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A 5 yr study in the semiarid wheat production region of eastern Washington documented the relative densities of pest aphids and their natural enemies in cereal production systems using on-farm replicated plots. The systems were reduced-tillage soft white winter wheat (SWW) (Triticum aestivum L.)—summer fallow rotation; no-till soft white spring wheat (SWS)— chemical fallow rotation; continuous no-till hard red spring wheat (HRS); and no-till HRS—no-till spring barley (SB) (Hordeum vulgare L.) rotation. The English grain aphid, Sitobion avenae (F.), was the dominant species, followed in abundance by the Russian wheat aphid, Diuraphis noxia (Mordvilko). The bird cherry-oat aphid, Rhopalosiphum padi (L.), and rose grass aphid, Metopolophium dirhodum (Walter), were infrequently encountered. Overall, aphid densities were low, with aphids rare or absent in SWW and SB plots. The data revealed no clear and consistent effects of cereal production systems on aphid densities, but it did reveal, based on analysis of data from continuous HRS plots, high among-year variability in S. avenae and D. noxia densities. Only in 1996 and only in continuous HRS was it necessary to chemically control damaging populations of D. noxia. English grain aphid densities never approached threshold levels. S. avenae parasitism averaged .16% in some spring wheat systems in 1998 and 2000, while only two mummified D. noxia were observed. Coccinellid beetle counts in all plots totaled 143 in 1998 and 163 in 2000, with 90.2% and 94.5% in the genus Hippodamia, respectively. The ladybird beetle Coccinella septempunctata L. comprised 9.8% (1998) and 5.5% (2000) of the populations. The results suggest that damaging aphid populations are unlikely to develop in winter wheat, but populations in spring cereals warrant monitoring because they fluctuate from year-to-year and can be damaging. |
