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This year, many acorns in our area (Germany, Schartzwald) fell prematurely, plagued by some kind of warts or gall-nuts which I see for the first time:

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Some of the acorns are covered by two or three warts at a time, so that the actual nut itself is practically non-existent.

Judging by the hole present in all the warts, I'm guessing those were houses for some insects or larvae. Unfortunately, warts on acorns available for inspection (those which already fell) are already abandoned by their hosts.

Does anyone know who those guys are and how they look like?

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  • $\begingroup$ tiny vespids that lay an egg in the acorn, produce an enzymatic reaction by specific chemical release that mutates the acorn into a certain shape, depending on the species, it can be that shape, or a round ball of wood, and many other galls depending on species. if you cut it open you will see a small grub or it's silk cocoon, and a little tunnel where it lived and escaped from. $\endgroup$
    – bandybabboon
    Commented Apr 14, 2017 at 18:12

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The structure on the oak nut is a knopper gall.

It is a distortion of growing acorns on pedunculate oak (Quercus robur L.) trees, caused by gall wasps(Andricus quercuscalicis), which lay eggs in buds with their ovipositor.

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An Andricus quercuscalicis larva in an open gall of an acorn: Link

Here's a video by BBC on the same you may find it interesting.

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