Sitophilus granarius (Linnaeus, 1875) - Granary Weevil

Summary

The granary weevil is found throughout the temperate regions of the world and in cool upland areas of the tropics. It is a pest of stored maize and a variety of stored products. Both adults and larvae feed on internally on maize grains.

Common Names

Granary weevil, grain weevil.

Synonyms

Calandra granaria (Fabricius) Gistl, 1848;Curculio contractus Geoffroy, 1785;Curculio granarius Linnaeus, 1758

Taxonomic Position

Phylum: Arthropoda; Class: Hexapoda (Insecta); Order: Coleoptera; Family: Dryophthoridae

Origin and Distribution

The granary weevil is found throughout the temperate regions of the world and in cool upland areas of the tropics.

Description

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. 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.

It is possible to confuse the granary weevil with other storage insect pests such as the larger grain borer – LGB (Prostephanus truncatus). The end of the body of the maize weevil is also more rounded that of the LGB and its head is elongated into a distinctive “beak-like snout? and its antennae are elbowed.

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. In warm conditions the life cycle can be completed within 4-6 weeks, but this can up to 21 weeks in the winter. 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.

Pest Destructive Stage

Adult and larva feed on grain and thus damage it.

Host Range

The hosts for the granary weevil include the following: maize, barley, wheat, groundnut, oats, chickpea, sunflower, rice, millets, rye, sorghum, triticale, broad bean and a variety of dried stored products.

Host Lifestage Affected

Post-harvest and storage

Host Plant Part Affected

Seeds and grains

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.

Pest Management

Detection methods
Because the granary weevil larvae develop inside the grain it is difficult to detect the pest by visual inspection unless its numbers are very high.
 
Cultural practices
The severity of a granary weevil infestation can be reduced by good store hygiene: cleaning the store between harvests, removing and burning infested residues, fumigating the store to eliminate residual infestations and the selection of only uninfested material for storage. Harvesting the crop as soon as possible after it has reached maturity will reduce the chances of attack by granary weevil and other storage pests. The use of resistant cultivars may also reduce the severity of an infestation.
 
Physical control
The removal of adult insects from the grain by sieving can reduce populations but this is very labour-intensive. The addition of inert dusts such as ash and clay to the grain can reduce insect numbers by causing the insects to die from desiccation.
 
Biological pest control
Biological control with parasites, parasitoids and predators has not been practiced against the granary weevil. Beauveria bassiana can be used as a biological insecticide to control granary weevil in stored maize. Bacillus thuringiensis can be used on adults
 
Controlled atmosphere
Where suitable infrastructure exists, low oxygen and carbon dioxide-enriched atmospheres can be used to control stored product pests.

Freezing and Heating
Where the infrastructure exists, freezing for several days and heating for 24 hours have proved to be effective control methods for stored product pests.

Chemical control
The granary weevil is susceptible to organophosphorus compounds such as fenitrothion and pirimiphos-methyl but is not very susceptible to synthetic pyrethroids. Grain stocks can be fumigated with phosphine or methyl bromide. 
Pesticides are poisons so it is essential to follow all safety precautions on labels.

Sources of Information and Links

CABI. (2007). Sitophilus granarius (granary weevil) datasheet. Crop Protection Compendium, 2007 Edition. © CAB International Publishing. Wallingford, UK.

Dent D. (2000). Insect pest management.CAB International Wallingford, UK

Infonet-biovision. http://www.infonet-biovision.org/default/ct/91/pests. Accessed on 28 Jan 2010.

Gaby, S. (1988) Natural crop protection in the tropics. Margraf Publishers Scientific books, Germany

Krischik V.A., Cuperus G. and Galliart D. (1995). Stored Product Management, 2nd Edition, Oklahoma State Univ. 204 pp.

http://www.grainscanada.gc.ca/storage-entrepose/pip-irp/lgb-ppg-eng.htm#d accessed on 15/5/2010.

PaDIL  – Plant Biosecurity Toolbox. Granary weevil Sitophilus granarius. http://www.padil.gov.au/pbt. Accessed on 12 Jun 2011.

Ridgway R.L., Silverstein R.M and Inscoe, M.N. (1990) Behaviour- modifying chemicals for insect management, Marcel Dekker Inc., New York.

Youdeowei A. (1993) Pest and vector management in the tropics. Longman.

Editors

Anne M. Akol, Makerere University; Maneno Y. Chidege, Tropical Pesticides Research Institute; Herbert A.L. Talwana, Makerere University; John R. Mauremootoo, BioNET-INTERNATIONAL Secretariat.

Acknowledgments

We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).

Contact

BioNET-EAFRINET Regional Coordinator: eafrinet@africaonline.co.ke