Sitophilus granarius

Sitophilus granarius

The wheat weevil or grain weevil or granary weevil (Sitophilus granarius, Linnaeus, 1758) is a small insect belonging to the Dryophthoridae family.

Systematics –
From a systematic point of view it belongs to:
Eukaryota Domain,
Kingdom Animalia,
Sub-kingdom Eumetazoa,
Bilateria branch,
Phylum Arthropoda,
Subphylum Tracheata,
Superclass Hexapoda,
Insecta class,
Subclass Pterygota,
Endopterygota cohort,
Superorder Oligoneoptera,
Coleopteroid section,
Order Coleoptera,
Suborder Polyphaga,
Infraorder Cucujiformia,
Curculionoidea superfamily,
Dryophthoridae family,
Rhynchophorinae subfamily,
Litosomini tribe,
Genus Sitophilus,
S. granarius species.
The terms are synonymous:
– Calandra frumentarius Stephens, 1829;
– Calandra granaria (Clairville and Schellenberg, 1798);
– Calandra laevicosta Philippi and Philippi, 1864;
– Calandra remotepunctata (Gyllenhaal, 1838);
– Calandra remotepunctatus (Gyllenhaal, 1838);
– Cordyle granarius (Thunberg, 1815);
– Curculio granarius Linnaeus, 1758;
– Curculio pulicarius Panzer, 1798;
– Curculio segetis Linnaeus, 1758;
– Curculio unicolor Marsham, 1802;
– Rhynchaenus segetis (Latreille, 1804);
– Rhynchophorus granarius (Herbst, 1795);
– Sitophilus remotepunctatus (Gyllenhaal, 1838).

Geographic Distribution and Habitat –
Sitophilus granarius is a cosmopolitan beetle infesting foodstuffs that prefers areas with a temperate and cold climate. This beetle is easily found inside grain storage deposits or warehouses.
This insect causes serious damage to the kernels by eating infested cereals, both in the adult stage and in that of the larva and by not flying, it prefers to insinuate itself under the surface of the infested food; also loves the dark and shuns the light.

Morphology –
Sitophilus granarius is a small beetle of about 2.5-4.5 mm in length, with blackish-brown elytra, with varying light or dark shades.
The pronotum is dotted, like the elytra; these have a localized punctuation in longitudinal furrows.
It has a long rostrum, longer in the females than in the males, and reddish geniculate antennae.
The hind wings are absent, so it is unable to fly.
The legs are reddish-brown in color.
The larvae have a typical “C” shape and are 3.5–4.0 mm long, yellowish-white in color, with a dark brown head.

Attitude and Life Cycle –
The wheat weevil overwinters at the adult stage, inside the warehouses.
The females, after having dug a hole in the caryopsis with the rostrum, lay an egg for each seed; during the oviposition, which can last even a few months, they lay more than 200 eggs.
After about one or two weeks from the laying of the eggs, an apoda larva emerges which, once it reaches maturity, always inside the seed, the larva pupates.
Larval development lasts 57–71 days, during which the larvae burrow into the seed, passing through four moults; completed the fourth stage form a pupal cell; pupation lasts 5–16 days, at the end of which the adult emerges from the semen.
At least three generations occur during the year but in optimal conditions there can be up to 6 generations.

Ecological Role –
Sitophilus granarius is an insect known since ancient Roman times as a pest of granaries.
This pest attacks various cereals in stock, with a particular predilection for wheat, barley, corn, chickpeas, but can also infest oats, rye, millet and buckwheat. The damage occurs on the kernels and is determined by the adults, but above all by the larvae.
We also remember the species Sitophilus oryzae L., a phytophagous of Rice similar to S. granarius.
Prevention against this insect is based on some criteria:
– the premises intended for storage must be perfectly impenetrable by insects. In this sense, both doors and windows must be able to be hermetically sealed. The same building must also be isolated in the foundations, to allow any disinfestation fumigations, even under pressure; windows must be equipped with metal or nylon nets, with fine mesh, to prevent the entry of adult insects.
Furthermore, the following can be effectively applied in warehouses and processing rooms:
– food traps;
– electric discharge light traps;
– sexual traps, particularly effective against Lepidoptera. With these traps you can get different results:
– massive capture: in this way the numerical consistency of the population is reduced, by capturing the males who can no longer make mating;
– monitoring capture: in this way the size of the population is identified and the development cycle is followed in order to determine the intervention threshold. This makes it possible to identify the most propitious moment to intervene with pesticides and only when the size of the population is such as to cause real economic damage.
In order to evaluate the intervention threshold it is necessary to consider the presence of 1 to 2 insects per trap depending on the phytophagous considered.
Once it has been decided whether the conditions for exceeding the threshold are present, a disinfestation must be carried out with fumigants or with residual action insecticides; it can be done both with infestations in progress and with empty rooms, for preventive purposes.
Please note that fumigations must be carried out by specialized personnel, subject to the authorization of the Police Headquarters, the A.S.L., or the Harbor Master’s Office.
The doses and periods of exposure must be strictly respected to prevent the stored product from taking on odors which are then also transmitted to bread and other derivatives.
It is also possible to intervene with insecticides that act by contact.
The application of the doses must be rigorous to avoid the onset of resistance phenomena, it is also advisable to alternate the use of the active ingredients, to reduce these phenomena.
However, in order to avoid the use of insecticides which, in any case and always, cause harmful effects on the surrounding ecosystem and on humans, the conservation of foodstuffs is currently focusing on the use of two new technologies: the controlled atmosphere and refrigeration; these new techniques which tend to replace chemical products allow to limit infestations and to obtain preserved products without chemical residues.
Obviously these techniques involve the creation of specially constructed and naturally watertight environments.
Finally, it should be remembered that Sitophilus granarius is often parasitized by the hymenoptera Anisopteromalus calandrae (Pteromalidae).

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– Russo G., 1976. Agricultural Entomology. Special Part. Liguori Editore, Naples.
– Pollini A., 2002. Manual of applied entomology. Edagricole, Bologna.
– Tremblay E., 1997. Applied entomology. Liguori Editore, Naples.