Spatial variation in occurrence of biological control agents of the noxious weed Cirsium arvense

Background/Question/Methods

Canada thistle (Cirsium arvense) is a Eurasian perennial weed invasive across Canada and the United States. Biological control has been attempted in North America by introducing a variety of insect herbivores from this thistle's native range, with very mixed results. One source of this variation may be escape of isolated or marginal populations from their specialist herbivores. To document patterns of attack and escape, and to determine their potential consequences for plant performance, an approximately 815 km latitudinal transect was sampled from agricultural southern Ontario north to boreal James Bay. Stem thickness, plant height, number of flowers, seed production, and number of shoots were quantified as estimates of plant performance. Leaf damage was quantified as percent area affected, stem damage was determined by scoring for stem galls, and flower heads were measured and scored for evidence of seed parasitizing insects.

Results/Conclusions

Initial data suggest that (1) plants in southern Ontario are heavily damaged by insects originally introduced for biocontrol purposes, (2) leaf and stem damage are greatly reduced at more northern and isolated sites, and (3) seed herbivory shows great spatial variation related to land use type and population isolation. The most southerly site sampled (Newmarket) produced significantly fewer viable seeds than any other location (p<0.0002), largely as a result of attack by the biocontrol weevil Larinus planus; leaf damage by the tortoise beetle Cassida rubiginosa was also very severe. Additionally, this was the only region to show evidence of stem galls by the biocontrol fly Urophora cardui. The most northern populations (Moosonee) also exhibited high rates of seed herbivory, but the causal organism (likely a tephritid fly) differed from that of southern high-damage sites. Isolated mid-latitude populations (Algonquin Park) showed no evidence of seed, stem, or leaf herbivory, likely because populations of C. arvense were very sparsely distributed. These results suggest that there is a critical host abundance required to support C. arvense's natural enemies, which therefore are often absent from range margins and/or isolated sites.