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

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(R); 2,4-D ester) and biocontrol with an introduced beetleLochmaea 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.