EFFECT OF PLANT PHENOLOGY AND CLIMATE ON RUSSIAN WHEAT APHID (HOMOPTERA, APHIDIDAE) DAMAGE TO WINTER-WHEAT

Wheat, Triticum aestivum (L.),plant response to Russian wheat aphid, D iuraphis noxia (Mordvilko), infestation varies at different plant grow th stages. Also, the range of climatic zones is great over the distrib ution of the Russian wheat aphid in the United States. Therefore, the effects of wheat growth stage and climate on Russian wheat aphid damag e potential were determined. Experiments were conducted in 4 states (T exas, Colorado, Montana, and Washington) encompassing the Great Plains and Pacific Northwest to determine the damage potential and economic injury level for the Russian wheat aphid. The growth stages included 2 leaves (Zadoks growth stage 12),2 tillers (stage 22), beginning of sp ring regrowth (approximate to stage 29), st node (stage 31), and prebo ot (stage 45). Three levels of Russian wheat aphid infestation were us ed: 5, 10, and 20 aphids per plant for stage 12 plants and 20, 40, and 80 aphids at all other growth stages. Also, there was a no Russian wh eat aphid check treatment. This paper reports on damage potential data . Russian wheal aphid densities and percentage of the tillers that wer e infested or damaged were lower in the fall than in the spring, infes tations in the fall reduced bulk grain weight and number of spikes per plant. These losses were higher in the northern states than in Texas. We suspect that this difference was due to a reduction in winter hard iness in the northern states. In the spring, Russian wheat aphid damag e resulted in a reduction in number of seeds per spike, 200-seed weigh t, and foliage dry weight. Plants damaged in the spring produced as ma ny spikes as uninfested plants. However, the quantity and quality of s eed were reduced by spring infestations.