Native-range research assists risk analysis for non-targets in weed biological control: the cautionary tale of the broom seed beetle

This case study considers the broom seed beetle, Bruchidius villosus, a narrowly oligophagous species within the Fabaceae, subtribe Genistinae for which in-depth native-range studies have been vital to help understand the likely field host specificity following release. Bruchidius villosus has been used in three countries as a classical biological control agent against Scotch broom, Cytisus scoparius. Original host-specificity testing of a UK population, where this species had only been observed developing on C. scoparius, suggested this population was specific to the target. The beetle was released in New Zealand. Following release, however, the agent exhibited a broader host range than in the tests, but not a broader host range than that of the species as a whole. Subsequent studies in the native range using surveys and field testing have helped to show why B. villosus populations exhibit higher specificity in the native range than would be expected from the species' host range. This case is used to illustrate the contribution native-range studies can make to science-based risk analysis of biological control agents against weeds. By doing so, they also highlight the associated risks of ignoring native-range studies and adopting of a 'grab-and-run' approach to obtaining classical biological control agents.