The impact of peat-free growing media and the emergence of sciarid flies (Sciaridae) as a pest or just a nuisance in Scotland

Reducing peat use in horticulture is critical for the future health of Scotland’s peatland
habitats (IUCN UK Peatland Programme, 2024). More sustainable peat-free growing media
options are available (Holmes & Bain, 2021; Royal Horticultural Society, 2024a), but the
biosecurity status of some of these materials is currently unclear. In response to anecdotal
stakeholder reports of higher numbers of fungus gnats (Sciaridae) in peat-free materials, this
study engaged with stakeholders to understand their perceptions on fungus gnat (Sciaridae)
prevalence in the growing media they are using and also reviewed the evidence available in
literature.
Fungus gnats (Bradysia spp.) (also commonly known as sciarid fly) are small, black, slowflying insects which are commonly encountered in the glasshouse environment (Marín Cruz,
2022), although for most people they are best known as a nuisance on houseplants. There are
at least 250 species of fungus gnat in the UK (Royal Horticultural Society, 2024b), and their
larvae are small and transparent which makes them difficult to identify on, or in, growing
media and other organic material.
In UK horticulture, fungus gnats have long been thought of as a nuisance and a low-level pest,
but little research has been conducted on fungus gnats in the UK. This scoping study therefore:
• Interviewed stakeholders who are growing plants in the glasshouse context to
understand the impacts that they are seeing with fungus gnat infestations.
• Reviewed the literature to establish what is known about fungus gnat issues in
Scotland, the UK, and internationally.
• Engaged with biological control suppliers to establish what species are currently
deployed to manage fungus gnats.