Can anyone tell me what these red marks are on this sycamore leaf, Acer pseudoplatanus? The marks are about 2mm in height and can cover much of the leaf. They are abundant in the woodland and some can be seen from the public footpath.
We have another gall here on this sycamore leaf, this time caused by a mite called Aceria macrorhynchus. Mites are related to spiders, being members of the Class Arachnida and Subclass Acari, which includes the mites and ticks. The galls are known as the sycamore red pustule gall.
The mites are almost exclusively female and reproduce parthenogenetically (without the need of a male). The adult female mites emerge in the spring to feed on young leaves, after overwintering in bark crevices. The feeding activity within the leaves forms the galls, with each mite being capable of producing multiple galls. When ready to reproduce in May, the mites access the galls through small homes on the underside of the leaf, under each gall. Each hole is fringed with tiny hairs. The eggs are laid inside the gall and on hatching, the larvae feed within the gall. The mites are tiny and are rarely seen, giving away their presence by the obvious galls on leaves.
The galls can be seen from April through to the end of the summer and appear as raised pustules on sycamore leaves, turning red coloured at the tips, with tiny entrance holes on the leaf underside, fringed with hairs.
Look out also for a similar species, Aceria aceriscampestris, that causes red pustule galls on Field Maple, Acer campestre, which is related to the sycamore. Both species are common and widespread in the UK, wherever the host species is present.