Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Three New Agents Approved for Two Weeds

Field horsetail weevil

Field horsetail weevil

The release of agents to biologically control two serious weeds, tutsan (Hypericum androsaemum) and field horsetail (Equisetum arvense), is a step closer after we received approvals from the Environmental Protection Authority (EPA) in May this year. The new agents for tutsan, which we developed for the Tutsan Action Group (a farmer-led group supported by Horizons Regional Council), include a leaf-tying moth (Lathronympha strigana) and a leaf beetle (Chrysolina abchasica). The moth attacks the stem, shoot-tips and seed-pods of the plant, and the beetle feeds on the foliage. Tutsan has become a significant pest of pasture and conservation land, particularly in the central North Island.

Hugh Gourlay, who is leading the work with the tutsan agents, has recently received confirmation that the leaf beetles are disease free and have been correctly identified, enabling him to apply to the Ministry for Primary Industries (MPI) to take them out of containment. However, the beetles have not been breeding as quickly as anticipated. “This is possibly due to changes in food quality and/or because they undergo a lengthy hibernation during the winter and we are still trying to understand what conditions are optimal for their survival in captivity,” said Hugh. “Once we can take the beetles out of containment, we are hoping they will be happier and breed more readily, so we can start to seriously mass rear them for release,” added Hugh. All going well, Hugh is hoping to make the first releases of both new agents simultaneously near Taumarunui in late spring this year.

Lindsay Smith has been leading the project to find agents to biologically control field horsetail (Equisetum arvense) for another farmer-led group, the Lower Rangitikei Horsetail Control Group. Field horsetail is particularly problematic in the lower North Island. Potential agents sourced from the UK were host-range tested at the Lincoln containment facility during 2013–15. These included a flea beetle (Hippuriphila modeeri), a weevil (Grypus equiseti), and two sawfly species (Dolerus germanicus and D. eversmanni). “After studying all four insects we decided the weevil offered the most potential to control the plant,” said Lindsay. Now that EPA approval to release the horsetail weevil has been granted, the challenge for this project is also to rear sufficient numbers to allow field releases to begin.

The great thing about the horsetail weevil is that both adults and larvae feed on the plant. Young weevil larvae munch their way down the stem and into the large underground root system, reducing its ability to produce new shoots and lowering the plant’s ability to invade new habitats. “This is a significant advantage because it will help minimise the extent to which the plant is shifted unintentionally from site to site in soil,” said Craig Davey from Horizons Regional Council, which has also supported the group behind this project. “Biocontrol will add another tool to the toolbox putting us in a better position to provide best practice advice to land managers who are struggling to keep the plant under control,” explained Craig. “What is required is long-term persistent control that will minimise the need for chemical input and reduce the risk of spread region-wide,” he added.

“We only have small numbers of larvae at this stage, but once they have pupated and new adults emerge in spring we will apply to MPI for approval to remove them from containment and make the first field release in the Rangitikei region,” said Lindsay. Some additional adult weevils were recently shipped from the UK to help boost the population. “These adults are currently producing plenty of eggs in containment, but their offspring will need to be rephased to our southern hemisphere seasons before they can be released,” said Lindsay. It is not difficult to grow field horsetail in containment, but in winter plants die back, making it difficult to maintain a breeding population of weevils shipped in from the northern hemisphere summer needing fresh horsetail growth. However, it is hoped that from these small beginnings a lot of farmers are ultimately going to be very happy.

Both the field horsetail and tutsan projects are funded by the Ministry for Primary Industries’ Sustainable Farming Fund, with co-funding provided by a range of other organisations, including the National Biocontrol Collective.