Pine needle scale is an armored scale and because of the way armored scales feed (on the contents of meshophyll cells, mostly, not fluids of the phloem) most systemic insecticides do not reach them.

The exception is dinotefuran, sold as Safari, Transtect, and Zylam - probably others. It is the most water soluble and mobile of the systemic insecticides in plants and can get to armored scales. It is also so mobile that it can be applied as a trunk spray, moving into the plant through thin areas of the bark.   A "basal trunk spray" application. Alternately, it can be injected into the soil. However, if you do the latter, the soil must be kept moist long enough for the insecticide to be picked up by the roots.  So, if they can not irrigate, soil applications are out.

I would like some background on this, if you know about it. Are these properties that have been sprayed a lot, perhaps for mountain pine beetle? The reason I ask is that a lot of times pine needle scale outbreaks occur when insecticide use has devastated the natural enemies of the insect, causing it to explode in numbers. So if these are trees that are getting regular sprays of insecticide - and there is no good reason for it (e.g., the mountain pine beetle outbreak ended years ago) - then the first thing I would recommend is to stop spraying insecticides on the tree. This will allow the natural enemies of the pine needle scale to recover and do their job, which they normally do very well.

If sprays are used for pine needle scale then I suggest products that are conservative of the natural enemies. Oil sprays are quite good, and probably an application after the crawlers have hatched would help a lot. There is also an insecticide that is a "insect growth regulator", known as pyriproxifen (Distance, Fulcrum) that is excellent on scale insects and largely spares scale natural enemies.

Oils and/or pyriproxifen are the recommended sprays, A basal trunk spray or soil injection of dinotefuran should also help, as this is also conservative of natural enemies.