The larger grain borer: more trouble in store
MetadataShow full item record
CTA. 1991. The larger grain borer: more trouble in store. Spore 34. CTA, Wageningen, The Netherlands.
Permanent link to this item: http://hdl.handle.net/10568/45558
The larger grain borer, a beetle pest introduced accidentally into Africa, is relentlesly spreading through maize growing areas where, if unchecked, it can decimate stocks of stored maize and dried cassava. Until 1981 the larger grain borer (LGB),...
The larger grain borer, a beetle pest introduced accidentally into Africa, is relentlesly spreading through maize growing areas where, if unchecked, it can decimate stocks of stored maize and dried cassava. Until 1981 the larger grain borer (LGB), Prostephanus truncatus, was known as a pest of stored maize and dried cassava only in Central America. However in that year the beetle was confirmed as being responsible for the destruction of these crops stored on farms in central Tanzania. Following its accidental introduction into Tanzania, LGB has become firmly established on farms throughout most maize growing regions of the country, where it is now the major storage pest. Losses can be total The beetle is extremely destructive. It exhibits a preference for maize on the cob and dried cassava and, by its continual boring through the food, it reduces them to dust. LGB is also capable of damaging stored wheat, sorghum, sweet potato and groundnuts, and it will even tunnel into wooden utensils and storage structures. Maize grains and dried cassava are often so heavily damaged as to be rendered totally unfit for human consumption. The levels of loss can be exceptionally high; weight losses of 34% have been recorded in cob maize in a crib after 3-6 months of storage and up to 70% of dried cassava was considered lost after only 4 months' farm storage. Although the beetle is quite capable of flying between individual farm stores, much of its spread has been assisted by the movement of infested maize, dried cassava and perhaps used grain bags in local, regional and international trade. Infestation can be carried to uninfested areas via maize and cassava bought at rural and urban markets. The distance travelled may extend to many hundreds of kilometres along major bus and lorry routes. LGB may be spread from one country to another by similar means: pests do not respect national borders. In some areas there is much local movement of small amounts of grain across borders as head-loads, and passengers travelling on long distance buses and lorries frequently carry small quantities of maize and cassava for gifts or personal consumption. On a larger scale there are commercial export shipments of food, feed or seed maize and, to a lesser extent, dried cassava and other grains. Through various routes LGB has spread over much of Tanzania and across the borders with neighbouring countries to become established in Burundi and parts of Kenya. In 1984, as a consequence of a second accidental introduction, this time into West Africa, LGB became established in Togo. From there it has spread to neighbouring Benin, Ghana and Burkina Faso. Very recently the beetle has also been found in Guinea Conakry. It is now recognized that Prostephanus has the potential to spread to all the major maize-producing regions of Africa. There is now increasing evidence that it may survive outside the storage environment. Containment and control A package of recommendations for maize has been devised which involves shelling dried cob maize as soon as possible after harvest and mixing a suitable insecticide powder with the shelled maize before storage. Following intensive extension campaigns, farmers are now able to combat the pest and to contain the losses. The success of the control programme is dependent upon providing a timely supply of the insecticide and ensuring its correct application. A supplementary approach to control, without employing insecticide, is currently under investigation. This involves the use of another beetle, Teretriosoma niqrescens, which in Central America is a natural predator on Prostephanus. Teretriosoma does not yet occur in Africa but it is hoped that, following the very recent release of the beetle there, the ravages of Prostephanus maybe checked through biological control (see pages 13,15). International implications To date, the elimination of LGB from infested areas has not proved possible, despite sustained control programmes. One consequence of this problem is the difficulty of trading grain from countries in surplus, where LGB is a recognized pest, to countries not known to have the pest. LGB-infested countries with surplus grain stocks find that they are unable to secure commercial markets for their maize because prospective buyers fear the risk of introducing the beetle. In addition, international organizations involved in financing and distributing 'triangular' food aid in the African region are naturally concerned not to contribute to the risks. A major study of the implications of LGB in ten countries in Eastern and Southern Africa was recently sponsored by the European Commission. A strategy has been proposed which would have two major objectives: firstly, to prevent the distribution of LGB in long-distance commodity movements and secondly to encourage governments, exporters and food aid donors to adopt stricter specifications for the purchase and movement of potentially infested grain. For those countries where LGB is already established, the study makes technical proposals to ensure that exported maize is free from live LGB in order to be able to reassure potential buyers. Further information: For further advice on the recognition and control of the larger grain borer write to Peter Tyler at the Natural Resources Institute, Chatham Maritime, Chatham, Kent, ME4 4TB, UK