I've never had much luck killing mealies with insecticidal soap - They've got some sort of waxy coating that makes the soap ineffective. I have found isopropol alcohol to be very effective, even at penetrating the wooly nests, but it can cause the plant to become dessicated if it isn't washed off after a few minutes. If your yews aren't too big, you could just dump some isopropol alcohol into a sprayer and douse the tree with that, then hose it down after a minute. The alcohol kills anything it's going to kill right away, so there's no point in leaving it on. If you do this every week for a month you should have killed most of them off, as their life cycle is about 30 days.

There is a product called Safer's Fine Oil Spray, which is supposed to dissolve the waxy coating, making them susceptable to the insecticidal soap. I've never used it, or even seen it for sale for that matter, but if you can find it, it might be worth a try. Particularly if you don't want to use the alcohol

BP

I always add alcohol to my spray to control the mealy bugs. But you are talking a large area that needs to be sprayed every couple of days. Still, it may be worth it.

A thought - since your yews are big enough to make spraying a pain in the rear, what about beneficial insects? Ladybug, lacewing and syrphid fly larvae all eat mealies. There are probably others you could try too, those are just the ones I know eat them for sure.

They all appear on their own in my garden, so they might show up in yours too. But it might be worth ordering a bunch, just to get a good head start on the natural populations establishing themselves. Of course then you couldn't spray anything.

BP

NOt much lady bugs in my area. In fact I think the reason these nasties appeared is because my spider living in that bush for many years now, disappeared. Possibly died during during winter. I'll try the alcohol method. right now just thinking of tackling hte problem 1 branch at a time. Using perhaps a large garbage bag to engulf each branch to prevent recontamination from the other affect branches. If any of you spot a weird person wrapping up her bushes this way - that would be me.

I had surprisingly good luck at finally getting rid of mealy bugs on some indoor plants by making a mix of Murphy's Oil soaps and End-All (pyrethrin) concentrate in a litre spray bottle - used about a capful of each in a litre of water. I had tried alcohol forever to get rid of the bugs - only killed off the plants.
Don't know if you can do this with such a large area - winter will kill them off.