Biological control of weeds in New Zealand
From Wikipedia, the free encyclopedia
New Zealand is regarded as one of the weediest countries in the world[1] and classical biological control (biocontrol) has long been used as one method to combat New Zealand's most widespread and damaging weeds.[2] Classical weed biocontrol is where an agent specialised to feed on a weed species, typically a species of herbivorous insect or plant pathogen, is introduced into a country or region in an attempt partially or fully control the weed population. New Zealand is recognised as one of the five countries/regions of the world that has been most active in weed biological control (the others being Australia, Hawaii, North America, and South Africa).[3] Since weed biocontrol began in NZ in the 1920s, 69 biocontrol agent species have been released against 28 weed species.[4] Recently New Zealand has been releasing biocontrol agents faster than any other country, which has been partially attributed to the magnitude of the country's weed problem and its broad public support for weed control.[2]
Good weed control in New Zealand has been achieved using biocontrol for mist flower (Ageratina riparia), St John's wort (Hypericum perforatum), and ragwort (Jacobaea vulgaris) (the latter most successfully controlled on NZ's drier east coast).[2] Partial control has been achieved for Mexican devil weed (Ageratina adenophora), alligator weed (Alternanthera philoxeroides), heather (Calluna vulgaris), nodding thistle (Carduus nutans), and Scotch broom (Cytisus scoparius).[2] Other weeds, notably gorse (Ulex europaeus), have remained widespread and abundant despite several introductions of biological control agents.
Economic assessments suggest that weed biocontrol in New Zealand overall has been strongly cost beneficial. For example, the 2022 investment in weed biocontrol for production landscapes was NZ$0.69 million and yielded an estimated annual benefit of NZ$85 million.[4] The calculated economic savings to New Zealand from the successful biocontrol of St John's wort, by itself, is greater than the country's total investment in all weed biocontrol programmes.[2][5] St John's wort used to be one of NZ's worst pasture weeds until being successfully biocontrolled by two now widespread introduced beetles (Chrysolina hyperici and Chrysolina quadrigemina) with assistance in the upper South Island from an introduced gall-forming fly (Zeuxidiplosis giardi).[2][6]
It is too early to judge the success of some programmes as it can takes years to decades for agents to reach densities that have significant impact on weed populations.[2] A notable example of a delay that can sometimes occur in agent success has been the biocontrol of heather (Calluna vulgaris) using the heather beetle (Lochmaea suturalis).[7] Heather had spread widely on the central volcanic plateau in the North Island.[8] The Department of Conservation initiated a heather biocontrol programme that led to L. suturalis being released at Tongariro National Park in 1990.[7] It was over a decade later, in 2001, that heather beetle outbreaks began killing patches of heather.[7] By 2007, the heather beetle outbreak was spreading quickly and it was demonstrated that heather beetles were more effective at killing heather than herbicide use, and led to an increase in cover of native dicot plants while these decreased in cover and species richness with herbicide application.[7]
With so many weeds wild in New Zealand, there had been the concern that biologically controlling one weed species on conservation land would only lead to replacement by another weed instead of benefiting the native flora.[9] This was shown not to be the case with the successful biocontrol of mist flower, which had become a dense and widespread forest riparian weed in northern New Zealand, and then was successfully biologically controlled by the release of a white smut fungus (Entyloma ageratinae) and a gall fly (Procecidochares alani).[9] Over five years of monitoring, the species richness and percentage cover of other introduced plants was unaltered while the species richness and percentage cover of native plants increased.[9]
Weed biocontrol research has been done by successive generations of New Zealand scientists, beginning in 1925 at the Cawthron Institute in Nelson.[10] The first weeds targeted were blackberry (Rubus fruticosus agg.), foxglove (Digitalis purpurea), gorse (Ulex europaeus), ragwort (Jacobaea vulgaris), and one endemic native NZ plant, piripiri (Acaena anserinifolia).[10] Of the 17 insects imported for study, three were released and of these two established (the ragwort cinnabar moth (Tyria jacobaeae and the gorse seed weevil (Exapion ulicis),) while the Chilean piripiri sawfly (Ucona acaenae) failed.[11]
In the 1940s through to the 1980s, New Zealand weed biocontrol work was led by the Department of Scientific and Industrial Research, followed by the Crown Research Institutes from 1992.[2] There was a wane in biocontrol investment between 1931 and 1965 due to a focus on new-generation herbicides, followed by a resurgence in interest in weed biocontrol following increased awareness of herbicides' non-target impacts and issues with resistance.[2] To date, the decades with the most weed biocontrol introductions into New Zealand have been the 1990s and 2000s (with 15 and 13 agents introduced, respectively).[2]
Weed biocontrol programmes in NZ have typically been initiated by, and often partially funded by, primary production industries or agencies responsible for environmental management like regional councils and the Department of Conservation.[2]
Agents used
New Zealand's Better Border Biosecurity (B3) maintains a searchable database of all biocontrol releases into New Zealand, including for weed control.[12] iNaturalist NZ maintains a checklist of weed biocontrol agents[14] and a project for public observations of weed biocontrol agents in the wild.[15]
Active research is ongoing into agents for the biocontrol of the following weeds: Acacia longifolia, Araujia sericifera, Berberis darwinii, Cirsium arvense, Clematis vitalba, Cytisus scoparius, Equisetum arvense, Iris pseudacorus, Lonicera japonica, Marrubium vulgare, Nassella neesiana, Passiflora spp. (banana passionfruit), Solanum mauritianum, Tropaeolum speciosum, Ulex europaeus.[13]
,_on_Japanese_honeysuckle_(Lonicera_japonica).jpg)
