I wish there was a soil injectable pesticide that took care of everything, but there isnt as of yet. The increased spider mite activity in hemlocks after being injected with merit, I personally think is false. It doesnt make sense to me.
Right plant in the right location works best for me.
We do use floramite with success, but repeated applications require you to switch to another miticide such as Avid as the mites will build a tolerance to one product.
If I remember correctly, I heard Dr. Raup say at the IPM short class @ University of Maryland a few years ago that Merit can actually make "super" spider mites,and I am paraphrasing...and I don't mean to speak for nor represent Dr. Raup...I'm no where near his level of expertise!
Merit has been known to increase spider mite populations. Remember that Merit is an insecticide and will not work to control Spider Mites and it also doesn't work well against your caterpillars. You need a miticide for spider mites. Spider mites and insects are two different beasts when it comes to control. Some manufacturers do have a miticide/insecticide but these are mostly non selective. Go with a miticide that won't wipe out the beneficial mites. Also remember to rotate your miticides so the mites do not become resistant. Some miticides are labled to only be used once per season so it is very important to read the label.