You have thrips, and that sucks monkey balls, by now all of your plants are most likely infected.
The good news is that it's possible to grow successfully with thrips :)
The bad news is that it's almost impossible to get rid of them without doing a full reset.
First of all, the leaves are not yellow because of the thrips, that's other issues with either soil, pH, pot size, or nutes. Thrip damage shows up as grey-ish discolorations.
Most bugs are only able to establish a bridgehead on weak leaves, keeping plants healthy and removing dying/dead leaves from the plant and grow room is a good idea.
What to do depends on your situation:
1. If you're not in a hurry to get a harvest done, then I recommend going protocol zero and doing a full reset = kill all plants, throw out all soil, used pots etc., disinfect your tents and equipment thoroughly with chlorine diluted in water several times over a few weeks (wait at least 14 days until starting a new grow).
2. If you need this harvest to fill your jars, then spray ALL the plants for a week or two with something that will kill thrips but won't hurt the plants too much. Then flip and keep spraying for another 2 weeks or until you see flowers.
Also cover the soil surface of ALL plants with perlite or diatomaceous earth.
This treatment probably won't kill them all, but set them back enough to not be fully back until mid/late flower where the damage they do won't be as big of a problem.
You can introduce a predator like the predatorial mite (Amblyseius cucumeris) or the guys that eat pretty much everything, Lacewing Larvae (Chrysoperla carnea)
When you harvest you just wash your plants using the Bud Wash method and then you can enjoy bug and dirt free ganja, and no it won't knock off trichs :)
3. If you have a perpetual setup with several tents and mother plants, then you either learn to live with thrips or do a full reset, been there, and believe me the reset is worth it :)