2024 - What I did today

Sounds like mites - they are too small for me to see: I know they are there by the damage they cause and when it is really bad, the webbing, leaf on the right. The one on the left is just starting to show damage.

You might be right. The leaves look very similar to mine, although there's no sign of webbing. Next trip to the greenhouse I'll take powerful lens to see if they are visible. Next question, what do you do about them?

There are 2 products I have come across for controling spider mites: SB Plant Invigorator, sold by simply seeds among others and Neem Oil which isn't recommended in the UK as an insecticide. My understanding is that it would be harmful to bees if in direct contact so using on young plants and foliage only should be no problem to bees. I got some neem oil from superdrug but there are other suppliers that could be cheaper (e.g. https://thenaturalgardener.co.uk/product...cts_id=288).

The SB Plant Invigorator doesn't say what the active ingredients are and the safety notes on the internet just mention 'Sodium Lauryl Ether Sulphate' which is a detergent.

This article discusses neem oil use by bee-keepers for pest control in hives in the US as well as gardening.  https://pollenpaths.com/does-neem-oil-kill-bees/

I've pulled off all the dead/badly infected leaves on my cucumbers and created quite a long bare stem. They have new vines from near the bottom. I sprayed it with a mixture made up of the plant invigorator (2L.) and added 10ml. of neem oil along the stem and the leaves at top and bottom, there were no flowers on it at the time. The detergent in the SB Plant Invigorator acts to disperse the neem oil in solution.  I've sprayed it once since and quite happy with the results.

Ideally though, I'd like to get rid of them from the greenhouses as both products are not cheap and it takes my time from other things. They are not in the polytunnel.  I cleaned them with Jayes fluid last winter but obviously that wasn't enough. This winter, I'll do the same but also steam clean all the nooks and cranies and see if that works.