We investigated the single and combined effects of two biological control agents, the stem-mining weevil Hadroplontus litura and the pathogen Pseudomonas syringae pv. tagetis, with a herbicide (reduced or full application of glyphosate: 0.63 kg ae ha−1, or 3.78 kg ae ha−1, respectively) on the growth of Canada thistle, Cirsium arvense. We hypothesized that first, although each control method would have a negative effect on Canada thistle shoot biomass, root biomass, and shoot number, the integration of more than one control method would have greater impact than individual control methods. Second, we hypothesized that the order in which control methods are applied affects the outcome of the management program, with a pathogen application following weevil infestation being more effective than one prior to it. Although control methods impacted Canada thistle growth (P < 0.001, expect for a nonsignificant impact of glyphosate on shoot number), the combined effect of the three control methods behaved, generally, in an additive manner. A marginal interaction between the pathogen and the herbicide (P = 0.052) indicated a slight antagonistic interaction between these control methods. An interaction between the two biological control agents tested (P < 0.001) indicated that application of a pathogen prior to the release of weevil larvae could be more deleterious to Canada thistle than a late application. The observed, mostly additive, relationship between biological control agents and herbicides implies that integrating control methods rather than using a single approach could lead to greater Canada thistle control.
Nomenclature: Glyphosate; Pseudomonas syringae pv. tagetis (Hellmers) Young et al.; stem-boring weevil, Hadroplontus litura (Fabricius); Canada thistle, Cirsium arvense (L.) Scop
Interpretative Summary: Canada thistle (Cirsium arvense) is one of the most problematic invasive plant species infesting crops, pastures, rangelands, roadsides, and noncrop areas in the United States, Canada, northern Europe, and New Zealand. Although many single methods have been developed to minimize Canada thistle spread and impact, this persistent invasive species continues causing problems in temperate regions of the world. Field data and mathematical models have shown that, if properly designed, integrated weed management can provide sustainable, economically viable, and successful weed control. In an integrated weed management program, control methods can interact synergistically, additively, or antagonistically, but few studies have specifically evaluated the existence and patterns of such relations. In repeated greenhouse experiments, we investigated the potential for suppressing Canada thistle using a combination of the stem-mining weevil Hadroplontus litura, the pathogen Pseudomonas syringae pv. tagetis, and reduced or full-rate applications of glyphosate (0.63 kg ae ha−1, or 3.78 kg ae ha−1, respectively). In general, individual control methods impacted Canada thistle growth and the combined effect of the three control methods behaved in an additive manner. A marginal interaction between the pathogen and the herbicide indicated an antagonistic interaction between these control methods. An interaction between the two biological control agents tested (P < 0.001) indicated that application of a pathogen prior the release of weevil larvae could be more deleterious to Canada thistle than a late application. The observed mostly additive relationship between biological control agents and herbicides implies t