What’s this pest.

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A pest, rather than fern identification. Tiny black mites are flourishing on the fronds of a fern wall I created earlier this year. Asplenium scolopendriums are the most heavily affected but they are also present on Pteris species. The leaves feel weak and a bit dried out. The bugs look smaller and legless compared to a black aphid.
Need I worry or will they disappear with the colder temperatures. The wall is pretty sheltered and next to the house so may never get frosted.
What’s the best treatment, if needed.