Frequently one sees white webs on gorse bushes out on the Dorset heaths. I had always been led to believe that these were the work of a moth, probably the lackey moth, but one day I noticed a brown rusty colour in one of these webs and so I stopped for a closer look. The photograph is poor as it was difficult to get the image magnified enough and yet remain in focus. Still unable to work out what was happening I collected some of the 'rust' and brought it home a put it under a microscope. I was amazed to see what looked like little spiders! It took a while but I eventually tracked it down to being a colony of gorse spider-mites (Tetranychus lintearius).
As its name implies it lives entirely on gorse and can do some damage to gorse bushes to the degree that it had been introduced in some places (especially New Zealand) as a biological control to stop gorse dominating everything else as it is such a vigorous plant. Since the initial find I have encountered gorse spider-mites quite frequently but we have so much gorse here in Dorset I think the mites are struggling to take control!
Not a gall or a deformity but this fits with mites that do create such things so I am including them in my plant galls and deformities series.