Combined impacts of Ceutorhynchus litura and herbicide treatments for Canada thistle suppression

In repeated greenhouse experiments, we investigated the potential for suppressing Canada thistle, Cirsium arvense, using a combination of the stem-mining weevil Ceutorhynchus litura and late-bud to early-flower stage application of one of three herbicides: 2,4-D, glyphosate or clopyralid. We hypothesized that weevil damage combined with a subsequent herbicide application would synergistically reduce Canada thistle biomass by reducing the ability of thistle plants to compensate for herbivory. We found that the effects of weevil attack and herbicide treatment varied considerably across the experimental trials. Contrary to our hypothesis, the effects of weevil attack and herbicide application were additive on a natural log scale, indicating independent, non-synergistic effects of the two factors. In one of the three experimental trials, however, neither weevil attack nor glyphosate treatment alone significantly affected thistle shoot biomass but the combination of the two factors reduced shoot biomass considerably. Overall, combining weevil attack with clopyralid led to the most consistent and greatest suppression of root biomass of Canada thistle.