Background/Question/Methods Wheat stem sawfly, Fusarium crown rot, and cheatgrass form a multitrophic pest complex that threatens the economic and environmental sustainability of wheat cropping systems in the Northern Great Plains. The increased pressure of herbicide resistance, lack of a viable pesticide options for wheat stem sawfly and Fusarium crown rot, and contradictions among some of the current single-pest management recommendations challenge the traditional chemically-centered single pest management paradigm. This study aims to 1) pinpoint first principles related to multitrophic interactions in this pest complex as its members may influence each other in both positive and negative ways and 2) evaluate the joint impact of simple, easy to adopt, management practices such as crop variety, stand density and herbicide application rate on these pests. In a commercial field near Churchill, Montana, we seeded 81 plots, each 53.5 m², with three wheat varieties (Choteau, McNeal, and Reeder) that vary in competitive ability and sensitivity to sawfly and Fusarium, at either half, recommended, or double seeding density. Herbicide was applied at 0, 0.4, or 0.8 the label recommended rate.

Results/Conclusions Overall yields were 20% and 24% lower for Choteau and Reeder compared with McNeal. The high seeding rate produced the lowest yield, while low and medium rates produced 13% and 27% more yield, respectively. Herbicide application increased yields non-linearly by 17% and 23.5% for the 0.4 and 0.8 rates, respectively. However, the existence of interactions between pest groups and in response to management practices confounded some of the above results. For example Choteau, a solid stem variety, was less infested by sawfly than the hollow stem varieties, thus we would expect higher yields. However, due its low tolerance to cheatgrass competition, yields were 11% and 43% lower than for McNeal at the low and high seeding densities, respectively.  Although yields and test weights generally decreased as a function of cheatgrass cover, plots at the high and low seeding rates, treated with 0.8 label herbicide rate had similar or lower test weight and yields as plots treated with only 0.4 of the label rate, probably due to interactions with Fusarium infection levels that are known to vary with soil moisture. These results highlight the need to update current single-pest management recommendations that may lead to undesirable responses with a total-system approach to minimizing losses due to synergetic impacts of these pests.