Horehound Agents Ready for Action
A grant from the Ministry for Primary Industries’ Sustainable Farming Fund has enabled the importation of two agents to help bring horehound (Marrubium vulgare) under control. Horehound is predominantly found on hill country farms, where it affects the quality of wool, lowering profit margins. The meat can also be tainted if sheep are grazing in areas where it is abundant. Some farmers are having to retire areas of their land due to extremely high horehound infestations because it is no longer suitable for grazing. Horehound is also troublesome in lucerne crops, which are susceptible to the heavy-duty herbicides needed to control this weed. Current costs of horehound to dryland sheep farmers are estimated to be in excess of $6.85 million per annum.
Two biocontrol agents were recently sourced from Australia. The plume moth (Wheeleria spilodactylus) attacks the above-ground vegetation, while the clearwing moth (Chamaesphecia mysiniformis) attacks the roots.
Landowner and chair of the Horehound Biocontrol Group, Gavin Loxton, has been involved with the project from the start and has taken a personal interest in seeing the agents released. “Once we started to gather information about the extent of horehound, we realised that we had underestimated its true distribution,” said Gavin.
Sourcing the agents has been relatively straightforward compared with other biocontrol programmes. “Satisfactory host-range testing had already been completed by our Australian colleagues before the moths were released there in 1994. An application to release the same biocontrol agents was made to the Environmental Protection Authority (EPA) in May 2018,” said Ronny Groenteman, who is leading the scientific aspects of the project for the Horehound Biocontrol Group.
During the pre-application consultation process concerns were raised by the Herb Federation of New Zealand and the New Zealand Association of Medical Herbalists regarding the impacts of biocontrol agents on wild horehound, which is harvested for medicinal purposes. Ronny met with members of these groups to listen to their concerns and explain how they could participate in the decision-making process undertaken by the EPA. On balance, the EPA decided that the circumstances faced by farmers, including costs from having the plant on their land and the shortcomings of other management options, outweighed the concerns of the herbalists, which could be mitigated. Therefore, approval was granted by the EPA to release the moths. “We are continuing the dialogue with the herbalist industry to develop techniques to mechanically protect some horehound plants from the moths and we are hopeful that a win-win solution can be found,” said Ronny.
Clearwing moths require specific conditions, such as high temperatures during the summer months, to mate and reproduce. The moths also require sheltered sites, as they rely on pheromones to attract mates and these are easily diluted in the wind. “Initially we were uncertain if we would be able to create the right conditions for the clearwing moths inside the containment facility. Fortunately, the moths responded well to the natural light available in the facility, and started reproducing prolifically,” said Ronny. A novel technique for releasing the moths in the field developed by the Australians has been adopted here. “We learned that gluing the eggs to the top of a cocktail stick/toothpick and sticking that onto a cut stem of horehound allows the newly hatched larvae to crawl down on to the cut stems, eventually reaching the roots of the plant – a winning formula,” said Ronny. Once the moth has successfully established there is no need to repeat the fiddly egg-gluing exercise, since infected root material can then be transferred to other farms.
Thankfully the plume moth is easier to rear in containment, and much more straightforward to release in the field too. Our Australian colleagues predict that the plume moth will easily establish anywhere horehound is present in New Zealand. In Australia establishment was successful wherever annual rainfall was above 450 mm, which describes most of New Zealand.
Shortly before Christmas, Manaaki Whenua – Landcare Research was granted permission to remove the horehound moths from containment, and in the 2 weeks that followed, releases were made in Marlborough, North Canterbury and the Mackenzie District. Altogether, five releases of the clearwing moth and eight of the plume moth were made, including one on a farm close to Lincoln, where we are aiming to create a ‘nursery site’ for future redistribution.
“Most landowners have been very receptive to the idea of biocontrol and we hope that the programme will be at least as successful as other biocontrol projects, such as the one for ragwort(Jacobaea vulgaris),” said Gavin. Gavin has been monitoring horehound on Sawdon Station in Tekapō for 3 years to gain a better understanding of the population dynamics. He found that cocksfoot (Dactylis glomerata) is one of the main competitors for horehound, and he expects that once the vigour of the horehound is reduced it will be quickly replaced. “Horehound plants can produce up to 20,000 seeds per year, and the seeds remain viable in the ground for 7 years or more,” Gavin explained. Gavin is encouraging farmers to consider temporarily replacing horehound-infected lucerne crops with cocksfoot until such time as the horehound seed bank is exhausted. But the biocontrol agents will, hopefully, in time, offer good control and expand into areas that are not able to be managed using other methods.
For now, though, it is a matter of waiting to confirm that the moths have established. The plume moth has two or three generations per year, so with a bit of luck it may be possible to find some sign of them next spring. The clearwing moth, which has only one generation per year, will take a bit longer. Even if moths can be seen flying about at release sites next summer, it will probably take a few years to determine whether conditions are consistently suitable for them to mate and produce offspring. Here’s hoping!
A big thanks to John Weiss of Agriculture Victoria and Jean-Louis Sagliocco for their valuable assistance, including securing populations of the horehound moths in Australia and rearing and release advice.
Contact: Ronny Groenteman (groentemanr@landcareresearch.co.nz)