Biocontrol – slow and steady wins the race

Back Published 24 November 2020

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As is well known, invasive non-native plant species are a threat to New Zealand’s native biodiversity and ecosystems – and the list of invaders is long. Herbicides are widely used in the management of invasive plants, but they have non-target effects on native plants in treated areas and are expensive to apply (especially from the air). It is also thought that they slow down the recovery of native plant communities after treatment. Widespread aerial herbicide application also risks public disapproval linked to perceived effects on human and wider ecosystem health.

Heather beetle ([Lochmaea suturalis]) a biocontrol agent introduced in 1996 is shown to only attack its target plant heather ([Calluna vulgaris]).

Paul Peterson and colleagues from Manaaki Whenua, Massey University and the Open Polytechnic recently published the findings from a long-term, carefully controlled study, partly funded by SSIF, comparing herbicide use with biocontrol methods for the control of invasive heather in and near Tongariro National Park. Their work was named ‘Paper of the Month’ for June 2020 by the Journal of Applied Ecology.

Heather (Calluna vulgaris) was deliberately introduced into the park from Europe in 1912, subsequently becoming the most invasive weed there, dominating over 50,000 hectares. The heather beetle (Lochmaea suturalis) was introduced from the UK as a biocontrol agent to control heather in 1996. This study, which began in 2007, followed the outcomes of four treatments: (a) control (insecticide spray to protect vegetation from heather beetle feeding, (b) biocontrol (to expose heather to beetle feeding only), (c) herbicide (herbicide + insecticide to protect vegetation from beetle feeding but expose it to herbicide), and (d) biocontrol + herbicide (to expose vegetation to beetle feeding and herbicide).

The herbicide-only, biocontrol-only, and herbicide + biocontrol plots all showed effective reduction in heather cover from around 50% to near zero. However, after 5 years, plots treated with herbicide also showed reductions in cover and species richness of native plant species, whereas these non-target effects were not seen in biocontrol-only plots; biocontrol agents are carefully chosen to ensure they only attack the target plants. In fact, native plant species even started to recover in biocontrol-only plots.

The authors concluded that control strategies for the management of invasive plants should more readily consider biocontrol methods in the future. “Where invasive non-native plant species problems are widespread and threaten indigenous plant communities over large tracts of land, as they do in New Zealand, biocontrol is likely to be more appropriate than widespread herbicide application, mechanical, grazing or burning methods.”

Biocontrol is not a quick fix, however. It takes many years to select, approve, breed and release biocontrol agents, and the sector is scrupulously regulated through the Environmental Protection Authority. Nonetheless, over time Manaaki Whenua’s scientists have built up considerable expertise, experience – and large reserves of patience! – to ensure the successful application of biocontrol methods to enhance New Zealand’s biodiversity.