Chirosia betuleti is a species of fly, which causes knotting gall in ferns.

The tip of the frond rolls upwards into a loose, obvious knot or mop-head structure involving many pinnae; inside, a white larva mines along the rachis, eating the trichomes, causing it to coil.

The pupae drop from the gall and remain from autumn and winter to emerge in the spring.

[3] The knotting gall is found in Cornwall, Wiltshire, Worcester, Hants, Norfolk, Warwickshire, Berkshire, Cardiganshire, Cheshire, Denbighshire, East Norfolk, Suffolk, Easterness, Elgin, Flintshire, North Ayrshire, Herefordshire, Merionethshire, Perth, Yorkshire, Montgomeryshire, Lincolnshire, Somerset, Oxfordshire, Shropshire, Devon, Hampshire, Yorkshire, Stafford, Surrey, Gloucestershire, Kent, Suffolk, and Worcestershire.

[4] As shown, the knotting gall has been recorded throughout the United Kingdom, in the Netherlands, the Czech Republic, Denmark, Finland, France, Germany, Italy, Norway, Poland, Russia – north and north-west, Slovakia, Spain, and Sweden.