The larva of the midge Contarinia coryli causes galls to form in the catkins of Hazel. The galls contain white jumping midge larvae; the galls must be opened to confirm whether mites or gall midge larvae are present. 

Very similar galls on Hazel catkins are caused by a mite, Phyllocoptruta coryli.   The mites are very small and need high magnification under a microscope to see them

Gall Adult  

The gall-causing larvae must be found.

Galls must be opened and photographed to confirm presence of midge larvae.  

Anywhere that Hazel catkins are found.

Galls may be seen at any time that catkins are present on Hazel.

Probably widespread and fairly frequent, but not well recorded in Britain.

Status in Leicestershire and Rutland not known.