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 truncat us was investigated in environments containing both uninfested maize grains and grains previously infested by the maize weevil, Sitophilus zeamais. Th e grain-host selection preference of P. truncatus was reflected in the numb er of adults in each type of grain and the number of damaged grains. Result s 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 uninf ested grains were exposed to P. truncatus. The larger grain borer damaged s ignificantly more uninfested grains relative to infested ones. Infested gra ins seemed to have a deterrent effect on the larger grain borer. We hypothe sised that this behaviour was due to larval activity and to deterrent subst ances 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 uninfeste d grains were more damaged by P. truncatus. These observations indicated a close relationship between the development of S. zeamais larvae inside maiz e grains and the selection of grain-host by 19 truncatus. To test the secon d hypothesis, adults of P. truncatus were placed together with uninfested m aize grains, and with infested grains that had been cleaned of substances d eposited by S. zeamais using different organic solvents (n-hexane, ether-di ethylene and methanol). Results showed that the larger grain borer damaged indiscriminately both infested and uninfested grains. Therefore, the deterr ent effect of S. zeamais against P. truncatus was also associated with the existence of substances deposited on the grain surface by adult weevils.