Sometimes I am too accepting of things which should be challenged and investigated.
I regularly look at Holm Oak (Quercus ilex) leaves and notice the felt galls caused by the mite, Aceria ilicis. But someone on a social media gall group made me question more deeply what I have noticed before: we use the name Aceria ilicis to cover two quite different types of galls.
Type 1: the pimple gall
The upperside of the leaf shows distinct raised bumps or pimples with corresponding cavities on the underside filled with rusty brown erinea (hairy patches or felt).
When put under the microscope, these patches show themselves to be very dense patches of rusty brown, mostly tightly curled ‘hairs’.
Type 2: the felty patch gall
In ‘type 2’, the upperside of the afflicted leaf may show some general bulging, but there are no distinct ‘pimples’ as with ‘type 1’.
Unlike in ‘type 1’, ‘type 2’ galls appear to be have darker brown erinea and to be quite extensively covered by felt patches.
When looked at under greater magnification, the erineum itself appears slightly different.
The erineum is not only a different colour, but it appears to be growing less densely than in ‘type 1’ examples and the ‘hairs’ are less likely to be tightly curled.
Without detailed study of the mites themselves – beyond my powers and powers of my microscope, I doubt a conclusion can be made.
However, I am not convinced by the argument that ‘type 2’ merely displays more mature and extensive galling. Even discounting differences in colour of the erinea patches, the lack of pimples in ‘type 2’ seems odd.
Could ‘type 1’ and ‘type 2’ be caused by different species of mite? Possibly.
One final interesting observation is that all the photos above, of ‘type 1’ and ‘type 2’ galls were taken from two leaves growing on the same tree host.