Comparing biocontrol and herbicide for managing an invasive non‐native plant species: Efficacy, non‐target effects and secondary invasion

Abstract Globally, invasive non‐native plants are an increasing threat to indigenous biodiversity and ecosystems, but management can be compromised by poor efficacy of control methods, harmful non‐target effects or secondary invasions by other non‐native plant species. A 5‐year field trial compared two stakeholder‐selected control methods for heather, a European plant invading native ecosystems in and adjoining Tongariro National Park in New Zealand. The control methods were a selective herbicide (Pasture Kleen ® ; 2,4‐D ester) and biocontrol with an introduced beetle Lochmaea suturalis (Coleoptera: Chrysomelidae). Biocontrol reduced mean heather cover by 97%, slightly more than herbicide at 87%, compared with a 20% increase in heather under no management. Cover of native dicots, the most species‐rich plant group, increased following biocontrol. In contrast, herbicide application had major non‐target effects on native dicots, reducing their percentage cover and species richness. Native monocot cover and species richness increased following both herbicide and biocontrol treatments. A similar eightfold increase in non‐native monocots occurred following both biocontrol and herbicide treatments. Overall, secondary invasion was greatest with biocontrol because non‐native dicot cover also increased, whereas herbicide almost eliminated non‐native dicots. Synthesis and applications . Biocontrol and herbicide treatments both controlled heather but herbicide application was associated with severe non‐target impacts on native dicots. Benefits to the native flora were consequently greatest in the biocontrol treatment, despite greater secondary invasion. Control strategies for management of widespread non‐native plants to optimize ecosystem outcomes should include more consideration of biocontrol.