Spatial and biotic interactions of the wheat stem sawfly with wild oat and Montana dryland spring wheat

The wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), has been a significant pest of wheat in the northern Great Plains since the inception of small grain production by homesteading farmers. Biological constraints associated with both the larval and adult stages of the sawfly have thwarted the development of effective chemical control of this stem-boring pest. To date, management of sawfly relies on cultural and ecologically-based strategies. Foremost among these was the development of sawfly-resistant wheat varieties.; Sawfly damage is estimated to have reduced profits to Montana wheat producers by {dollar}20.4 million during the 1995–1996 growing season. Many Montana spring wheat fields are also impacted by the grassy weed wild oat, Avena fatua L. (Poeaceae: Gramineae). Reduced yield, grain dockage and herbicide treatment costs are the primary sources of economic loss due to wild oat infestation of wheat fields and cost Montana farmers approximately {dollar}20 million annually.; An accurate characterization of the spatial pattern and distribution of adult and larval stages of sawfly in sawfly-resistant spring wheat was developed to enhance existing management tools. Contrary to the conventionally held perception that sawfly is an edge-effect pest, sawfly infestation was not found to be predominantly focused at field margins. The influence of wild oat on sawfly spatial distribution was also examined. Although sawfly larval mortality was 100% in all infested wild oat stems examined, sawfly acceptance of this host plant was not rare. However, sawfly host plant preference was significantly higher for spring wheat than for wild oat hosts based on the relative densities of each species. Sawfly attack on the small number of spring wheat stems within wild oat patches at a higher rate than spring wheat in weed-free areas further emphasizes sawfly host plant preference for spring wheat.