Question
My yews are turning dark and dropping their needles and there is a white substance sticking to the stems and needles.  It's spread to at least 6 older shrubs.

Answer
Dyanne,

The white substance is actually an insect called mealy bugs. Mealy bugs feed on the juices of the yews, slowly killing them. I would recommend spraying the inside of the plants with horticultural oil to suffocate the insects and fertilizing them to encourage new growth.

Good luck,
Ed Gulliksen