Biological Invasions

, Volume 21, Issue 3, pp 1021–1031 | Cite as

Limits to biocontrol: the effects of urbanization and elevation on Bruchidius villosus and Exapion fuscirostre—two biological control agents of Cytisus scoparius

  • Robert Frederick BodeEmail author
  • Sara Grove
  • Nathan Krueger
Original Paper


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.


Plant–insect interactions Urban ecosystems Elevation Invasive species 



We would like to thank Rebecca Tong, Danner Linhart and Emily Prall for helping in seed collection. Jennifer Andreas provided us with an early release of her biological control identification book, which helped students learn the difference between a weevil and a beetle. She also looked over an early version and provided helpful feedback. We also thank Mikala Marbach, Leslie Maya, Sheridan Menard, Catherine and Emily Pham for counting seeds in 2016. Guilia Perini, Gia Sheppard, Karlee Kawasaki and Hasley Villadelgado helped count seeds in 2017. Saint Martin’s University provided laboratory space and equipment. A special thanks to the Murdock Charitable Trust (Grant Number 2015277) for funding the elevation portion of this research.

Supplementary material

10530_2018_1882_MOESM1_ESM.docx (20 kb)
Supplementary material 1 (DOCX 21 kb)


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Copyright information

© Springer Nature Switzerland AG 2018

Authors and Affiliations

  1. 1.Saint Martin’s UniversityLaceyUSA
  2. 2.University of California, Santa CruzSanta CruzUSA

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