The Knotting gall, is caused by the dipteron fly Chirosia betuleti (Ringdahl, 1935). The gall develops in the terminal shoots of ferns, such as Broad buckler fern Dryopteris dilatata, Male shield fern Dryopteris filix-mas

Dryopteris filix-mas

Dryopteris filix-mas is one of the most common ferns of the temperate Northern Hemisphere, occurring throughout much of Europe, Asia, and North America. It favours damp shaded areas and is common in the understory of woodlands, but is also found in shady places on hedge-banks, rocks, and screes...

, Lady fern Athyrium filix-femina

Athyrium filix-femina

Athyrium filix-femina is a large, feathery species of fern native throughout most of the temperate Northern Hemisphere, where it is often abundant in damp, shady woodland environments and is often grown for decoration.The plant is caespitose Athyrium filix-femina (Lady Fern or Common Lady-fern)...

, and the Ostrich fern, Matteuccia struthiopteris.

The physical appearance of the galls

The fly larva mines the leaves and stems of the fern's frond at the apex. 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. Usually only one larva is present in the leaf tip, sometimes two. An elongated white egg shell is visible at the centre of the mass

Life-cycle

Eggs are laid in the unfurling fronds and the hatched larvae feed on the trichome

Trichome

Trichomes are fine outgrowths or appendages on plants and certain protists. These are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae.- Algal trichomes :...

s in the groove of the rachis causing the frond to curl inwards. The pupae drop from the gall and remain from autumn and winter to emerge in the spring. Galling rates of up to nine fronds out of 13 on a single plnat have been noted.

Distribution

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.

As shown, the Knotting gall has been recorded throughout the United Kingdom, in the Netherlands, Czech Republic, Denmark, Finland, France, Germany, Italy, Norway, Poland, Russia - North and Northwest, Slovakia, Spain and Sweden.

Parasitoids

Braconid wasps of the genus Aphaereta and eulophid wasps of the genera Dimmockia and Elachertus have been identified as parasitoids of C. betuleti.