Enhancing pre-release studies for weed biocontrol agents: A review of existing and emerging tools

Thorough pre-release studies to determine the environmental safety of biocontrol agents and to predict their effectiveness in controlling the target weed are crucial parts of any weed biocontrol project. Over the last five to six decades, the scientific rigor of pre-release investigations has constantly improved, reflected in a continuous decrease of nontarget attack incidences. Success rates have also increased, but are more variable, and the proportion of biocontrol agent releases leading to heavy impact has remained stagnant since the 1950 s at an approximate maximum of 25 %. In this paper, we review four methodological approaches that can further enhance environmental safety testing of weed biocontrol agents prior to release. These methodological approaches are (i) chemical ecology, (ii) predictability of field host use through relative performance analysis, (iii) quantification of field host range data, and (iv) experimental evolutionary studies. These approaches are not meant to substitute traditional host specificity testing, which remains the foundation of environmental safety assessments of weed biocontrol agents. Instead, they are meant to provide additional quantitative, behavioral, physiological and evolutionary insights to more accurately predict the realized host range of agents, and to further improve interpretation of host specificity data. We also review four methodological approaches that can facilitate increasing the effectiveness of weed biocontrol agents. These are (i) biocontrol agent and target weed trait analysis, (ii) experimental and field-based impact assessments, (iii) climate suitability studies, and (iv) target weed demography. The caveat of some of the tools and procedures is that they are context specific and not universally suitable for every weed biocontrol project. In addition, some of the approaches require substantial post-release data to corroborate their predictions. We are convinced that the tools and procedures reviewed here along with continued technological advances will refine predictions about the environmental safety and effectiveness of weed biocontrol agents. We also hope that this paper will motivate biocontrol scientists to not only adopt some of the tools more routinely, but ideally to improve them or to develop other novel methods outright. The more that analytical and explanatory research is conducted during weed biocontrol pre-release studies, the more credible the discipline will become.