TOLERANCE OF ANTIBIOTIC AND SUSCEPTIBLE CEREAL SEEDLINGS TO THE APHIDS METOPOLOPHIUM-DIRHODUM AND RHOPALOSIPHUM-PADI

Some cereal seedlings exhibit antibiotic and antixenotic resistance to the aphids Metopolophium dirhodum (Walker) and Rhopalosiphum padi (L. ), because the seedlings contain hydroxamic acids or gramine. The asso ciation between tolerance to aphids and aphid antibiosis was investiga ted for three cereals, Dollarbird wheat Vulcan wheat and Yagan barley. The dry biomass gained by the aphids and the simultaneous reduction i n the biomass of the plants (biomass conversion ratio) quantified tole rance. Biomass production and the density dependence of biomass produc tion by the aphids quantified antibiosis more effectively than fecundi ty. Vulcan wheat, which has more hydroxamic acid than Dollarbird wheat showed the highest level of antibiosis, and the barley was not antibi otic for either aphid. The biomass conversion ratio was a constant; th e biomass of an infested plant was reduced by 3 mg for each mg of aphi d biomass gained, regardless of aphid species, plant cultivar, or aphi d density. The three plants showed no differential tolerance to the ap hids, and therefore tolerance is not associated with antibiosis in thi s case.