How to recognise it
Lots of small red lumps (galls) which are about the size of a pinhead on the upper surface of young leaves, giving the appearance of a rash of pimples.
The galls are caused by tiny mites (less than 0.2mm long). The immature and adult mites emerge in the spring after overwintering in bark crevices and other tiny spaces. As they pierce individual cells to feed on the plant tissue the surrounding cells are prompted to divide, therefore enlarging and forming the gall. By mid-May the mites start to lay eggs within the galls and several generations can occur over a season. The adults leave the galls in autumn before leaf fall to overwinter.
Why it’s a problem
Other than the affect to the appearance of the leaves, this pest does little lasting damage.
Where you are likely to find it
Affects sycamores (Acer pseudoplatanus), some ornamental acers and hedge maples (Acer campestre). More prevalent on young trees.
The symptoms occur from April onwards.
How to deter it
Purchase new plants from reliable sources to ensure that you don’t introduce the mites to your garden.
How to get rid of it
Usually treatment is not necessary as the sycamore gall mite does little damage and, short of removing and destroying the affected plant, there is nothing that can be done to control it.
Is it good for anything?!
Other useful information
There are several species of gall mites which affect sycamores with these symptoms, including Aculops acericola, Aceria cephaloneus, Aceria macrorhyncha and Aceria aceriscampestris (although there is some debate as to whether these are all truly different species).
Field maples are also affected by Aceria macrochelus, which forms larger and hairy galls.