Hi
I don't have the polytunnel now (in the UK), but I used to buy the predators once a year in the spring when mites were first noticed but before numbers had really built up. The first year I had some improvement, enough to avoid using any sprays. The next few years it worked really well. I would think once a year should be enough, as long as you accept that very minor damage is inevitable. You could top up the predator population if the mites seem to be getting out of control again. I was growing fruit and veg though, so a bit of cosmetic damage wasnt that important. What I did notice was after a few years I didn't need to use control every year, maybe there were enough predators around to keep things in balance.
I've been using Amblyseius andersoni to control thrips in my glasshouse this year for the first time. There seem to be less thrips around so far, time will tell!
cheers
