Actually pyrethrin breaks down VERY fast under intense light. You should always (veg and flower) try to use one that is JUST pyrethrins, because some contain organic activators- and I don't mean organic in the hippie sense, but in organic molecules- hydrocarbons- that scare the crap out of me- remember when they thought DDT was safe? yeah....
Even so I hate treating plants past week 3 of flower.
For your flowering girls, get the lower parts of the plant and the soil surface with neem. For one, the larvae migrate from the soil up the stem so halting that will keep them off the upper plant parts. Even a couple rings of double-sided sticky tape around the stalk should do wonders. Luckily thrips don't do the sort of damage high on the plant that other bugs can do. They are the WORST for seedlings and small clones, since so much of the plant is nice and low to the ground, just how thye littl bastard like it.