I’ve check many of my resources and can find two possibilities, pine needle scale or pine bark adelgid.

The female Pine Needle Scale (Chionaspis pinifoliae) has a white, waxy cover that measures up to 1/8 inch long. This covering is oystershell-shaped and has a yellow spot at one end. The adult female scale settles on a pine needle where it remains immobile, feeding and laying eggs. A heavy infestation can slow pine tree growth and cause dieback. Young, weak trees may suffer more severe injury. Prune off heavily infested portions of the pine and provide the tree with good cultural care, including adequate irrigation. Scales have many natural enemies that control this pest's numbers unless these beneficial predators and parasites are disrupted by dusty conditions or broad-spectrum, persistent pesticides.

Pine Bark Adelgids (Pineus strobi) weaken pine trees by sucking sap. Heavily infested trees grow poorly, and become discolored. Some trees may die or become weak and susceptible to other pests during dry periods. Look for discolored, stunted, weakened trees with small but conspicuous lumps of white woolly wax on the main stem and branches. The trunk may look whitewashed. You might see yellow or purplish insects, less than 1/25 inch long, under the woolly wax (use a hand lens). Mature females covered with woolly wax overwinter on the tree. Eggs laid in the spring produce wingless and winged forms that infest new hosts. Spraying trees with a dormant oil before growth starts in the spring or thoroughly spray tree with a registered insecticide in mid May when the insects are active.

Check these sites out to see if you can determine if one of these insects is the culprit. Because of the white on the trunk, it sounds to me like the adelgid. I would check with your arborist to see if he agrees.  It is true that the time to spray for either of these is in the spring. The arborist would know the best time.

Pine Needle Scale:

http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/insects/scale/pine-needle-scale.aspx

http://migarden.msu.edu/uploads/files/e2812.pdf

Pine Bark Adelgid:

http://entomology.osu.edu/bugdoc/Shetlar/factsheet/christmasstree/pine_bark_adelgid.htm

http://msue.anr.msu.edu/news/pine_bark_adelgid_becoming_active_in_white_pines

Hope this was helpful. Feel free to contact us again if you have further questions.