BIOLOGICAL INTERACTIONS BETWEEN THE CEREAL APHIDS RHOPALOSIPHUM PADI (L) AND SITOBION-AVENAE (F) (HOM, APHIDIDAE) ON WHEAT

Longevity and fecundity of the bird cherry-oat aphid, Rhopalosiphum padi (L.), and the English grain aphid, Sitobion avenae (F.), reared at various densities on caged wheat plants were onserved under laboratory conditions. On heading plants, R. padi responded to intraspecific competition by having a significantly shorter life and lower fecundity. When infested with 4 R. padi + 4 S. avenae on the same plant, S. avenae had either no effect or a positive effect on longevity and fecundity of R. padi. The presence of R. padi on wheat sedlings, however, appeared to increase longevity and fecundity of S. avenae. Both species had reduced longevity and fecundity when 16 R. padi + 16 S. avenae were placed on the same plant. The bird cherry-oat aphid preferred lower parts of the heading plant, while the English grain aphid preferred the plant head. The presence of the two species did not affect the spatial distribution of each aphid species on the plant. Only when crowded by its own species, S. avenae spread downward to avoid intraspecific competition on the wheat head.