During the summer of 2015, I noticed some distinct bulges upon a walnut’s (Juglans regia) foliage. With my curiosity raised, I took some photos and later that day identified the causal agent – Aceria erinea.
The walnut leaf gall mite, whose abundance (worldwide) really depends on how plentiful Juglans regia are within an area, overwinters in the buds of its host, and in the summer moves into the foliage. Here, it creates the distinct galling structures as shown below.
In terms of the damage it causes to the host, I admit I struggled to find anything. An internet search was not particularly fruitful, and consulting Google Scholar beought up few (if any) research articles that seemed focus exclusively on this insect. However, it can be stipulated that its presence is likely not going to be beneficial – the host walnut will need to synthesise chemicals (volatiles) that resist attack, which requires energy. Such volatiles will be herbivore-induced plant volatiles (HIPVs), and will serve to either resist herbivory by the gall mite, or attract predators of the mite to the tree. Additionally, the reduction in photosynthesis as a result of leaf damage may have adverse impacts, most notably if the host tree is experiencing other forms of stress.
Arboriculture 
We experienced a lot of these when I worked at a research woodland that heavily featured walnuts (Common, black and hybrids) and over the years we never saw a detrimental effect caused by these galls.
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Interesting, and thanks for the comment. Some insect outbreaks don’t tend to have much of a discernible effect, so perhaps this is one of those insects.
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