Caterpillar or larva on pines
These insects eating your trees are pine sawflies, insects which closely resemble caterpillars.These are not caterpillars.
Pine sawflies are voracious feeders which can strip all old needles from trees. New foliage is not eaten. If heavy defoliation (needle loss) occurs for 2 successive years, it can kill the pine.
Depending upon the size of the pines. you may be able to handpick the sawflies, then drop them into soapy water. Another option is to use a harsh water spray to knock them off.
Various pesticide sprays may be used. Most of them must touch the sawflies to be effective. Whatever your choice, follow label directions. Choices among active ingredients include azadirachtin, horticultural oils, insecticideal soap, pyrethrins, pyrethroids, and spinosad.
But if the trees are large, you may want to hire a pest control company. See "Selecting a Pest Control Company" for guidelines (http://npic.orst.edu/pest/selectpco.html)