Wheat genotype, early plant growth stage and infestation density effects on Russian wheat aphid (Homoptera: Aphididae) population increase and plant damage

Population increase of the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), and its effect on early plant growth and damage were studied under laboratory conditions on two winter wheat genotypes, susceptible 'Stephens' and resistant 10085-5. Three early plant growth stages (one-, two-, and three-leaf stage), and three insect densities (0, 5 and 20 aphids per plant) were compared. Insect counts, plant damage assessments, plant height, leaf number and dry weight measurements were made. Diuraphis noxia population increase was greater on 'Stephens' than on 10085-5, but this was highly influenced by plant growth stage at time of infestation and initial aphid density. Host quality of 'Stephens' decreased rapidly when plants were infested at the I-left stage, especially with the high initial density (20 aphids), resulting in low aphid population increase. For all plant developmental stages and genotypes, D, noxia per capita population increase was lower at initial densities of 20 compared to 5 aphids per plant, probably due to a density-dependent reduction in reproductive rate associated with a reduction in host-plant quality and/or crowding. Diuraphis noxia significantly affected plant growth, but the magnitude of the effect was influenced by genotype. in general, susceptible 'Stephens' had significantly more damage and a greater reduction in growth than resistant 10085-5.