The use of ecological theory in the management of agricultural weeds has historically lagged behind the management of other pest groups, perhaps due to the relative ease and convenience of applying herbicides. Due to environmental and societal concerns, there has been a recent emerging interest in the development of Integrated Weed Management (IWM) strategies that help reduce the dispersal and impact of weeds though the application of multiple control methods at different times of the plant’s life cycle. The expectation of an IWM program is of synergistic or additive interactions among control practices. However, integrating tactics can yield unexpected results, namely the existence of antagonism between practices. In this study, we use Canada thistle (Cirsium arvense (L.) Scop.), an aggressive, introduced creeping perennial weed, as a model species to test for the presence and type of interactions between a stem-boring weevil (Hadroplontus litura), a pathogen (Pseudomonas syringae pv. tagetis - PST), and a herbicide (glyphosate). Specifically, the objective of this work was to determine whether the relationship between the weevils, the pathogen, and glyphosate was antagonistic, additive, or synergistic in Canada thistle control.

Results/Conclusions Experiments were carried out in a greenhouse setting using glyphosate at either a reduced rate (0.63 kg ae/ha) or a full labeled rate (3.78 kg ae/ha), the stem-boring weevils, and PST to evaluate their single and combined effects on Canada thistle shoot biomass, root biomass, and shoot number. Both rates of glyphosate, weevils, and PST individually reduced shoot biomass by 11-51%, root biomass by 17-88% and shoot number by 12-32%. Combining weevils with PST led to a 94% reduction in shoot biomass, a 90% reduction in root biomass, and a 91% reduction in shoot number. Finally, combining either rate of glyphosate with the weevils and PST led to an 83-86% reduction in shoot biomass, a 94-98% reduction in root biomass, and a 78-91% reduction in shoot number. There was a negative interaction on the combined effect of PST and glyphosate on shoot biomass. This antagonism could be attributed to glyphosate killing the PST-infected shoot tissue and/or PST metabolizing or degrading some of the herbicide’s active ingredient. Despite this antagonistic interaction, this study indicates that as a general trend the relationship between control agents is not synergistic but additive, and that integrating management tactics yields greater Canada thistle control than any singular approach.