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This weevil along with other seed feeders will further reduce the seed that spotted and diffuse knapweed are dependent on for reproduction , dispersal , and survival . Even so , a ship 's store of hard tack was often infested with small bugs called weevils . Like other broadnosed weevils , Polydrusus can not fly . The weevil needs high soil temperature to complete development and cannot establish in areas with a short growing season . The weevil , after these tests , is considered to be host specific to Centaurea species in the subgenera Acrolophus and Jacea . " Weevils can reduce yields by 25 to 30 per cent , " says Dr Higgins . Metarhizium spp. is being tested as a natural enemy of corn rootworm , white grubs ( scarabs ) , and some root weevils . These included the white pine weevil ( Pissodes strobi ) , the yellow poplar weevil ( Odontopus calceatus ) , and the Japanese weevil ( Callirhopalus bifasciatus ) . Bark beetles or root collar weevils are sometimes associated with the dying trees , but are not considered to be the primary cause of mortality .