Population growth rate and relative virulence of the two South African biotypes of Russian wheat aphid, Diuraphis noxia, and bird cherry‐oat aphid, Rhopalosiphum padi, on resistant and non‐resistant barley

In South Africa a new biotype of the Russian wheat aphid (RWA), Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), RWASA2, has appeared which exhibits an improved performance compared to the original biotype (RWASA1 ) on wheat containing the Dn1 resistance gene. We examined population growth rates as well as damage caused by RWASA1 and RWASA2, in addition to a different aphid species, the bird cherry-oat aphid (BCA), Rhopalosiphum padi L. (Hemiptera: Aphididae), on three RWA-resistant barley [Hordeum vulgare L. (Poaceae)] lines (STARS-9577B, STARS-0502B, and STARS-9301B) and one susceptible control (PUMA). RWASA2 had a higher reproductive rate than RWASA1 on all barley lines tested, which is consistent with previous results on wheat. Two of the RWA-resistant lines (STARS-0502B and STARS-9301B) also exhibited a similar resistance phenotype against BCA. In our experiments, severe chlorosis and leaf roll appeared earlier on the control PUMA barley variety as a result of RWASA2 feeding than was the case with RWASA1, probably due to the differences in reproductive rate. Although chlorosis appeared earlier on resistant plants after RWASA2 feeding, this symptom developed much faster during RWASA1 feeding on all three resistant lines tested. As chlorosis did not correlate well with aphid population numbers, we surmise that the differential chlorosis effects may be related to differences in the amount of saliva introduced by the two aphid clones during feeding. Our results indicate that the difference between RWASA2 and RWASA1 are broader than a 'gene for gene' interaction with the Dn1 resistance (R) gene in wheat, and that these biotypes also differ in important aspects of their biology.