Emerging concern highlighting non-target impacts in classical biological control of arthropods and weeds
has heightened awareness of these risks but raised the risk of obscuring beneficial effects. This review
applied a retrospective assessment of the benefits from weed biological control in New Zealand, using
the framework designed for pre-clearance assessment of classical biological control. Of those agents
released which can be assessed because of sufficient passage of time (n = 33), their impact has been
assessed according to the modern criteria for judging beneficial effects used by New Zealand’s Environmental
Protection Authority (negligible, minimal, minor, moderate, major and massive). Cases with negligible
benefit (n = 12) included failures to establish self-sustaining populations, while cases with
minimal benefit (n = 11) included some where predation reduced the realized benefit of established
organisms. The remaining cases offered massive (n = 2), major (n = 1), moderate (n = 5) or minor (n = 2)
benefit. Suppression of ragwort (Jacobaea vulgaris Gaertn, 1754), and St. Johns wort (Hypericum perforatum
L.) were considered to be massive in magnitude, offering long term ecosystem benefits of controlling
invasive weeds. Improved clarity around risk and benefit could help improve the quality of debate on biological
control, and the five step scale used in New Zealand may prove more widely useful elsewhere.