TERETRIUS NIGRESCENS AGAINST LARGER GRAIN BORER PROSTEPHANUS TRUNCATUS IN AFRICAN MAIZE STORES: BIOLOGICAL CONTROL AT WORK?

Authors: HOLST, NIELS - DANISH INST AG SCI; Meikle, William

Submitted to: Journal of Applied Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2002
Publication Date: 4/1/2003
Citation: HOLST, N., MEIKLE, W.G. TERETRIUS NIGRESCENS AGAINST LARGER GRAIN BORER PROSTEPHANUS TRUNCATUS IN AFRICAN MAIZE STORES: BIOLOGICAL CONTROL AT WORK?. JOURNAL OF APPLIED ECOLOGY. 2003.

Interpretive Summary: 1. Following the accidental introduction of the bostrichid beetle Prostephanus truncatus into East and West Africa in the early 1980s, a classical biological control campaign was launched in 1991; the histerid beetle Teretrius nigrescens was released as a biocontrol agent to prevent the destructive outbreak of the pest in small-farm maize stores. However, while the campaign has been ongoing, so has the discussion in the scientific community about the merits of this campaign and its chances of success. 2. From an analysis of the insect population dynamics inside rural maize stores, we conclude that (i) T. nigrescens reduced significantly the population growth rate of both P. truncatus and the non-target pest, the weevil Sitophilus zeamais; (ii) T. nigrescens displayed a positive numerical response to both prey species, P. truncatus and S. zeamais; (iii) asymmetric competition existed between the two prey species; S. zeamais was negatively affected by P. truncatus but not vice versa; (iv) T. nigrescens and S. zeamais displayed negative intra-specific density-dependence, whereas P. truncatus was resource-limited. 3. Transforming the regression equations into differential equations, subsequent simulations showed that (i) the presence of T. nigrescens in the store would delay serious loss, defined as the period until 10% weight loss of the stock, by at most one month; (ii) inundative releases, even at the unrealistically high rate of one T. nigrescens released per kg maize kernels per day, would delay serious loss by less than one month; and (iii) in the best-case scenarios, T. nigrescens over 8 months of stocking time would reduce the final percentage of kernels damaged by 30 percentage points to a level of 30-50%. 4. Since complete pest control through classical biological control with T. nigrescens does not seem likely, neither from our study or from field evidence published elsewhere, we recommend that further research and practical guidelines should consider the farmer as an active agent managing the store according to integrated pest control principles. 5. The statistical methodology applied in this study should be generally applicable for the detection of competition and predation among interacting populations.

Technical Abstract: 1. Problem. A great deal of work has been done to attempt to evaluate the importance of Teretrius nigrescens as a classical biological control of Prostephanus truncatus in Africa. Some workers have declared that T. nigrescens has solved the P. truncatus problem, while other workers are still reporting serious losses to P. truncatus. Understanding the role of T. nigrescens is important to organizations who funded and conducted the research to identify, produce and release the natural enemy, as well as to farmers and researchers interested in knowing whether P. truncatus should still be considered a serious pest of stored products. 2. Approach. Using datasets from field experiments in Benin, the relationship of T. nigrescens density (insects per ear of maize) to the densities of both P. truncatus and S. zeamais was statistically tested using regression procedures. The resulting relationships were then incorporated into simulation models, developed and published earlier by the authors, to examine the impact of the natural enemy both in terms of pest densities and in terms of grain damage and loss. 3. Results. The authors found that the presence of T. nigrescens in the store would delay serious loss by at most one month, that inundative releases even at unrealistically high rates would delay serious loss by less than one month and that even in the best-case scenarios, T. nigrescens would reduce, over 8 months, the percentage of kernels damaged by 30% to a level of 30-50%, which is still almost a complete loss of the grain. Since complete pest control of P. truncatus through classical biological control with T. nigrescens does not seem likely, neither from this study or from field evidence published elsewhere, the authors recommended that further research and practical guidelines should be developed to help farmers manage their grain stores according to integrated pest control principles.