PLANT RESISTANCE Plant Responses to Seven Russian Wheat Aphid (Hemiptera: Aphididae) Biotypes Found in the United States

The Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), is a serious wheat, Triticum aestivum L., and barley, Hordeum vulgare L., pest throughout the small grain-producing areas in the western United States. The virulency and classiÞcation of recently described Russian wheat aphid biotypes 1Ð7 (RWA1Ð7) were clariÞed using 24 plant differentials. These seven biotypes had been described previously using various methods and test environments; therefore, the purpose of this study was to test them all under uniform environmental conditions. RWA1 was the least virulent of the biotypes tested, with susceptible ratings observed in Þve plant differentials and intermediate ratings observed in four plant differentials. RWA4, RWA5, RWA6, and RWA7had intermediatevirulence.RWA4,RWA5, andRWA7share similar responses,with susceptible responses in six plant differentials and intermediate responses in Þve plant differentials. Small differenceswithin a few plant differentials separate RWA4, RWA5, andRWA7. RWA6has susceptible responses with only four plant differentials, but 10 plant differentials had intermediate responses. RWA3 was highly virulent, with susceptible responses in 10 plant differentials and intermediate responses in Þve plant differentials. RWA2 was the most virulent strain tested with susceptible responses to 12 plant differentials and intermediate responses to Þve plant differentials. This study has demonstrated that RWA1Ð7 have different combinations of virulence to the plant differentials tested, thereby conÞrming previous Russian wheat aphid biotype designations.