I spray mine with a systemic pretty regularly as well.

What brands are y'all using? I think the scale and mealies have gotten immune to my spray because I didn't mix it up with other types. I'm assuming the spider mites are similarly immune, thought they've pretty much disappeared since going outside.

Well, for the mites I use an unidentified bottle from a friend. Yeah, not good practice, but by god it works! Foer when I run out, I have a litre bottle of Cal-ex Abamectina fro Cheminova.
For scale and mealies, I use Imacloporid, or if I am reallly upset, Chas 48 aka Dursban.
One tip, (at the risk of teaching my grandmother to suck eggs) when you spray the plant, wash the roots, and with normal plants, soak the soil.
I had a run of the bastards on the plants on the terrace, -not orchids, and spray as I might, back they came, over and over.
Then last time, after spraying I went round all of them with a watering can full when they needed watering, and soaked them all. They haven't come back after that.

Yes I spray and drench with Orthene wettable powder, smells like death but very effective.