Field biology of Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae) on wheats differing in categories of resistance

The Russian wheat aphid, Diuraphis noxia (Mordvilko), is a serious pest of small grains in certain regions of the United States. An economically and environmentally effective option for controlling this pest is the use of resistant cultivars. Two resistant genotypes (CO 940626 that contains resistance gene Dn4 and CO 940252 that contains resistance gene Dn6) were tested for their effects on Russian wheat aphid field biology in 1996, 1998 and 1999. Russian wheat aphid densities were low on resistant wheat genotypes as compared to those of the susceptible wheat cultivar TAM 107. Based on Russian wheat aphid age structure, the two resistant genotypes affected the Russian wheat aphid developmental biology. A higher proportion of first instars was produced, but lower proportions of second-third, fourth instar and adults were observed, indicating an antibiosis resistance category. Both of these resistant genotypes had similar effects on aphid development. The infestation levels were distinct over the study period in all three years. The susceptible cultivar had more parasitization of the Russian wheat aphid than the resistant genotypes. Seed quality was affected by the highest level of infestation.; Biotic and abiotic factors were examined for their effects on Russian wheat aphid on the resistant and susceptible wheat genotypes in 1997–1998 and in 1998–1999. Combined natural enemies did not affect Russian wheat aphid mortality on resistant wheats but did on the susceptible cultivar. Exclusion of natural enemies with insecticide treatments had no effect on activity of natural enemies. Rainfall affected the Russian wheat aphid significantly more on resistant wheats than on the susceptible, because of the flat leaves of resistant wheats. Accumulative degree hours below 0°C reduced aphid population densities during winter. On the other hand, accumulative degree hours above 20°C negatively affected aphid densities later in the season.; A laboratory experiment showed antibiosis as the primary resistance category in the Zadoks 13 growth stage, antixenosis and some indication of tolerance also were observed. However, tolerance was not clearly indicated as one of the category.