The insect has 2 or three generations per year, and has the semi-odd habit of going into a summer estivation (sleep-mode). Thus it seems to be better to spray (if using a high-quality horticultural oil) at least twice per annum, maybe thrice.
Late summer / early Fall and early Spring seem to be the most effective IMO. It is possible to have great effect with oil so I do not recommend going to more aggressive chemicals such as imidacloprid (Merit). Tree companies push the Merit because they can charge more for it.
This is an easy bug to control as long as it is possible to really drench the tree with sprays - trees too tall (over 50') or too close to obstructions need other treatments.
My Eastern Hemlocks were treated successfully w/ horticultural oil - applied in May in Zone 6b. I used an arborist with a specialized boom sparyer system to reach 60'+. I think he was going after them in the egg laying stage. In my case, I took action late - maybe 2 or 3 years after first noticing the "white stuff." I'm no expert, but preventative spraying probably isn't necessary as the "white stuff" is easy to spot. If you've already had a W.A. infestation, be sure to watch out for a secondary infestation of spider mites (oils & soap will work, but proper timing is everything). They are opportunistic and attack already weakend trees. The arborist also suggested using soak hose irrigation during dry spells as Hemlocks often grow near water. He also recommended root zone fertilization to help them recover. It all worked out well and there has been no re-infestation 3 years later. I suggest you consult someone w/ specific W.A. experience.
Other Hemlock & W.A. observations and factoids:
1) There are experimental programs using a predatory Asian lady bug (I think) to control the W.A.
2) In my area, native Hemlocks, which are in bad shape, line many pristine trout streams. It is predicted that the trout (& the entire stream ecosystem) will be seriously harmed - even wiped out in some streams, b/c when the shade cover is gone, water temp rises, reducing dissolved oxygen, thus killing the fish. Since they're near water, our state DNR won't spray anything on them.
3) I have a friend who has a lone hemlock in a 10 year old sub-division that has never been infected or treated. It seems that they attack in areas of high population and can "skip" isolated specimens.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.