Predicting outcomes and ecosystem impacts of classical weed biocontrol

Reliable prediction of weed biocontrol outcomes is desirable because developing new programmes can be costly and success is not guaranteed. In this review, we outline how advances in both agent and weed-target selection are improving programme efficiency without compromising environmental safety. Better prediction of candidate-agent impact and host specificity, aided by advances in molecular and modelling approaches, including plant-insect genotype and climate matching, has advanced agent selection. There has also been renewed interest in predicting biocontrol outcomes following recognition that the Enemy Release Hypothesis does not explain all plant invasions. Because classical weed biocontrol is expected to be most effective where enemy release contributes substantially to invasion success, identifying invasions facilitated by enemy release should improve the prediction of weed biocontrol outcomes and inform prioritisation systems that rank weed targets. Although studies of ecosystem recovery following successful weed suppression remain limited, there is growing recognition that weed suppression alone does not always restore heavily degraded ecosystems without additional interventions, some of which may be impractical or prohibitively expensive at large scales. Nevertheless, biocontrol benefits are likely to be greater in less heavily invaded areas, where restoration outcomes are typically better. Consequently, more proactive use of biocontrol, targeting weeds earlier in the invasion curve, should deliver more ecological and economic benefits.