March 1998, Volume 19 No. 1

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• General News
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General News

LGB - All's Well That Ends Well?

Results from recent research in both East and West Africa* have indicated that classical biological control may once again be playing a significant part in containing a threat to food security posed by an introduced pest, this time the larger grain borer, Prostephanus truncatus (LGB). An introduced histerid predator, Teretriosoma nigrescens, is having a resounding effect on LGB populations in the natural environment, and in West Africa it has been demonstrated to reduce drastically both LGB populations and maize damage in stores.

The bostrichid LGB from Mesoamerica was accidentally introduced into Africa in the 1980s: it was first recorded in Africa at Tabora in Tanzania in 1981 and was recorded from West Africa for the first time in Togo in 1984. It spread from its sites of introduction, but its appearance in Guinea (Conakry) in 1987 may represent a third and separate introduction. It was immediately identified as a serious threat to food security in Africa where it attacks stored maize and cassava, the staple crops relied on by some 500 million people across the continent, and it seems inevitable that it will eventually spread throughout sub-Saharan Africa. It has caused very high losses of stored maize to individual subsistence farmers, and made storage of maize and cassava more difficult and more expensive. In East Africa, it was farmers in Tabora who, faced with a dramatic increase in losses of maize in stores, first alerted officials; losses for maize stored for three months without pesticide application typically averaged 10%, but some farmers were losing more than 30%, losses far in excess of those experienced before the arrival of LGB. Thus began the process that led to an increase in research on pest management in traditional stores over the last 15 years, and to LGB becoming one of the most thoroughly researched storage pests of recent years.

The initial response to the LGB outbreak, co-ordinated by FAO (the Food and Agriculture Organization of the UN), concentrated on conventional containment and control efforts. Researchers from Britain's Tropical Products Institute (now part of the Natural Resources Institute - NRI), working with the Tanzanian Ministry of Agriculture, found that a mixture of pirimiphos-methyl and permethrin provided adequate control of both LGB and other, more widespread, storage pests. However, for control to be fully effective, farmers had to be persuaded to shell their maize and modify their storage structures to contain the loose grain - considerable changes to their traditional system that not all farmers were prepared to make. In West Africa, researchers from the German agency GTZ (Gesellschaft für Technische Zusammenarbeit) working with the Togolese plant protection service found that a similar 'binary' product could be applied directly to maize stored on-the-cob. Again, some control was provided, although lack of adoption suggests that the method was not entirely satisfactory: on-farm surveys carried out some years later, at the height of the LGB attack in Benin indicated that less than 10% of farmers actually used the binary product recommended by the extension service.

FAO worked with affected countries and their neighbours to establish inspection and quarantine measures which achieved some success in allowing regional trade in maize to continue. But the pest continued to spread... by 1996 it had been officially recorded from Tanzania, Kenya, Uganda, Malawi, Zambia, Rwanda and Burundi (and unofficially from Zaire) in the eastern outbreak area and from Togo, Benin, Ghana, Nigeria, Niger and Guinea (Conakry) in West Africa.

Ironically, perhaps, it was a spin-off from the initial containment effort that subsequently provided a vital key to understanding the ecology of the insect and the true nature of `the LGB problem'. A synthetic aggregation pheromone was developed by NRI with the intention of improving the detection of LGB in infested stores and shipments of maize, so that they could be fumigated. It was only some years later that it was discovered that the pheromone did not attract insects already settled in a satisfactory substrate - and indeed had probably served only to attract flying insects into the shipments where lures had been placed! In the meantime, however, the lures were put to good use in Mexico by entomologists from NRI and INIFAP (Instituto Nacional de Investigaciones Forestales y Agropecuarias - the national agricultural research organization)... who found that the pheromone-baited traps caught large numbers of LGB in various types of forest, from sea level to well over 3000 m altitude, and sometimes far away from any maize cultivation.

Another Mexican researcher then found the LGB attacking dying tree branches that had been girdled by lamiine cerambycids - a situation in which the bostrichid apparently benefits from the higher-than-usual nutrient content of the wood. Researchers from IIBC and KARI (the Kenya Agricultural Research Institute) confirmed that a similar association is formed by LGB with locally occurring ceram bycids in East Africa and went on to show that LGB can breed in the wood of many species of trees provided that moisture content and nutrients are within an adequate range. Studies by German university researchers and IITA (the International Institute of Tropical Agriculture) in West Africa** have helped to show how the pheromone production and aggregation behaviour of this wood feeding insect are well adapted to bring the insect into the limited niche suitable for its reproduction... including, by accident and most unfortunately, human food stores.

