Regional-scale impact of the weed biocontrol agent Mecinus janthinus on Dalmatian toadflax (Linaria dalmatica)

Dalmatian toadflax (Linaria dalmatica (L.) Mill.) is an important invasive plant on rangelands throughout western North America. In 1991, the stem-mining weevil, Mecinus janthinus Germar, was introduced into Canada from Europe as a classical biological control agent to reduce toadflax densities and improve rangelands, particularly in British Columbia. To determine if the program was a success at a regional level, this paper answers three key questions: (1) has M. janthinus spread throughout the study area, (2) is M. janthinus causing a decline in toadflax plant size or density at the regional scale, and (3) has the distribution of toadflax plants changed following M. janthinus introduction? These questions are answered by combining historical survey data and mensurative experimental data on plant and weevil densities across a 40,000 km² area in southern British Columbia. The results show that through a combination of intentional redistribution and natural dispersal weevils have spread throughout the study area. Stem densities at naturally colonized sites and historical release sites were equally low. Across weevil populations between 3 and 10 years old, weevil densities peaked in the eighth year, and there was a negative relationship between weevil density and stem length. Between 2000 and 2007, toadflax patches were found to both decrease in density and become more fragmented over time, with 15% of patches disappearing completely. These findings show that M. janthinus has had a significant negative impact on both the density and distribution of Dalmatian toadflax throughout a large part of its range in British Columbia.