We (Stewart Rosell and Rhona Watson) found this gall on the Walnut tree (Juglans regia c.1950) by North Court on 27th July 2021. This seems to be the third recorded UK sighting of this gall. A. brachytarsus is known from sightings in California and there have been occasional records of sightings from Iran and mainland Europe, but there were none from the UK until July 2021.
Sam Buckton made the first UK recorded sighting in Downing College on 2nd July 2021, and then he found it again in Cambridge University Botanic Gardens during their Bioblitz on 24th July. He then passed on the details to us and we checked our large Walnut and we found it was there as well. The Downing College galls have been verified by several experts and Sam has looked at pictures of our galls and agreed that they are A. brachytarsus.
On the 28th July I checked the Black walnut (Juglans nigra) c.1912 and I could not see any galls, although there were no low branches so I was unable to check properly, but the young (c1998) Walnut (Juglans regia) in the Orchard by the Fellows’ Garden also has A. brachytarsus.
Walnut has two other known Aceris galls; A. erinea and A. tristriata, and both were recorded on the North Court tree in 2021.
Aceris brachytarsus is sufficiently different from other Walnut galls for it to be identified easily, as it has a conical projection on the underside of the leaf, whereas A. erinea has large (1cm or more) ‘blisters’ on the leaf, and A. tristriata has a myriad of tiny (up to 2mm) round pustules all over the leaf. (There is a third UK gall that may be found on Walnut, Microstroma juglandis, but it is visible as yellowish blotches and so it cannot be confused with A. brachytarsus).
Aceria are gall mites; the mites themselves can only be seen using a high magnification, but their presence can be assumed due to the galls that are present on the leaves of trees. The mites always produce similar galls and so it can be an easy way of increasing the species list of an area if you can identify the host plant and the gall itself.
If any of you have Walnuts in your area then it might be worth checking to see what galls you have too.
Interesting fact: This gall mite is new to the UK and so it is not on irecord (so if you are recording an example, record it under Aceria species and write in the Comments field). To get it added to irecord we have had to contact the Natural History Museum to ask for the gall mite to be added to the official UK Species Inventory – hopefully they will agree and it will be added soon.