As the most active field lepidopterist in Calderdale I have set a little challange for you. I found twice only the cases of 563 Coleophora argentula on Yarrow (Achillea millefolium ) in Calderdale. I found this at Cromwell Bottom and Lowfields Business Park (before they built on it) and at that time thought I had bagged a new Yorkshire record (its slowly moving north) but Harry Beaumont beat me to it by a year :-(( I am not sure if it occurs on the garden forms of Yarrow as I never found it on any I searched, but there again I only ever found it on two clumps of Yarrow in the wild. It must occur I am sure in other parts of Calderdale but as Yarrow is not exactly abundant it might take some searching for. I have found it today here on the Llyn but as Yarrow is very abundant I was not surprised at finding it. The cases can be found right through winter but I suspect many go to ground later this month to astivate.
The reason I ask for you to look for it is that sadly BOTH patches of the plant were destroyed, built on at lowfields and buried by the countryside unit at Cromwell Bottom :-((
PHOTO: Courtesy of Ben Smart