Horehound has had a big effect on lucerne crops in parts of New Zealand, but a couple of Australian helpers have been enrolled in the battle. Tom Ward reports.

Farmers afflicted with the production-limiting weed horehound received an early Christmas present last year with the release of two bio-control agents into North Canterbury and the Mackenzie Basin.

Once established, it is hoped the bio-control agents – two moths – will reduce horehound infestations to manageable levels.

It is estimated that horehound costs sheep and beef farmers $6.85 million annually in lost production and weed control – and this doesn’t take into account the extra processing required for horehound seed-affected fleeces.

The release of the moths was something of a victory for MacKenzie Country farmer Snow Loxton who led the campaign.

The owner of Sawdon Station near Lake Tekapo had discovered two moths had been used successfully on horehound in Australia as bio-control agents (parasite).

The two moths are the Plume (foliar feeder) and the Clearwing (root feeder), both introduced to Australia from Europe.

Horehound has long been a nuisance to farmers in dry and semi-arid parts of New Zealand. Its seeds form burrs, which damage wool, and it infests lucerne crops in which it is costly and difficult to spray out. The plant has an alkaloid taste so is unpalatable to livestock, taints meat if animals are forced to eat it, and can be a fire hazard. However, horehound is valued by herbalists and it’s assumed the herb was purposely introduced to NZ for that purpose.

In New Zealand the horehound biocontrol group (HBG) was formed to investigate the possible introduction of the moths from Australia.

The project began in November 2017 with a Landcare report to the HBG, and on 7 December 2018 MPI agreed to allow the release of the moths.

The two moths inflict significant damage to different parts of the plant. The Plume moth larvae defoliate the stem, reducing the amount of seed produced, and the Clearwing moth, feeding on the roots, disrupts the vascular flow and introduces infection from pathogens.

Spraying and mechanical control, while very effective against above-ground horehound, are not only expensive, but severely affect other desirable species. These techniques often open the ground to erosion and infestation by noxious weeds, the principal offender often being horehound itself. In contrast, horehound in a state weakened by insect attack is vulnerable to competition from tussock and cocksfoot.

The EPA disagreed with herbalists that a reduced supply of wild horehound would adversely affect their business. By December 21 moth releases at 13 sites had been completed, five of the Clearwing and eight of the Plume, spread from Marlborough to the MacKenzie.

Establishing the Plume moth is easy – sprinkle the larvae on fresh horehound stems and leaves, then scatter the mixture on established, healthy horehound plants. That’s it!

Establishing Clearwing moth is much more difficult – the freshly laid eggs from the larvae held in confinement were glued to toothpicks and the toothpicks glued to cut horehound stems at 800 per site, a major achievement requiring many volunteers. In the future it is expected infected plants will be dug up and transported to a new site.

The weed is not as problematic in Europe where farming conditions are different, but also where horehound has always had competition from specific insects and diseases.

In Victoria, six million hectares were infected with horehound by 1980, including 3.5% of conservation lands. The establishment of the moth in Australia was confirmed in 2001. By 2003 more than 50% of the original release sites were infested by the larvae. In Australia there did not appear to be any measurable economic benefits by 2008, but by 2012 it was suggested no other control would be required. There has not been a formal review of the moth release in Australia. However, in November 2018 Snow Loxton and Australian entomologist John Weiss, who managed the moth release in Australia 20 years ago, observed many of the moth release sites in Victoria and South Australia and were impressed with the reduction in horehound in those areas.

In New Zealand the 2017-18 HBG survey results showed 112,000ha of hill and high country (nearly all of this is in the South Island and including 5700ha of lucerne) were infested with horehound, with 98,000ha of this area assessed to be under non-chemical control. Total costs of control (chemical and non-chemical) were assessed at $3.35m per annum and with another $3.5m in lost production the annual costs would total $6.85m. This is a conservative figure with no allowance being made for increased wool processing costs and for the opportunity costs of avoiding lucerne.

The area of horehound is expected to double every three years on the surveyed farms.

Biological control won’t eradicate the target weed (nor should it as that would kill off the agent) but may reduce the weed to levels at which control is not required. One indication of the potential efficacy of the moth release is the level of release and initial establishment, which in NZ is very high.

In Australia, there were some significant failures of establishment, partly because of the difficulties of bringing the agent (moth larvae) from the Northern Hemisphere. Also, our horehound is healthier (greener for longer) encouraging better establishment of the moths. Although rapid expansion occurs when air temperature is above 22oC, health of the target plant population is seen as more important.

Biocontrol has been very successful where other grasses can outcompete with a weakening horehound and is not so good among native vegetation. Further releases in spring 2019 are expected to spread the moth throughout the country.

Beyond control of horehound, Snow is warning farmers to avoid planting lucerne in horehound-infested paddocks for 10 years, because the horehound seed will continue to germinate for that period of time.

  • Tom Ward is an Ashburton-based farm consultant.