Virulence of Two Russian Wheat Aphid Biotypes to Eight Wheat Cultivars at Two Temperatures

Biotype 2 of the Russian wheat aphid, Diuraphis noxia (Mordvilko), is virulent to both sources of resistance presently available in commercial wheat, Triticum aestivum L. The performance of biotype 2 was compared with that of biotype 1 on eight wheat cultivars at two constant temperatures, and the plants were evaluated for overall damage and leaf rolling. Colonies of biotype 2 grew an average of 2.3 and 24.9 times faster in the first and second generation, respectively, than did their biotype 1 counterparts at 20°C, reaching 80 to 125 aphids per plant after 20 d, compared with 10 to 31. The no. of aphids per plant at 10 and 20 d after infestation displayed a significant biotype-temperature interaction. There was also a biotype-temperature interaction for plant damage at 10 d, and for damage and leaf rolling at 30 d. After 20 d at 24°C, damage ratings ranged from 7.3 to 8.6 on a scale of 1.0 to 9.0, and leaf rolling ranged from 2.4 to 2.9 on a scale of 1.0 to 3.0 for biotype 2, whereas values for biotype 1 ranged from 2.8 to 5.1 and 1.4 to 2.2, respectively. There were no differences among cultivars in plant damage or leaf rolling induced by biotype 2, and ratings of both were higher than for biotype 1 in all cultivar-temperature combinations. Biotype 2 D. noxia has overcome both Dn4- and Dny-based sources of resistance, was more virulent than biotype 1 to all the cultivars tested, and induced plant injury more rapidly than biotype 1, especially at higher temperatures.