere storage, can also be used to control S. granarius in stored grain, although the weevil is more resistant to this treatment than other storage pest species (Kishino et al., 1996). Inadequate fumigation or controlled-atmosphere treatments are likely to result in some survival. Gamma radiation, at doses of 30-500 Gy, prevented the development of eggs and larvae of S. granarius in grain (Aldryhim and Adam, 1999); although this method of control is relatively expensive.

Biological Control

Biological control has not been practised against S. granarius. There may be some potential for the development of pest management strategies that favour the action of natural parasites. Lariophagus distinguendus, an ectoparasitoid that prefers older larvae and pupae as hosts, appears particularly promising as a biocontrol agent (Steidle, 1998).

Cultural Control and Sanitary Methods

Good storage hygiene plays an important role in limiting infestation by S. granarius. The removal of infested residues from the previous season's harvest is essential. Ensuring grain is well dried at intake is very important. Moisture content of 10-12% is desirable, but most grain purchasers have an upper limit of 14% and do not find it practical to operate below this limit.

Infested grain can be treated with hot air, at an inlet temperature of 300-350°C, as an alternative to fumigation. Good weevil control has been obtained by this method, with heat exposure times (around 6 seconds) that do not unduly harm the grain (Mourier and Poulsen, 2000).