Integrated weed management strategies (IWM) are being advocated and employed to control invasive plants species. In this study, we compared three management strategies (biological control alone [BC], BC with fire [BC F], and BC with mowing [BC M]) to determine if physical controls reduce seed production by Scotch broom and interfere with the action of the biological control agent—the Scotch broom seed weevil. We measured seed production and seed predation by the weevil at both pod and plant scale, and seed bank density over two field seasons. We found no difference in the number of seeds per pod among management strategies. However, combining management strategies (BC M and BC F) resulted in significant reductions in pods per plant, mature seeds per plant, and seed bank density relative to biological control alone. We did not find differences among management strategies in number of weevils per pod or proportion of seeds predated by the weevil at either pod or whole-plant scale. However, combining management strategies (BC M and BC F) resulted in a significant reduction in healthy mature seeds per plant relative to biological control alone. Although both integrated strategies outperformed biological control alone in reducing seed production and the seed bank, with no statistical difference between them, we propose that short-rotation prescribed fire could prove to be a more effective strategy for long-term management of Scotch broom due to its potential for slightly greater depletion of the seed bank.

Nomenclature: Scotch broom, Cytisus scoparius (L.) Link; Scotch broom seed weevil, Exapion fuscirostre Fabricius.

Management Implications: High fecundity and large seed banks have limited the success of biological control of Scotch broom in the United States. However, there is awareness among land managers that effective control of weeds requires the integration of biological control with other control techniques. For example, 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 examined if the integration of physical controls with biological control by the Scotch broom seed weevil could reduce seed production by Scotch broom and interfere with the action of the biological control agent.

We measured the impact of three management strategies, biological control alone (BC), and combination of BC with either fire (BC F) or mowing (BC M), on seed production and seed bank size, and seed predation by the weevil at both the pod and plant scale. Combining management strategies (BC M and BC F) resulted in significant reductions in the number of pods per plant, mature seeds per plant, and seed bank density relative to biological control alone. Weevil seed predation rates were enhanced in BC M and BC F plots, compared to BC-alone plots, but these differences were not consistently statistically significant. However, there was a reduction in number of healthy mature seeds per plant in both BC M and BC F plots, compared to BC-alone plots. We found no differences between BC M and BC F plots for any of these variables. There also was no difference among management strategies in the number of weevils per pod. These results show that management of Scotch broom necessitates the integration of biological control with repeated applications of physical control methods. Although both integrated strategies outperformed BC alone in reducing seed production and the seed bank, short-rotation prescribed fire might be more effective than mowing for long-term management of Scotch broom due to its potential for slightly greater depletion of the seed bank.