Selection of grain-hosts by Prostephanus truncatus (Coleoptera, Bostrychidae) in the presence of Sitophilus zeamais (Coleoptera, Curculionidae) previously infested grains

The selection of grain-hosts by the larger grain borer Prostephanus truncatus was investigated in environments containing both uninfested maize grains and grains previously infested by the maize weevil, Sitophilus zeamais. The grain-host selection preference of P. truncatus was reflected in the number of adults in each type of grain and the number of damaged grains. Results showed that P. truncatus individuals were significantly more attracted to uninfested grains than to infested grains. Several densities (according to egg density) of S. zeamais infested maize grains mixed together with uninfested grains were exposed to P. truncatus. The larger grain borer damaged significantly more uninfested grains relative to infested ones. Infested grains seemed to have a deterrent effect on the larger grain borer. We hypothesised that this behaviour was due to larval activity and to deterrent substances deposited on maize by adults of the maize weevil. To verify the first hypothesis, adults of P. truncatus were placed with uninfested grains and infested grains bearing immature stages of S. zeamais. Results showed that the infested grains were preferentially damaged during the first two weeks, while three and four weeks after infestation with S. zeamais the uninfested grains were more damaged by P. truncatus. These observations indicated a close relationship between the development of S. zeamais larvae inside maize grains and the selection of grain-host by 19 truncatus. To test the second hypothesis, adults of P. truncatus were placed together with uninfested maize grains, and with infested grains that had been cleaned of substances deposited by S. zeamais using different organic solvents (n-hexane, ether-diethylene and methanol). Results showed that the larger grain borer damaged indiscriminately both infested and uninfested grains. Therefore, the deterrent effect of S. zeamais against P. truncatus was also associated with the existence of substances deposited on the grain surface by adult weevils.