There's a white powdery kind of stuff on some of my hemlocks (and I've seen it on some around the block as well); a nearby landscaper says it's a disease, and must be treated, but a tree person took a look and said it's an insect. He proposed two applications of an oil, at a total cost of $270. Any thoughts?

 

Woolly Hemlock Adelgid crawlers love hemlocks. These crawling suckers' eggs form a powdery mildew at base of needles. They can stunt growth, discolor needles and eventually cause limb loss, usually beginning in topmost branches.

Horticultural oil will smother insects. Care should be ongoing. Height of tree usually makes it impossible for the average homeowner to properly care for hemlocks. Dense foliage usually can't be properly pentrated with water hose applications by the DIYer.

Spider mites is also a hemlock pest. You can shake a branch over a piece of white paper to see if you shake off black specks that when crushed produce a reddish streak.




Hemlock Wooly Adelgids
Photo Credit: Ontario Trees and Shrubs