Designation of Two New Greenbug (Homoptera: Aphididae) Biotypes G and H

Major plant sources of greenbug, Schizaphis graminum (Rondani), resistance were evaluated against two new greenbug isolates, Oklahoma (SCO) and Texas (WCT), to demonstrate that these isolates should be designated as biotypes. Host plant reactions to the isolates were compared with those of biotypes E and F. The SCO isolate (biotype G) had two plant resistance relationships distinct from any previously described biotypes. Biotype G damaged all known sources of greenbug resistance in wheat, Triticum aestivum L., but did not damage ‘Wintermalt,’ a barley, Hordeum vulgare L., variety previously reported to be susceptible to all greenbug biotypes. The WCT isolate (biotype H) caused host plant responses on wheat that were similar to those of biotype E. However, biotype H was avirulent on all sorghum, Sorghum bicolor (L.) Moench, varieties and severely damaged ‘Post,’ a barley variety resistant to all previously described biotypes. Damage ratings for varieties infested with biotypes E and F were consistent with other studies and verified greenbug resistance in our plant sources. Both biotypes G and H lack the middorsal dark green abdominal stripe characteristic of biotypes B, C, and E but have the general appearance of biotype F.