The effect of wild grasses on lepidopterous stem borer infestation on maize was studied in western Africa from 1991 to 1999. First, countrywide surveys in roadside fields revealed strong negative relationships between grass abundance in the vicinity of a maize field and stem borer incidence in maize. It was hypothesized that this may be due to plant induced mortality and/or higher natural enemy activity if grasses are present. Oviposition experiments in the greenhouse showed that Sesamia calamistis Hampson and Eldana saccharina Walker females had equal preferences for most grass species and maize, but both had a high preference for Pennisetum polystachion L. Life-table studies showed that on the most common grass species, with exception of Sorghum arundinaceum (Desv.), immature mortality for both borer species was nearly 100%. Thus wild grasses act as trap plants. Next, sites in the humid forest in Cameroon and the derived savanna in Benin were selected to evaluate the effect of planting some of these trap plant species as border rows on borer infestations. In Cameroon, Pennisetum purpureum Moench significantly lowered infestations by Busseola fusca Fuller, S. calamistis and E. saccharina. In Benin, grasses significantly increased egg and larval parasitism, and Panicum maximum Jacq. was the most efficient species for suppressing S. calamistis and Mussidia nigrivenella Ragonot infestations. The findings explain why stem borer problems are more severe in the forest zone, where grasses are scarce, than in the derived savanna where there is a continuum of wild and cultivated host plants.
Species 1: Lepidoptera Noctuidae Sesamia calamistis
Species 2: Lepidoptera Noctuidae Busseola fusca
Species 3: Lepidoptera Pyralidae Eldana saccharina
Keywords: Stem borers, trap plants
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