Hybridization of invasive Diuraphis noxia, Russian wheat aphid, with native Diuraphis tritici, western wheat aphid in the United States

The Russian wheat aphid, Diuraphis noxia, invaded the United States sometime around 1986 and rapidly dispersed throughout the hard red winter wheat growing region of the western United States. Biotypes of this species that are capable of destroying resistance genes affective against the original introduced strain, RWA biotype 1, began to appear in 2003.  Ecological studies that have been conducted since the introduction of D. noxia commonly found this aphid to be gynocyclic where only oviparous females occurred and no males were produced.  These observations led to the common belief that D. noxia was asexual and new biotypes may have represented gene plasticity or new introductions into the United States.  Recently, a holocyclic population was discovered in grasses and adjacent wheat at high elevations in western Colorado.  Assessment of 93 progeny on 16 Russian wheat aphid resistant and susceptible cereal entries revealed 41% of the progeny were unique biotypes.  Although the occurrence of sexual reproduction was rare it can result in tremendous biotypic diversity.  During ecological studies it the wheat producing areas in the high elevations along the Rocky Mountain states found that D. noxia occupied ecoregions shared with native Diuraphis species, D. mexicana, D. frequens and D. tritici.  The later species is most significant in that D. tritici and D. noxia utilized many of the same cultivated cereal and wild grass hosts in these regions. These two species also were found co-infesting the same wild grass hosts.   D. tritici is a holocyclic aphid and overwinters in the harsh winter conditions as eggs.  Because of the close association of these two species, laboratory methods were developed to conduct hybridization experiments to determine if hybrids were possible. Hybridization was successful and the resulting hybrids were tested to determine their fitness and potential threat to the wheat industry through testing on Russian wheat aphid resistant and susceptible cereal entries.