Prostephanus truncatus Horn - Larger Grain Borer (LGB)

Summary

The larger grain borer (LGB) was probably introduced to Tanzania in the late 1970s since when it has spread throughout the East African Region and to other parts of Africa. It is a pest of stored maize and dried cassava. Both adults and larvae feed 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: Larger grain borer, greater grain borer

Local: Kenya: Osama; Tanzania: Dumuzi, Scania

Synonyms

Dinoderus truncatus Horn, 1878; Stephanopachys truncatus (Horn, 1878)

Taxonomic Position

Phylum: Arthropoda; Class: Hexapoda (Insecta); Order: Coleoptera; Family: Bostrichidae (Bostrychidae is also a common spelling)

Origin and Distribution

LGB is native to Central America, tropical South America, and the extreme south of the USA. It was introduced into Tanzania, probably in the late 1970s where it has become a serious pest of stored maize and dried cassava. It has since spread into Kenya, Uganda, Burundi, Rwanda, Malawi, Zambia, Mozambique, Namibia and South Africa, and is almost certainly present but unreported from several other countries in Africa. It was first found in West Africa in Togo in 1984 and it has since spread to Benin, Nigeria, Ghana, Niger and Burkina Faso. This pest has spread long distances through its movement in infested grain.

Description

Eggs

Are white to yellow, with no surface features and have a broad ovoid (ellipsoidal) shape

 

Larva

The LGB larva is white, fleshy and has a sparse covering of hairs. It has a parallel-sided C-shaped (scarabaeiform) body. The legs are short and the head is small.

 

Adult

The adult beetle is 3 - 4.5 mm long, dark brown with a body that looks like a flattened tube, the end of which appears to have been cut straight. The body surface is pitted and has many small wart-like outgrowths (tubercles). The head is curved under the thorax so that the back of the head cannot be seen from above. The antennae have 10 segments, made up of a 7-segment 'stem' and a 3-segment 'club'.  

Similar Species

It is possible to confuse the LGB with other storage insect pests such as the lesser grain borer (Rhyzopertha dominica) and the maize weevil (Sitophilus zeamais). The lesser grain borer is about 2-3 mm long, dark red-brown with a body that is more rounded at the end and narrower than that of the LGB. The end of the body of the maize weevil is also more rounded that of the LGB and its mouthparts are 'beak-like' and antennae elbowed.

Life Cycle

Adult LGB eat into maize grains; females make small egg laying chambers at right angles to the main tunnels. Eggs are laid in batches of 20 and covered with finely chewed maize dust. The larvae hatch after about 3 days at 27°C and live on the maize dust produced by the adult's feeding activity. The last instar larva of LGB constructs a pupal case from frass stuck together with a larval secretion, either within the grain or in the surrounding dust. Development of the larva through to the adult stage on a maize diet and under the optimum conditions of 32°C and 80% humidity takes about 27 days. Adults disperse over short distances through flight. Females live longer (61 days) than males (45 days).

Pest Destructive Stage

Both the adults and larvae damage the grain though only the adults produce tunnels. Damage can be severe with weight losses of sometimes around 30% after 3-6 months storage.

Host Range

LGB is a serious pest of stored maize and dried cassava roots, but will also attack maize in the field just before harvest. Minor hosts include yam, sorghum, triticale and wheat. LGB is also present on a number of wild hosts and can infest wooden grain storage structures.

Host Lifestage Affected

Cobs just before harvest and post-harvest and storage.

Host Plant Part Affected

Seeds and grain. 

Damage Symptoms

Adult LGB produce neat round holes where they eat into the maize grains. They tunnel from grain to grain producing large quantities of maize dust. Damaged grains can be identified as they are usually covered by a layer of this dust.

Pest Management

Detection methods
Because the LGB 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 an LGB infestation can be reduced by good store hygiene: cleaning the store between harvests, removing and burning infested residues, immersing grain sacks in boiling water and removing wood from stores or 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 LGB and other storage pests. The use of resistant cultivars may also reduce the severity of an infestation. Varieties with a good husk cover are less likely to be attacked. If the maize stock can be sold soon after harvest it is likely to be relatively unaffected by LGB.

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
Only one biological control agent, a predatory beetle (Teretrius nigrescens) which only eats LGB (i.e. is specific) has been introduced to Africa specifically for the control of larger grain borer. It has reduced the problem in some areas.

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
LGB 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 which can be applied as a dilute dust insecticide control LGB very effectively if applied according to manufacturer's recommendations. However, these insecticides are not so effective against other storage pests such as grain weevils (Sitophilus species) and flour beetles (Tribolium castaneum), which are often found together (i.e. in the same species complex). These species are more susceptible to organophosphorus insecticides. Both types of insecticide can be applied in order to control the whole complex. Combinations such as pirimiphos-methyl and permethrin, deltamethrin and pirimiphos-methyl or fenitrothion and fenvalerate have been used successfully to protect farm-stored grain. A mixture of neem and pyrethrum known as "Nimpyr" seems to be effective against LGB and grain weevils. Fumigation with phosphine or methyl bromide is very effective in large-scale stores.  Pesticides are poisons so it is essential to follow all safety precautions on labels.

Sources of Information and Links

CABI (2010). Prostephanus truncatus (Larger Grain Borer) datasheet. Crop Protection Compendium, 2010 Edition. CAB International Publishing. Wallingford, UK. www.cabi.org/cpc. Accessed on 28 Jan 2010.

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

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

PaDIL – Plant Biosecurity Toolbox. Larger grain borer Prostephanus truncatus. http://www.padil.gov.au/pbt  Accessed on 12 Jun 2011.

Prostephanus truncatus (larger grain borer), Pests and Diseases Image Library, http://www.padil.gov.au/pbt/ Accessed 1st February 2011

Sekyembe S., Maurer G., Gatimu J., Nkanya J. and Injairu S.C. (1993). Training manual for the control of the greater (larger) grain borer (Prostephanus truncatus, Horn). Produced by the Agriculture and Food Protection Department, Food and Agriculture Organisation of the United Nations. http://www.fao.org/wairdocs/x5421e/x5421e00.HTM  Accessed on 28 Jan 2010.

Tigar B.J., Osborne P.E., Key G.E., Flores-S M.E. and Vasquez-A M. (1994). Distribution and abundance of Prostephanus truncatus (Coleoptera: Bostrichidae) and its predator Teretriosoma nigrescens (Coleoptera: Histeridae) in Mexico. Bulletin of Entomological Research 84: 555-565.

Uronu B. (1988). A review on the distribution and control strategy for the Larger Grain Borer, Prostephanus truncatus (Horn) in Northern Zone, Tanzania, pp 229-233. In: Moshi, A.J. and Ransom, J.K. (Eds), Proceedings of the First Tanzania National Maize Research Workshop, Arusha, 6-9 June, 1988, TARO, Tanzania.

Uronu B., Nyakunga Y.B., Riwa W.H. and Kirenga G. (2000). The distribution and control strategy for the Larger Grain Borer Prostephanus truncatus (Horn) in Tanzania pp. 98-105. In: Nahdy, S.M. and Agona, A. (Eds.); Proceedings of the 1st Conference on Stored Product Insect Pests: their status, coping strategies and control in Eastern, Central and Southern Africa. 

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