Wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), oviposition patterns and larval development in downy brome grass, Bromus tectorum L., and wheat, Triticum aestivum L., were compared in a commercially grown wheat field infested with downy brome grass in Montana. Seven weekly randomly selected samples of stems for each plant species were collected at points where both plants were growing together. The level of infestation in downy brome grass was almost two-fold higher than for wheat throughout the growing season. Larval mortality was not detected in either plant species early in the season, but mortality was more frequent in downy brome grass as the host plants matured. Mortality of late-instar larvae late in the season was significantly higher in mature downy brome grass than in mature wheat stems. The weight of these late-instar larvae from wheat was almost four-fold heavier than larvae from downy brome grass. Stem height and seed weight in wheat were significantly reduced by larval sawflies feeding. In contrast, stem height, stem diameter, seed weight, and seed number in grass stems were significantly greater in infested downy brome grass stems compared with uninfested plants. Our results suggest that downy brome grass, a serious weed in wheat cropping, may play an important role in the survival and dynamics of wheat stem sawfly populations in the northern Great Plains.
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