One of my customers showed me there peach tree growing in there greenhouse. The leaves were very mottled and were beginning to drop prematurely. The actual fruit did not seem to be progressing to there normal size.
This is one of the traditional signs of Red Spider Mite (Tetranychus urticae). Unfortunately my camera could not focus in close enough on the actual mite. But they form a very fine webbing over a shoot, not a traditional spiders web as such. But this webbing sort of protects the mite. Ultimately this webbing covers many shoots and all gets joined up into a mass of webbing. If not controlled adequately it can kill the plant.
This mite in this instance it was on a peach tree, but they infect virtually any plant. as it is a greenhouse pest it can attack tomatoes, cucumbers etc etc. Being so microscopically small in the early stages they are not detectable. They are typically 1mm long and bright red. Its only when you get the mottled leaves and leaf fall it is noticed.
They love hot dry greenhouses, so dampening down is recommended to keep the humidity up, and also ensure adequate ventilation. Sometimes here in the UK they can venture outside during the occasional long hot dry summer. They over winter in nooks and cranies and start there breeding cycle of laying eggs from March till October.