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The granary weevil, Sitophilus granarius. ©Pests and Diseases Image Library, Bugwood.org
Rostrum (beak) of the granary weevil. ©Pests and Diseases Image Library, Bugwood.org
Rectangular holes caused by the granary weevil. ©Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org
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.
Granary weevil, grain weevil.
Calandra granaria (Fabricius) Gistl, 1848;Curculio contractus Geoffroy, 1785;Curculio granarius Linnaeus, 1758
Phylum: Arthropoda; Class: Hexapoda (Insecta); Order: Coleoptera; Family: Dryophthoridae
The granary weevil is found throughout the temperate regions of the world and in cool upland areas of the tropics.
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.
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.
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.
Adult and larva feed on grain and thus damage it.
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.
Post-harvest and storage
Seeds and grains
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.
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.
Anne M. Akol, Makerere University; Maneno Y. Chidege, Tropical Pesticides Research Institute; Herbert A.L. Talwana, Makerere University; John R. Mauremootoo, BioNET-INTERNATIONAL Secretariat.
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).
BioNET-EAFRINET Regional Coordinator: [email protected]