Biological control of the larger grain borer Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) by its predator Teretrius nigrescens (Lewis) (Coleoptera: Histeridae) in Togo and Benin

Populations of the exotic bostrichid storage pest Prostephanus truncatus and its introduced histerid predator Teretrius nigrescens were monitored biweekly with pheromone traps at 124 sites in southern Togo and in the four agro-ecological zones of Benin, i.e., from south to north: the humid forest savanna (HFS), the forest–savanna mosaic (FSM), the southern Guinea savanna (SGS), and the northern Guinea savanna together with the Sudan savanna (NGS/SS), from mid 1995 to late 1997. Grain losses, pest and predator numbers as well as infestations by Sitophilus zeamais were, in addition, assessed monthly in nearby grain stores. In southern Togo and Benin, a comparison with previous trap catches showed a steady decline in P. truncatus numbers after introduction of T. nigrescens in 1992. At the same time, grain losses decreased to levels corresponding with the average losses reported prior to the arrival of P. truncatus, which were mainly caused by S. zeamais. P. truncatus trap catches were lowest in the HFS and around double in the three northern zones while T. nigrescens in traps decreased rapidly from FSM to NGS/SS. The latter was probably due to the later arrival of T. nigrescens in those regions; thus, the pest–predator system might not yet be at equilibrium. Similar to previous studies, yearly fluctuations in P. truncatus flight behavior could not only be attributed to T. nigrescens, but were also a result of changing weather patterns. Moreover, it is suggested that the role of wild woody habitats in suppressing the pest via its predator T. nigrescens has so far been underestimated.