From crisis to control: LGB in the spotlight
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CTA. 1996. From crisis to control: LGB in the spotlight. Spore 64. CTA, Wageningen, The Netherlands.
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Following its accidental introduction into Africa in the early 1980s, the Larger Grain Borer (LGB), Prostephanus truncatus, has spread rapidly and widely, becoming a serious pest of stored maize and dried cassava and a very real threat to food...
Following its accidental introduction into Africa in the early 1980s, the Larger Grain Borer (LGB), Prostephanus truncatus, has spread rapidly and widely, becoming a serious pest of stored maize and dried cassava and a very real threat to food security (see Spore 34 page 5). Consequently it has become the most intensively studied storage e pest. The initial spread of LGB in East Africa was rapid and loss levels in cob-stored maize in cribs in some places were as high as 34% after only six months' storage. Losses in dried cassava were also exceptionally high. While indigenous storage pests such as grain weevils (Sitopholus sip), grain moth (Sitotroga cerealella) and lesser grain borer (Rhyzopertha dominica), were a nuisance, they never reached the levels of damage caused by Prostephanus which, as an introduced species, had no natural enemies. In an effort to control this new pest, farmers were encouraged to change their storage practices and improve their store hygiene: to store maize shelled instead of on the cob, and to treat grain with a mix of insecticides. Large scale stored maize was fumigated with bromide or phosphine. These changes might have proved effective but severe drought affecting parts of Africa, necessitated large-scale inter regional movement of grain, often from infested to uninfested regions. So despite these and other safeguards LGB continued to spread, albeit at a slower rate. This control strategy, which was followed for a number of years, resulted in the apparent stabilization of the spread of LGB and some of the major maize producing countries such as Mozambique, Zimbabwe and South Africa remained unaffected. In the early '90s there was increasing evidence to show that LGB was established in the bush and and forest regions outside the storage environment. The question now was whether LGB evolved as a wood-boring insect and only later moved into maize and cassava, which subsequently became preferred hosts. If this proves to be the case, as researchers now strongly suspect, the spread of LGB into the rest of Africa is inevitable, by dispersal through the natural environment. Alternative control methods The fact that LGB will undoubtedly spread into those areas of Africa currently unaffected is not the unmitigated disaster that it might appear. Much has been learned since its initial arrival in Tanzania and, for the most part, losses have been brought down to manageable levels. Whilst LGB is not the only storage pest that farmers have to deal with, its occurence has encouraged people to review and renew many aspects of pre- and postharvest handling of crops, leading to improvement of storage as a whole. Unfortunately, the changes in storage practice such as the shelling of maize and its treatment with chemicals, which had proved effective control measures in East and Central Africa, were not really an option in West Africa, particularly in the humid zones. Due to the much higher humidity, shelled grain treated with chemicals became more prone to mould development. Moreover, in some countries in the region, chemicals were in such short supply that the need for alternatives became imperative and in the early '90s several research groups in West and East Africa began investigating integrated control of harvest pests in general and the LGB in particular. A search to find a natural enemy to LGB which began in the mid-'80s proved fruitful when researchers from GTZ (the German Technical Agency) and the UK's Natural Resources Institute (NRI) found a predatory beetle, the histerid Teretriosoma nigrescens in Central America, the original home of LGB. This was reared and released in Togo at the beginning of 1991 and subsequently spread to neighbouring areas of Benin where its movements have been assessed by pheromone trap monitoring. The results have been encouraging: in the coastal zones of both Benin and Togo, pheromone traps and rural stores have shown an increase in the numbers of 7. nigrescens and a drop in LGB numbers. Further releases are planned and, following impact assessments, there will be releases by IITA in Guinea Conakry and probably Zambia later in the year. Simultaneously, researchers at the International Institute of Biological Control (IIBC), part of CAB International, and the Kenya Agricultural Research Institute (KARI) also imported, screened and mass reared T. nigrescens for releases in Kenya beginning in May 1992. Again, continuous monitoring has shown encouraging levels of establishment and spread by the predatory beetle and a subsequent sharp fall in numbers of LGB monitored in pheromone traps. Information exchange Since the first outbreaks of LGB in the '80s there have been regular meetings to assess the impact and discuss new methods of control and containment of the pest. Last year a joint IITA/GTZ/ODA workshop was held for West Africa. As a follow-up, and to look more specifically at the problems of East and Central Africa, a Workshop on Storage Pest Management was held at Naivasha, Kenya in April of this year. This workshop was jointly organized and hosted by CAB International's Africa Regional Office and KARI. Leading crop storage specialists from the region and from FAO, GTZ, IIBC, IITA and NRI reviewed progress and discussed what steps still need to be taken in research, extension and the trade in stored products. The recommendations included the need to implement biological control of LGB in all affected countries in East Africa as a matter of urgency, and to develop and implement I PM strategies for storage pests using biological control of LGB as a base. Another strong recommandation was the farmer participatory research and extension should be adopted in future. There is also a need for regional coordination for training, information exchange and the coordination of research and extension methodologies. A warning was given that the use of phosphine as a fumigant should be controlled to avoid misuse and the risk of resistance developing. It is clear that LGB can be controlled, but that determined and regionally concerted action will be essential. The basis for such regional action is already available in the form of an agreed set of recommendations on future directions and a detailed list of research activities prepared by the Workshop. Structured in the form of a project planning matrix, these can now be used for project development and practical implementation by national programmes. Further irnormation froam: CABI Regional Office for Africa PO Box 76520 Nairobi, KENYA
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