Does mechanical control of Dalmatian toadflax impair its biological control?

Mecinus janthiniformis is a stem-mining weevil that was introduced into North America as a biological control agent of a weed, Dalmatian toadflax (Linaria dalmatica). Dalmatian toadflax can also be controlled by mechanical methods such as clipping and pulling. We are investigating whether clipping Dalmatian toadflax affects the abundance of M. janthiniformis. During 2012, we conducted a field experiment in which we clipped some of Dalmatian toadflax stems at treatment sites, while other sites were left as unmanipulated controls. During 2013, our experimental design involved clipping neighboring toadflax around selected plants at each site, and retaining the neighbors around control plants at each site. The site-level clipping treatment of 2012 increased the number of M. janthiniformis on the remaining toadflax plants compared to control plants during the early growing season (0.21 weevils/cm ± 0.04 at control sites versus 0.60 weevils/cm ± 0.14 at thinned sites; ANOVA: p = 0.008). In contrast, the smaller-scale clipping treatment of 2013 did not alter the abundance of M. janthiniformis (0.14 weevils/cm ± 0.03 at control sites versus 0.23 weevils/cm ± 0.05 at thinned sites; ANOVA: p = 0.155). Our results suggest that when applied early in the growing season, limited mechanical control does not impair the effectiveness of M. janthiniformis for biological control. Ongoing work is examining how clipping Dalmatian toadflax affects the dispersal and distribution of weevils within a site.