Limits to biocontrol: the effects of urbanization and elevation on Bruchidius villosus and Exapion fuscirostre - two biological control agents of Cytisus scoparius.
Both invasive species and their biological control agents face barriers to expansion, which provide opportunities to limit invasions or may enable target invasive species to exist in enemy-free space. A better understanding of the various barriers to the spread of insects introduced to control invasive plants will allow for more targeted release programs and potentially shorter lag times from introduction to management. In the Pacific Northwest of the United States, two seed eating beetles (Exapion fuscirostre and Bruchidius villosus) have been introduced to control the invasive plant Cytisus scoparius. These biological controls are predicted to be effective only at high rates of seed destruction, so any factors that limit their colonization or population sizes may allow C. scoparius populations to grow, leading to ecological and economic harm. In this study, we investigate relative impacts of biological control agents in relation to two barriers to insect movement: urbanization and elevation. We find that the impacts of B. villosus are not different between urban and rural sites, but that relative impacts of both biological control agents decrease with increasing elevation, a pattern consistent across 2 years of measurements. Cytisus scoparius populations experience substantial seed destruction in urban settings, strongly suggesting successful population control. The low seed destruction at high elevation sites could indicate that biological control agents are ineffective there, and that C. scoparius may exist in enemy-reduced space.