Field Horsetail Project Forges Ahead
Sawfly (Dolerus germanicus) laying eggs on horsetail in containment.
Last year, the Lower Rangitikei Horsetail Control Group successfully applied to the Sustainable Farming Fund for a grant to investigate biological control options for field horsetail (Equisetum arvense). Field horsetail is an ancient fern-like vascular plant that is a significant weed in New Zealand as well as other Southern Hemisphere countries including Madagascar, South Africa, South America, and Australia. It made its way to New Zealand in the early 1900s from Eurasia (possibly as a passenger with iris root stock from Japan). Like many of New Zealand’s weeds, it is toxic and unpalatable to stock, reducing pasture quality. The stems contain silica, which is not digestible, but more serious is the condition of ‘equisetosis’, which is brought on by grazing the plant, leading to acute thiamine deficiency in horses and cattle. Field horsetail also prevents recruitment of native seedlings and it grows well around river margins, blocking waterways and causing flooding. The plant reproduces and spreads by producing spores (as other ferns do), but also vegetatively via stolons and tubers. In some areas field horsetail has been unwittingly spread around in gravel extracted from infested areas. Now designated an unwanted organism, it is illegal to knowingly grow or transport the plant within New Zealand. Two other closely-related species have also found their way here – E. hyemale (rough horsetail) and E. fluviatile. Rough horsetail has not shown the invasive tendencies seen by field horsetail and E. fluviatile has been successfully eradicated.
Field horsetail prefers the wetter regions of New Zealand and is now widespread in Whanganui, Rangitikei, Taranaki, parts of Wellington and the West Coast of the South Island. But it has also been recorded on the east coast in Havelock North, Marlborough, Canterbury and Otago. Traditional control measures using herbicide are costly and not always successful. “It is not a plant that can be controlled easily by applying herbicide,” explained Lindsay Smith, who is leading the project. “Because the plant has a deep root system, it is difficult to find chemicals that penetrate deep enough into the roots to be effective,” he added. As a result, field horsetail is not able to be controlled adequately and biocontrol might be the only hope of managing the plant and preventing further spread. New Zealand is the first country to investigate the use of biocontrol agents to manage this plant.
Recently we have completed an important first step, a survey to see what natural enemies are already attacking field horsetail in New Zealand. “It is important to get an idea of what is already here, what niches are already occupied and which organisms may compete with, parasitise or predate on any introduced agents,” explained Lindsay. The survey mainly focused on field horsetail but two rough horsetail sites were also sampled (see map).
The invertebrate samples were taken by beating field horsetail plants over a sheet and collecting what was dislodged from the plant. The samples produced only 19 generalist herbivorous species and there were extremely low levels of damage seen on the plants. It was a similar story with the pathogens collected, with most being generalist species that occur commonly in commercial crops (e.g. Phoma spp.). A total of 38 pathogens were recovered by collecting plants with disease symptoms and then cultivating the fungal colonies on potato dextrose agar plates so they could be identified using DNA sequence analysis. Two species found might have some potential to be developed into mycoherbicides, but the size of the market for such a product is likely to be too small for this to be economic; and so classical biocontrol options look more promising at this stage and are being investigated first.
It was clear from the survey, that compared with other weeds in New Zealand, the invertebrate fauna associated with field horsetail is depauperate. This was supported by the lack of damage seen on the plants, with <1% of the foliage showing any signs of feeding by insects. There were few predatory insects but an abundance of spiders, which were likely to be using the field horsetail plants as a convenient habitat. Therefore there is considerable scope for the introduction of host-specific invertebrate biocontrol agents that could markedly reduce the vigour of field horsetail in New Zealand.
Four potential insect agents from the UK are currently undergoing host-range testing at the Lincoln containment facility. They include a flea beetle (Hippuriphila modeeri), a weevil (Grypus equiseti), and two sawfly species (Dolerus germanicus and D. vestigialis). Despite their misleading name, sawflies are in fact a type of herbivorous wasp, and are notoriously difficult to rear in captivity. As adults the males and females look quite different and can easily be told apart. They hibernate over winter and can emerge from this diapause at different times depending on environmental factors. As if this isn’t enough, they have quite elaborate courtship rituals requiring the right environment to get the females “in the mood”! Figuring out these requirements and replicating them in a containment facility can be a big challenge. “So far, though, we have managed to produce enough sawflies to get host-testing underway, and the larvae appear to be capable of inflicting severe damage to above-ground foliage, grazing it down to ground level,” said Lindsay.
The flea beetle is easier to manage in containment but there are questions about whether it would be an effective agent. The larvae mine the thin needle-like fronds of the plant, which stunts plant growth but doesn’t compare favourably to the damage caused by the other agents.
The weevil appears to be the most promising of all. Adults lay their eggs near the top of the plant and the larvae then bore into and down the central stem, killing all of the above-ground material. Larvae continue to bore down into the root system, causing complete collapse of the stem. “Initial host-testing results are promising and the weevil is so effective that we are beginning to wonder whether any other agents would be needed; however, it is good to have the other species as a back-up. Also, depending on the phenology and emergence rates of the weevil, the sawfly larvae could inadvertently eat the weevil eggs or disrupt the larvae, which would be counterproductive,” said Simon Fowler. All of these agents co-exist in the UK but the environmental conditions are slightly different here and subtle differences in the climate or the niche-overlap might constrain their ability to colonise the plants. We plan to study likely interactions further.
Two fungal pathogens, Stamnaria persoonii and Ascochyta equiseti, available overseas appear to be specific to field horsetail and look like promising agents, and could also be considered later if needed.
To conclude, the prospect of finding agents to manage field horsetail looks very encouraging. There aren’t any plants even closely related to field horsetail in New Zealand’s indigenous flora, which makes the logistics of host-range testing simpler and the chance of non-target attack minimal. Although the plant has been grown for medicinal properties overseas, it hasn’t been used here for this purpose, which will reduce the hurdles required to gain approval to release biocontrol agents. The agents that have been selected are likely to be host-specific and all have been shown to damage field horsetail to varying degrees. With host-testing proceeding at a good pace, it is likely that an application will be made to EPA to release field horsetail agents in 2015.
The Lower Rangitikei Horsetail Control Group represents a diverse group of landowners and managers who have a significant problem with field horsetail, and who have come together to try to find a better solution. Alastair Robertson chairs the group, which also includes other arable and pastoral farmers, and representatives from the aggregate extraction industry, district and regional councils, the New Zealand Landcare Trust, and Landcare Research as the science advisor.
