Early establishment and dispersal of the weevil, Mogulones cruciger (Coleoptera: Curculionidae) for biological control of houndstongue (Cynoglossum officinale) in British Columbia, Canada

First released in Canada in 1997 to control the invasive rangeland weed, houndstongue (Cynoglossum officinale), the European root weevil, Mogulones cruciger, is showing early potential as a successful biocontrol agent. Out of 22 experimental releases in southeastern British Columbia, Canada, 100% established, regardless of initial release size (range 100-400). These founding populations persisted beyond 2 years, and quickly dispersed through a treed and variable landscape to colonise new houndstongue patches surrounding the original release patches. Within 3 years, the weevil had moved 1.42 km. Both initial, within-patch and later, between-patch dispersal followed a similar pattern, with M. cruciger adults and/or their feeding/ oviposition damage being more concentrated nearest the original points of release and declining with distance. Within-patch, this pattern of distribution and also the rate of spread of weevils were similar regardless of initial release size; suggesting density-independence in dispersal behaviour at this scale. Closer investigation of the sequence of invasion at a larger spatial scale suggested that the weevil arrived at, colonized and increased in number on the closest host patches first. Three years post release, both distance from release patch and the number of M. cruciger released, were significant predictors of the amount of feeding/oviposition damage, and presumably weevil population size, within newly colonized houndstongue patches surrounding the original releases. These data, and the finding that more weevils were retrieved from patches where higher numbers of weevils were released in the previous year (i.e., 300 or 400 vs 100 or 200), suggest that release number and placement of releases within a landscape can be manipulated for effective houndstongue control.