Biotypic Variation in a Worldwide Collection of Russian Wheat Aphid (Homoptera: Aphididae)

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Abstract
Eight isolates of Russian wheat aphid, Diuraphis noxia (Mordvilko), from a worldwide collection were evaluated on resistant and susceptible barley, triticale, and wheat entries. Aphid population levels and damage ratings for leaf chlorosis, plant stunting, and leaf rolling were used to determine ifbiotypic variation occurs within this species. Overall, percentage leaf chlorosis was the best criterion for detecting biotypic variation in D. noxia on cereals; however, the differing mechanisms of resistance expressed by these cereals makes it important to consider other plant and insect factors. Each plant entry responded differently to the D. noxia isolates. Seven of the eight aphid isolates had unique virulence profiles across entries indicating a high degree ofbiotypic diversity. Moreover, isolates differed biotypically in countries where more than one isolate was collected (France and former USSR). An isolate from the former USSR was the most virulent, whereas an isolate from Turkey was the least virulent across all plant entries. Discriminant analysis showed that the U.S. isolate was most similar to a French isolate. The entries that performed best against the D. !loxia collection were the resistant triticales PI 386148 and PI 386156 and the resistant barley PI 366450. However, resistant plant germplasm will have geographical limits because of biotypic variation in D. noxia .

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