Farmers seek biocontrol for smelly weed
Horehound in 4 year old pasture in Central Otago.
Recently Ronny Groenteman was contacted by a high country merino farmer, Gavin Loxton, asking about the possibility of biocontrol for horehound (Marrubium vulgare), a putrid-smelling weed that reduces lucerne yields and wool quality. Although there are no biocontrol agents available for this weed in New Zealand there are some available just across the ditch. Biocontrol for horehound in Australia was developed during the 1990s and two moths were released: a plume moth (Wheeleria spilodactylus) that attacks the above ground vegetation, and a clearwing moth (Chamaesphecia mysiniformis) that attacks the roots. The moths provide excellent control of horehound in Australia in many situations. Other potentially good agents that could also be considered were identified, but not released.
Horehound is a perennial shrub resembling mint, native to temperate Eurasia, Europe, the Middle East and the Mediterranean region, including North Africa. As well as Australia and New Zealand, horehound has become a weed in southern USA and South America. Recorded as naturalised here in 1867 horehound was first classified as a weed in 1902. It is frost resistant and drought tolerant but also occurs in higher rainfall areas. Horehound occurs in eastern parts of New Zealand from Northland to Southland, particularly in Canterbury and Otago.
Horehound has become an increasing problem on dryland farms across the country over recent years and is now recognised as one of the worst weeds in lucerne crops. Chemical control of horehound is problematic in lucerne stands for a number of reasons. The waxy coating on the leaves provides horehound with some protection against herbicides. The most cost-effective chemical, Metsulfuron, has a long residual period and farmers must wait for 2–3 years before sowing any legumes in sprayed areas. During this time horehound can regrow from its long-lived seedbank. Also, young lucerne stands cannot tolerate Metsulfuron in their first 3-4 years and, to make matters worse, horehound is possibly beginning to develop resistance to this spray. Horehound also degrades pasture as it is unpalatable to livestock and its prickly burrs reduce the value of wool.
Gavin Loxton has first-hand experience battling the weed on his 8000 hectare property, Sawdon Station, near Lake Tekapo. Herbicides were just not working, Gavin said. They damaged clover, lucerne, and the soil. “They leave residual chemicals that stunt the lucerne and significantly reduce yields – as much as 30 per cent. If it happens to coincide with a dry year, you can also lose a lot of lucerne plants. They’re not actually solving the problem. You’re simply left with unproductive land.” Herbicides were also difficult and costly to apply in high terrain, Gavin said.
After talking to Ronny, Gavin formed the Horehound Biocontrol Group, which hopes to be able to release the two moths in New Zealand. Some host-testing may be required but, given the amount of work already done by Australia, this should be a relatively straightforward project. Soon after its formation the Horehound Biocontrol Group conducted a survey, in conjunction with Landcare Research, to get a better idea of the scale of the problem to ensure that biocontrol would be justified. Farmers were questioned about the size of the horehound infestation on their property, control measures they’re using and the effect it’s having on their lucerne crops. In a short time more than 60 responses were received. Many noted that horehound cover doubles every 2–3 years if left unattended. Others noted that horehound is now invading areas where it has not been seen before. The data suggest that the impact of horehound on lucerne crops alone costs around $29m to $39m per year.
“The survey made me realise that horehound is a much bigger problem than I envisaged,” said Gavin. “Farmers were disillusioned about finding a viable management option, and weren’t talking about horehound. The possibility of biocontrol brought back some hope.” An application for funding to develop biocontrol has now been submitted to the Ministry for Primary Industries Sustainable Farming Fund with strong financial support/commitment from affected farmers. “While horehound is still a relatively small problem in New Zealand, it would totally make sense to nip in the bud now,” concluded Ronny.
If you have a problem with horehound or are interested in supporting the Horehound Biocontrol Group please contact with Ronny or Gavin.
Ronny Groenteman: groentemanr@landcareresearch.co.nz
Gavin Loxton: sawdon@lupins.nz