I had a plague of spider mites on every grow since I let my first plants fraternize with the outdoor cucumbers. I was able to keep them down by spraying with citric acid + castille soap, and vacuuming tents off the colas with a shop-vac. At harvest I had to rinse the soap off by dipping in a bucket of water. My current grow is the first one where the mites didn't show up (Shhhh!). Before this grow I took a 3-month break and thoroughly cleaned the grow space. Here's what the adults looked like at 50x magnification:
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