Fairly early in the season, my greenhouse got infected with little red pests that I believe to be red spider mites.
I suspect the invasion arrived either when I bought some pepper seedlings from a local garden centre or when I started to leave the door and window open to stop the greenhouse from getting too hot. That's not important. What's important is that despite my best efforts at crushing the pests to death between finger and thumb, the problem has increased in size along with the growth in plants in my greenhouse.
I read around a bit on the topic and came to the conclusion that these things are persistent and difficult to deal with. Organic suggestions for killing them included crushing them (which I did), spraying them with water because they don't like being damp (which I did), blasting them with water (which I did outdoors on the worst affected plants using a hose), using neem oil (which I didn't) and clearing the greenhouse out very thoroughly with disinfectant after you've removed all the plants and all organic material at the end of the season (which we haven't got to yet).
I do actually own some neem oil. My problem with it is that every time I mix it with water (and washing up liquid to help it mix), it quickly destroys the spray bottle I put it in until all the bottle is capable of producing is a single fine line of spray in response to considerable pumping. It's done it often enough that I don't currently dare put it in my new spray bottles, making me owning the neem oil a little bit pointless. Maybe I'll be braver next year.
Anyway, what I can tell you is that although I believe my spraying with water and crushing and occasional blast with a hose to have kept the pest level lower than it would otherwise have been, it hasn't stopped it growing. The most effective thing I've done I actually did by accident: I transferred some of the plants there was no longer room for in the greenhouse outside and planted them in the front garden. They no longer have any sign of spider mites on them. This got me to thinking: what if I move all the plants in the greenhouse outside? So I did.
I took all the plants in the greenhouse (except the tomatoes, which were too huge for me to brave moving and seemed unaffected by mites) out of the greenhouse and lined them up along the path. The weather was so warm that I had no concerns at all that it would be to cool for them outside. My original plan was to leave them out there for a week. Unfortunately high winds and thunder storms got in the way, so I had to move them back inside after only a few days. At this point they were still infested with mites, but the infestation was much smaller than it had been originally.
I think what is probably happening is that mites are delicious, but things that find them delicious only live outside the greenhouse. And possibly also that these mites are a lot less keen on weather in the UK than greenhouse climates. When I moved them outside, I saw insects flocking to them, especially ladybirds.
I suspect that mite infestation is going to be a regular event for me in the greenhouse and that my regular solutions are going to involve crushing the mites by hand and moving the plants outside when it's sunny for other insects to feast on them. It should be noted that the strategy of moving the plants outside is not without risks. The plants are quite capable of picking up new types of infestation while out there - for instance green fly. And it was also possible that my infestation would simply spread to other plants in the garden. However, neither of these things appeared to have happened with my front garden peppers, so I decided to brave it and have been rewarded by a significant drop in my infestation level. Warning: this solution worked in the UK. It is possible that it is specific to something about the UK environment. If spider mites are a common problem outside of greenhouses in your country (or if the weather outdoors in your country would kill greenhouse plants), then this is unlikely to be of any help to you and may even worsen the problem.
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