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Pakeman, R.J. (2023). A Rapid Evidence Review of the Implications of Not Controlling Bracken with Asulam in Scotland. A report for the Scottish Government.


This report was requested to better understand the implications of not controlling bracken on biodiversity, rural productivity and public health, where asulam is traditionally used for its control in Scotland.

Asulam, sold under the trade name Asulox, is a relatively narrow-spectrum herbicide that has been the predominant chemical method of bracken control, particularly on rough or steep ground since the 1980s. Authorisation for Asulox was removed in 2012 with all recent use subject to emergency authorisation which has been given on an annual basis.

Emergency authorisation allows a plant protection product to be placed on the market, for limited and controlled use, in a situation where there is not a standard authorisation for that use. If granted, the authorisation period cannot exceed 120 days. Emergency authorisations may only be granted in special circumstances where control is necessary because of a danger to crops, other plants or to the environment, human or animal health. It must also be demonstrated that this danger cannot be contained by any other reasonable means. Applications are assessed against these requirements, including balancing the potential risks and benefits of using the requested product, to decide whether the benefits of granting the authorisation outweigh potential adverse effects.

The manufacturer of Asulox is continuing to seek approval of asulam through the standard process. It may take several years for all the required tests to be carried out. If the known data gaps can be filled, further assessment of asulam would be required to determine if it meets the regulatory conditions for approval.

Not having asulam available would remove the most effective method of bracken eradication, where initial blanket spraying is combined with a proper follow-up programme. It would also mean that control was not possible on steep ground (where the only option is aerial spraying), and it would remove the current forestry management practice of controlling areas of bracken to allow tree establishment. Cutting, while proven to be effective, is restricted to areas with safe vehicle access, is carbon intensive, and takes a longer time to significantly suppress bracken. Recent evidence shows rolling/bruising is ineffective when compared to cutting and asulox treatment. The establishment of woodland and forest habitat along with targeted habitat restoration presents a long-term bracken control option, but asulox is often used as a treatment to allow tree establishment. There is no information on the areas subject to different control methods or on where (aerial) spraying is the only option available. The updating of bracken control guidance is needed whether asulam continues in use or not.

Bracken has been shown to replace other habitats in Scotland, particularly acid grassland and heathland, but there is no data on these trends since 2007. Its presence is often on sites that were previously wooded where its competitive ability and the presence of grazing animals have prevented woodland regeneration. Climate change is highly likely to be behind the continuing expansion of bracken dominance, especially in areas where it was restricted in vigour due to cold temperatures. Combining information on the effectiveness of control and the amount of land treated suggests that past control with asulam has meant that several hundred square kilometres of land are now bracken free that otherwise would be under bracken with consequent benefits for biodiversity and rural productivity.

The expansion of bracken into semi-natural habitats will mean a loss of some of their associated biodiversity. Whilst some bracken stands maintain a woodland ground flora, hill ground taken over by bracken tends to have little value for biodiversity.

There is little scientific evidence available to assess the direct effects of bracken as a source of toxic or carcinogenic compounds within a Scottish context, but assessments at the UK level suggests risks are low. Some studies show that bracken harbours high densities of ticks. However, the evidence on whether levels are higher when compared to other habitats is not clear with tick numbers dependent on the presence of host mammals. More research is needed to assess if its presence and expansion could increase the risk of animal and human tick-borne diseases including Lyme disease.

We have no data on the impact of bracken on rural productivity. It may be possible to get a partial picture of control costs from grant support, but there is an absence of information on lost opportunity costs of bracken replacing grazing resources.

Not controlling bracken in areas without vehicle access where asulam was the previous control method has the following potential negative consequences:

  • Limited scope for other methods of control of bracken in rough or steep areas not accessible by vehicles.
  • Reliance on less effective bracken control measures and removal of the only effective option known to be capable of eradication.
  • Disruption of ongoing follow-up treatment to initial control spraying.
  • Implications for woodland and forestry establishment where asulam was a key bracken control method.
  • Potential implications for controlling the spread of bracken into acid grassland and health areas with sensitive biodiversity.
  • Continuing loss of grazing land.

The potential positive consequences are:

  • Removal of the risk of asulam and its breakdown products entering watercourses and soils and resultant potential impacts on non-target species, avoiding ecotoxicological concerns about its use.
  • Potential for a negative carbon balance from controlling bracken as the large carbon stores in litter and rhizomes are released.
  • Removal of asulox as a control method may drive innovation to find suitable alternatives for control.

A key set of data gaps need filling to provide the basic information for decision making, including:

  • Current trends in the area of bracken and transitions between vegetation types.
  • Satellite or aerial imagery-based land cover map that include bracken as a separate category. Combining this information with data on terrain slope (e.g., derived from a digital terrain model) would identify areas where aerial spraying is the only control option.
  • The long-term effectiveness of asulam-based or cutting-based control programmes.
  • A cost-benefit analysis of control for both biodiversity and improvements in grazing.
  • It is not known if any of the potential alternative herbicides can be used like asulam in repeated follow-up applications or if they give a long enough window for tree establishment.
  • Epidemiological studies of exposure to bracken that could be compared to the risks due to pesticide exposure.
  • Tick and tick host densities in different habitats and information on exposure for both humans and livestock.


Suggested citation:

Pakeman, R.J. (2023). A Rapid Evidence Review of the Implications of Not Controlling Bracken with Asulam in Scotland. A report for the Scottish Government. James Hutton Institute. pp22.

DOI: 10.5281/zenodo.8011214


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