Sitophilus zeamais Motschulsky, 1855 - Maize Weevil

Summary

The maize weevil is found in all warm and tropical parts of the world. It is a pest of stored maize, dried cassava, yam, common sorghum and wheat. Both adults and larvae feed on internally on maize grains and an infestation can start in the field (when the cob is still on the plant) but most damage occurs in storage.

Common Names

English: maize weevil, greater grain weevil, northern corn weevil, greater rice weevil.

Local name: Tanzania: Mdudu Tembo

Synonyms

Calandra chilensis Philippi and Philippi, 1864;Calandra platensis Zacher, 1922;Cossonus quadrimacula Walker, 1859

Taxonomic Position

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

Origin and Distribution

The origin of the maize weevil is not known but now it is found in all warm and tropical parts of the world.

Description

Adult maize weevils are 3 – 3.5 mm long, dark brown – black in colour and shiny 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 elytra also usually have four pale reddish-brown or orange-brown oval markings. The maize weevil has the characteristic rostrum (snout or beak) and elbowed antennae of the family Curculionidae (weevil family). 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 maize weevil (Sitophilus zeamais) can be separated from the granary weevil (S. granarius) by the presence of wings beneath the elytra (absent in S. granarius) and by having circular, rather than oval, punctures on the prothorax. The larvae of the two species are not easy to separate.

It is possible to confuse the maize 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 more rounded than that of the LGB, and its mouthparts are 'beak-like' and antennae elbowed.

Life Cycle

Females chew into maize grains where they lay their eggs throughout most of their adult life of up to one year, although 50% of their eggs may be laid in the first 4-5 weeks. Each female may lay up to 150 eggs in her lifetime. Development time ranges from about 35 days under optimal conditions to over 110 days in unfavourable conditions. Eggs, larval and pupal stages are all found within tunnels and chambers bored in the grain and are thus not normally seen. Because larval stages feed on the internal parts of the grain, it is difficult to detect infestations early. Adults emerge from the grain and can be seen walking over the grain surfaces. Adult emergence holes are large with irregular edges. Females release a sex pheromone which attracts males.

Pest Destructive Stage

Both adult and larva damage the grain by chewing. The infestation can start in the field but most damage occurs in storage.

Host Range

The maize weevil can develop on a range of cereal crops. It is a serious pest of stored maize, dried cassava roots, yam, common sorghum and wheat in the East African Region.

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 (insect waste). The large emergence holes with irregular edges are characteristic. Grains which float in water often indicate larval damage.

Pest Management

Detection methods
Because the maize 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 maize 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 maize as soon as possible after it has reached maturity will reduce the chances of attack by maize 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
There have been various studies on biological control agents for the maize weevil. Various parasitoids (Anisopteromalus calandrae, Cephalonomia tarsalis, Lariophagus distinguendus and Theocolax elegans) could be effective if introduced early in the storage period. The fungus Beauveria bassiana can be used as a biological insecticide to control maize weevil in stored maize. The bacterium 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
Maize weevil populations build up the longer the maize is kept in store so it is important to inspect the stock regularly. If the pest is found then some form of treatment will be required. Synthetic pyrethroid insecticides such as permethrin and deltamethrin are not very effective against maize weevils which are more susceptible to organophosphorus insecticides such as fenitrothion and pirimiphos-methyl. Fumigation with phosphine or methyl bromide is very effective in large-scale stores. Also grain stocks may be fumigated with phosphine. Pesticides are poisons so it is essential to follow all safety precautions on labels.

Sources of Information and Links

CABI. (2010). Sitophilus zeamais (maize weevil) datasheet. Crop Protection Compendium, 2010 Edition. CAB International Publishing. Wallingford, UK. www.cabi.org/cpc. Accessed on 28 Jan 2010.

Coombs CW, Billings CJ, Porter JE, (1977). The effect of yellow split-peas (Pisum sativum L.) and other pulses on the productivity of certain strains of Sitophilus oryzae (L.) (Col. Curculionidae) and the ability of other strains to breed thereon. Journal of Stored Products Research, 13(2):53-58.

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.

Ganesalingam VK, (1977). Insects in paddy and rice in storage in the Kandy District. Ceylon. Journal of Science, Biological Sciences, 12(2):169-176.

Krischik V.A., Cuperus G and Galliart D (eds.).1995. Stored Products Management, 2nd Ed. Oklahoma State Univ. 204 pp. http://www.grainscanada.gc.ca/storage-entrepose/pip-irp/lgb-ppg-eng.htm#d accessed on 15/5/2010.

Youdeowei A. (1993) Pest and vector management in the tropics, Longman group Ltd. England, 399pp.

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