The impacts of a stem boring weevil, Mecinus janthinus, on dalmatian toadflax, Linaria dalmatica
Classical biological control of weeds is generally considered an effective, safe, and cost effective tool for controlling widespread weeds in natural areas. However, only 60% of releases have become established and, of those, only 50% have led to control. Therefore, understanding the impacts of agents on target weeds across spatial scales, at different insect densities, and over time can give biological control practitioners the knowledge necessary to improve establishment and success rates. My studies characterized the impacts the biological control agent, Mecinus janthinus, on the rangeland weed Dalmatian toadflax (Linaria dalmatica) at individual plant and plant population scales. Individual plant studies were conducted in a garden and replicated on plants growing in the field, to measure the impact of agents on plant growth and primary physiology. The population study followed operational scale releases of M. janthinus for three to four years using intensive monitoring to characterize L. dalmatica cover, density, and population structure, and weevil establishment and population increase. I also evaluated which parameters were most important to measure to determine establishment and success. In the common garden experiment, M. janthinus injury was found to reduce relative plant growth, as well as root, stem, and reproductive biomass at medium and high adult densities. Trends of reduced photosynthetic, conductance, or transpiration rates with increasing M. janthinus density were observed. In the field experiment, M. janthinus injury led to reduced growth. Gas exchange rates decreased over the season and were lower in plants exposed to high larval and adult herbivore pressure. Field monitoring indicated successful M. janthinus establishment at releases across a range of elevations, slopes, and geographic locations, though high rates of overwintering mortality were observed at all sites. Linaria dalmatica cover was variable within watersheds and patches, and also differed between watersheds. Abundance of L. dalmatica decreased over time based on cover measurements, but did not change over time with respect to density. The proportion of mature L. dalmatica stems decreased over time. The majority of changes in L. dalmatica over time were the same in release and control transects and could not be attributed to M. janthinus.