Plant traits predict the success of weed biocontrol.
Biological control (biocontrol) can provide permanent cost-effective control of plant pests, but has variable success. The ability to predict the success of weed biocontrol should improve target prioritisation and the cost-benefit ratio of weed biocontrol. We compiled a data base of the quantitative impacts of weed biocontrol programmes against 80 weed species and tested hypotheses regarding weed traits that contribute to weed biocontrol success using generalised additive models. Modelling and cross-validation indicated that a model with three traits provided good ability to predict the responses of novel species in novel regions. Biocontrol impact varied according to whether a weed was reported to be a major weed in its native range, mode of reproduction (sexual or asexual) and ecosystem (aquatic or wetland versus terrestrial). Biocontrol appears to be highly effective against weeds with the best combination of factors for success (aquatic, asexual species that are not major weeds in their native ranges), whereas most, but not all, programmes against 'difficult targets', which possess the worst combination of factors for success, have failed to result in a substantial impact. An additional analysis provides a preliminary indication that the success of pioneering programmes predicts the success of repeat programmes against the same target weed in other regions. Synthesis and applications. Predictions generated by our model will assist weed prioritisation by improving the ability to predict the success of weed biocontrol. Nevertheless, prioritisation must also consider the importance of the candidate target weeds. Species that are predicted to be difficult targets could be targeted for biocontrol, provided that they are sufficiently important to offset the increased risk of failure against the greater benefits of successful control. Further investigation is needed to assess the ability of successful pioneering programmes to predict the success of repeat programmes in other locations, particularly in conjunction with plant traits.