I have an Oak (Quercus) in a container grown from seeds, must be 6-7 years old now. And every year it gets these yellow spots on the leaves. When I look under the leaves, I see tiny little red/brown dots, which are the suckers, but they are as small as grains of sand. I can't distinguish legs or body parts or anything. They are also too small for a photo, sorry I only have a camera in my phone.
Does anyone recognize these yellow spots, and know which animal are causing them? Are this Spidermites? I didn't see web yet. How can I treat them (preferably in an organic way), I have tried spraying diatomaceous earth but that did not work.
Maybe interesting, next to this plant is a horse-chestnut (Aesculus hippocastanum), which has no signs from this pest (but will get a leaf miner problem later this year, I expect).
The plants are on a balcony (facing east) in Amsterdam, the Netherlands.