I have had these guys in my growroom for a few years and I didn't know what they were. They are survivors, somehow surviving 18 months of attempts to eradicate thrips. I successfully eradicated thrips 15 months ago and these guys made it.
I just got around to doing a reverse image search and they are called Desjardin's beetle. They eat mold and mildew and are unfazed by torrent of water. If you see them in your greenhouse, no need to worry
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