The pheromone was also found to attract an histerid predator of the LGB. Fortunately, perhaps, while most international resources were, as usual, ploughed into conventional control efforts, GTZ decided at an early stage of the outbreak in Africa to look into the possibility of classical biological control as perhaps offering a more sustainable long-term solution. Comparative studies showed that grain losses were lower in Costa Rica (part of the pest's area of origin) than in the Togo and Tanzania outbreak areas... and the researchers concluded that the most likely agent responsible was the histerid beetle Teretriosoma nigrescens.

There are no previous records of the successful use of histerids in classical biological control. Indeed, when research by IITA and its collaborators in Mexico and Honduras indicated that severe damage can occur, even in the presence of the predator (which did not seem to show an adequate numerical response to its prey), the prospects of success did not look bright. However, the strong response of the predator to LGB's own aggregation pheromone suggested close, co-evolutionary adaptation to the target pest. Laboratory studies indicated that the predator showed a strong preference for feeding on the larvae of LGB and that, at least in the laboratory, it was able to suppress the pest population. When field-cage trials in Togo showed that the predator could also reduce pest population increase under realistic conditions, T. nigrescens was finally approved for introduction to Africa.

It was released in Togo in 1991 and in Kenya in 1992. However, although it established successfully and began to disperse from the release sites, trap catches were low and the expectations for control were not high. Moreover, initial attempts to follow up the impact of releases in Togo produced equivocal results - with pest populations seemingly lower in some release sites but higher in others - and the research effort there was then interrupted by political turmoil.

The first evidence of a possible classical biological control success came from the dry bushland of East Africa. KARI and IIBC staff found that populations of LGB adults caught in pheromone traps in Kenya's Tsavo National Park and Kibwezi Forest collapsed dramatically in 1995: seasonal peaks of trap catches were 80-90% lower than in previous years and have remained low ever since.

In West Africa, the reduction in trap catches was less precipitate, but the release of the predator has been followed by a progressive reduction in LGB population levels, at least in coastal regions, over the last several years. The situation could not be monitored closely until the predator spread across the border from Togo into Benin. There, a nation-wide trap network was established by researchers from IITA, GTZ and the national plant protection service. Regular sampling of stores allowed the impact of the predator on both dispersing pest populations and those infesting stores to be assessed. In 1996 in southern Benin, peak captures in flight traps were less than half those obtained in 1993 and have been even lower in 1997. Weight losses in experimental stores in the same region fell from some 60% before the predator's arrival to less than 10% in 1995. Surveys in farmers' stores show that LGB has not disappeared completely: 1997 sampling in Benin still detected the pest in about one-third of the stores. However, the current situation certainly represents a vast improvement over that in 1993, whether judged by research data or the farmers' own comments.

The vital question which remains to be answered is whether T. nigrescens will be able to control LGB adequately in all outbreak areas in Africa. Despite its spectacular success in lowland areas of East and West Africa, the slow rate of establishment of the predator at upland sites in Kenya, on the one hand, and the severity of LGB damage sometimes encountered at highland and mid-altitude sites in Mexico, on the other, provide some grounds for concern as to whether the predator may be slightly less versatile, ecologically, than the pest.

If so - and the situation is being monitored across Africa - researchers and plant protection services have some complementary strategies ready to deploy. NRI researchers in Ghana and Kenya and their GTZ and IITA counterparts in Benin have been working for some years, in close collaboration with national agricultural research and extension services, to develop and test IPM strategies to complement classical biological control of LGB - both as an insurance against any local failures of biological control and to moderate the continuing damage caused by other formidable storage pests such as Sitophilus and Sitotroga. A new alliance of entomologists and modellers from IITA, IIBC, and the Danish Institute of Agricultural Sciences are forming a network, beginning in Benin, Ghana and hopefully Kenya, to develop decision support tools (based on modelling and GIS approaches) which will encourage the further refinement of integrated control strategies and guide their deployment to those situations in Africa where they are most needed.

It is too early to declare total victory over the LGB - but thanks to an effective international effort, the initially devastating problems cause by this pest are subsiding to more manageable levels.

*Nang'ayo, F. L. O. (1996) Ecological studies on larger grain borer in savanna woodlands of Kenya. Imperial College of Science, Technology and Medicine, London, Ph. D. thesis, 179 pp.

*Borgemeister, C.; Djossou, F.; Schneider, H.; Djomamou, B.; Degbey, P.; Azoma, B.; Markham, R. H. (1997) Establishment, spread and impact of Teretriosoma nigrescens (Coleoptera: Histeridae), an exotic predator of the larger grain borer in southwestern Benin. Environmental Entomology 26(6) , December 1997.

**Scholz, D. (1997) Dispersal and host-finding behaviour of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae). University of Hannover, Germany, Ph. D. thesis, 146 pp.

Contacts: William Meikle, IITA, 08 BP 0932, Cotonou, Republic of Benin [Email: ] and Garry Hill, CABI Bioscience UK Centre (Ascot), Silwood Park, Buckhurst Road, Ascot, SL5 7TA, UK
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