I noticed some browning branches at the bottom of my Yew a few weeks ago and now its moving up the shurb. I think there is something eating it. I have attached a photo of a spidery web type material and I can see some small brownish shells on the branches. What can I spray on the yew to eliminate them?
Yew trees & shrubs are susceptible to a number of problems which could cause your issues.
Firstly, let’s mention the soil. Yews do not like two things – soil that is too wet or too acidic. When planted in well draining soil, a sloped location or a raised bed, they are much happier. The ideal pH for them is 6.5 so if you have acidic soil, the addition of limestone should make it more alkaline.
Secondly, the most common pests of yews are Black Vine Weevils, Mealybugs and Scale Insects. Judging from the picture you sent us, I suspect you have the latter, Scale Insects.
All this being said, I am attaching two links, see below, which will help you in reaching a definitive diagnosis of the problem and how to treat it.