Moorland grouse - Flubendazole use for parasitic worm control: preliminary environmental assessment

Executive Summary

The 2019 Scottish Government’s Grouse Moor Management Group’s report (“the Werritty report”) included an investigation into the use of medicated grit containing the active ingredient flubendazole for the treatment and prevention of the strongyle worm (Trichostrongylus tenuis) in red grouse. Based on concerns that there may be risks for the environment when best practice is not followed, the Werritty report made several recommendations in relation to the use of medicated grit as well as an additional recommendation that “SEPA should initiate a desk-based study to determine the appropriate nature and extent of a monitoring programme to ascertain whether flubendazole residues exist in water bodies on or downstream from where it is being used, including in association with grouse moors, to conduct such a monitoring programme and to report on its findings”.

SEPA committed to undertake a desk-based study to assess the potential impact on the wider environment of the use of flubendazole in medicated grit. This report is the result.

The active ingredient (a.i.) flubendazole is added to grit in a stearate coating, with around 50mg a.i. present in a 500g grit tray. Based on typical stocking densities, over a 3000 ha grouse moor there would be a total of around 100g flubendazole used each season following treatment recommendations.

Flubendazole will enter the environment via the faeces of treated grouse and by leaching from spilt or open piles of medicated grit if best practice is not followed. Based on these release pathways and the substance’s properties, flubendazole may be found in soil, surface waters and sediments on or near moorlands. No measured environmental data exist in relation to this use of flubendazole.

Exposure in animals that may predate or scavenge on grouse carcasses is likely to be low, as are levels in soil, based on typical dose rates. However, predicted levels in surface water and sediments are less certain because of missing information on the importance of particle-associated transport during runoff following rainfall. It is clear though that levels in watercourses are likely to be higher in cases of malpractice where grit is placed too near a watercourse. These variables also mean that monitoring in practice would not be a good tool to pick up examples of poor practice in a systematic way.

It has not been possible to derive a toxicity threshold for the protection of the water environment because of missing data for some organisms. The available data show that aquatic invertebrates, e.g. water fleas, are more sensitive to the substance than aquatic plants. Comparing the most sensitive aquatic toxicity test result to predicted levels shows that the risk to surface water organisms in watercourses on or near grouse moors is likely to be low when best practice is followed. The biggest areas of uncertainty in this conclusion relate to poor grit placement practice and to a lesser extent inputs associated with particulate runoff. Extrapolating the aquatic toxicity test result to sediments leads to the same conclusion on risk, with similar caveats. Further extrapolating the aquatic toxicity test result to soil-dwelling organisms indicates that the risk again is likely low.

Although the assessment in this report has generally presented a low environmental risk from the use of flubendazole in medicated grit on grouse moors, the uncertainties in both the estimated environmental concentrations and ecotoxicological effects data are high enough for us to recommend:

• An investigation into levels of flubendazole in surface waters (and sediments) and potentially impacted invertebrate communities near or on moorlands using medicated grit be considered to confirm that best practice is protective of the local environment. Sampling for chemical analysis should be conducted to reflect both dry and wet periods and be carried out during the main periods of use (winter – early summer)
• If there is a need to investigate instances of grit malpractice, “walk overs” or other visual inspection methods (eg unmanned aerial reconnaissance) should be considered since ecological and chemical monitoring are unlikely to give much information on this.
• Refining this report should new information on ecotoxicity or modelling approaches relevant for particulate run off become available to improve predictions of levels in water and sediments and their potential impact on wildlife.