|Knotting gall on Broad buckler fern|
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, Lady fern Athyrium filix-femina, 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
Eggs are laid in the unfurling fronds and the hatched larvae feed on the trichomes 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 plant have been noted.
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.
Braconid wasps of the genus Aphaereta and eulophid wasps of the genera Dimmockia and Elachertus have been identified as parasitoids of C. betuleti.
- Darlington, Arnold (1968). The Pocket Encyclopaedia of Plant Galls in colour. Pub. Blandford Press. Dorset. ISBN 0-7137-0748-8
- Hancy, Rex (2000). The Study of Plant Galls in Norfolk. The Norfolk and Norwich Naturalist's Society.
- Redfern, Margaret & Shirley, Peter (2002). British Plant Galls. Identification of galls on plants & fungi. AIDGAP. Shrewsbury : Field Studies Council. ISBN 1-85153-214-5.
- Stubbs, F. B. Edit. (1986) Provisional Keys to British Plant Galls. Pub. Brit Plant Gall Soc. ISBN 0-9511582-0-1.
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