At our cabin near Sonora CA, elev. 3000', Sunset zone 7, I had planted a row of Leyland Cypresses between our house and the neighbor's house about 12 years ago. The tree screen had slowly grown to about 15-20' tall until about 2 years ago when one rapidly turned brown and died. Since then 2 more have also died so that now there are just 4 trees. There are signs of borers on the trunks and larger branches. A local nurseryman said borers had become a problem with Leylands and no one was planting them anymore. He thought a systemic like Bayers Rose granules would help. I had been using that before the last 2 died.

I have put in little Deodoras where the dead Leylands were. Any advice on what might save the last 4 Leylands? The soil's thin and rocky and it took a long time to get the otherwise fast-growing Leylands this big.

Thanks