Interspecific competitive interactions between the larger grain borer Prostephanus truncatus (horn) (Coleoptera: Bostrichidae) and the maize weevil sitophilus zeamais motschulsky (Coleoptera: Curculionidae) in maize stores

Abstract

Interspecific interactions between the larger grain borer Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) and the maize weevil Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) were studied during two storage seasons: 1994-95 (a short-term) and 1995-96 storage (long-term) in maize stores, in Benin. Maize cobs were randomly sampled from stores in order to examine (1) colonisation patterns of P. truncatus and 5. zearnais; (2) how each species influences distribution of the other, (3) other factors which may influence P. truncatus and S. zeamais. For both storage seasons, P. truncatus and S. zeamais populations were initially aggregated on few cobs, but not associated with each other. The degree and strength of association increased with each sampling occasion with respect to the Ochiai, Jaccard and Dice indices of association. P. truncatus was found on most cobs and on some cobs in very high numbers (>300 insects). Almost all cobs with P. truncatus contained at least a few S. zeamais individuals, but many cobs with S. zeamais contained no P. truncatus. By regressing Taylor's b coefficients of one species on the mean adult and reared out densities of the other, aggregation of S. zeamais was found to have no significant correlation with mean density of adult P. truncatus and a weakly positive one with reared-out densities of P. truncatus, Aggregation of P. truncatus showed a negative relationship with mean and reared-out densities of S. zeamais, The same procedure with Lloyd's mean crowding index (m*), the aggregation of S. zeamais also showed a similar relationship with mean and reared-out densities of P. truncatus as observed with Taylor's b coefficient. P. truncatus aggregation, however, was not found related to mean and reared-out S. zeamais density, contrary to results using Taylor's b coefficient. Preliminary analysis of each species' rate of reproduction, as measured by the ratio of F 1 adults with parent adults, showed a mutual negative relationship. Husk number and extension past ear, number of visible holes on [he husk, density of earworms and other insect species within cobs were found to play a significant role in initial colonisation of cobs