ARS | Publication request: Integrated management of Scotch broom (Cytisus scoparius) using biological control

Submitted to: Journal of Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 2, 2011
Publication Date: January 2, 2012
Citation: Reddy, A.M., Carruthers, R.I., Mills, N.J. 2012. Integrated management of Scotch broom (Cytisus scoparius) using biological control. Journal of Invasive Plant Science and Management. 5(1):69-82. DOI: 10.1614/IPSM-D-11-00048.1.

Interpretive Summary: Plant life history traits, such as high fecundity and large seed banks, have limited the success of biological control of Scotch broom [Cytisus scoparius (L.) Link] in the U.S. However, there is an increasing awareness among land managers that effective control of weeds such as Scotch broom require the integration of biological control with other control techniques. For example, aside from biological control, land managers at Fort Lewis in Washington have used repeated prescribed fire and mechanical removal since the late 1980s to manage Scotch broom. In this study we examine if the integration of physical controls with biological control (seed weevil Exapion fuscirostre Fabricius) enhances the management of Scotch broom at Fort Lewis. We measured the impact of three management strategies, biological control alone (BC), and the combination of BC with either fire (BC + F) or mowing (BC + M), on seed production and seed bank size, seed predation by the weevil at both the pod and plant scale. There were 71% fewer pods per plant, 79% fewer mature seeds per plant, and an 82% reduction in seed bank density in the BC + M plots, and 55% fewer pods per plant, 69% fewer mature seeds per plant, and a 93% reduction in seed bank density in the BC + F plots compared to the BC alone plots. We also found no difference among management strategies in the number of weevils per pod or the proportion of seeds predated by the weevil at either the pod or whole plant scale. These results show that effective management of Scotch broom necessitates the integration of biological control with traditional control methods, as well as repeated applications. Furthermore, while both integrated strategies outperformed BC alone in reducing seed production and the seed bank, short-rotation prescribed fire may be more effective than mowing for long-term management of Scotch broom due to its potential for slightly greater depletion of the seed bank.

Technical Abstract: Integrated weed management (IWM) strategies are being advocated and employed to control invasive plants species. In this study, we compared the impact of three management strategies [biological control alone (BC), BC with fire (BC + F), and BC with mowing (BC + M)] to determine if combining fire or mowing with biological control enhances Scotch broom control. We measured seed production by Scotch broom and seed predation by the weevil at both the pod and plant scale, and seed bank density over two field seasons. There was no difference in the number of seeds per pod among management strategies. However, there were 71% fewer pods per plant, 79% fewer mature seeds per plant, and an 82% reduction in seed bank density in the BC + M plots, and 55% fewer pods per plant, 69% fewer mature seeds per plant, and a 93% reduction in seed bank density in the BC + F plots compared to the BC alone plots. We found no difference among management strategies in the number of weevils per pod or the proportion of seeds predated by the weevil at either the pod or whole plant scale. While both integrated strategies outperformed BC alone in reducing seed production and the seed bank, with no statistical difference between them, we propose that short-rotation prescribed fire may prove to be the more effective strategy for long-term management of Scotch broom as its potential for slightly greater depletion of the seed bank.