Question
Discovered something was munching my bristlecone pine tree in yard -- looked more like deer were nibbling, but futher investigation, found bright green worms, some small and maturing to 3/4" length with legs (6?) toward the front of worm rather than on full length of worm.  NOTE: It's a small tree (only about 5' high)
LOCATION: Old Snowmass, CO; 8500' elev; natural vegitation is oak brush and sage, some small aspen on no aspects.  I planted this tree near my house.
* WHat kind of worm is it?
* How do I control or get rid of the tree of worm (safest product, natural products or concoctions). Don't want to harm pets, wildlife.  

Answer
These are called sawflies. Despite the name "sawfly", sawflies are not a type of fly. Rather, sawflies are taxonomically related to ants, bees and wasps. Newly emerged larvae immediately begin their feeding activities on "old needle growth". Larvae are gregarious, feeding in tightly clustered groups.
There several different sawflies the one you describe sounds similar to the European Pine Sawfly. But it does not matter which one they all do the same damage and are controlled the same way.

pine sawfly can be controlled with a number of products that contain "natural" insecticidal materials. Azadirachtin, a bioinsecticide derived from neem trees, works as an insect growth regulator. Horticultural oils and horticultural soaps effectively control the soft-bodied sawfly larvae. But with oils and soaps, what you spray is what you get. That is, once dried, oils and soaps do not provide residual control. So if initial spray treatment coverage was not thorough, an additional treatment might be necessary to "clean up" larvae that were untouched and have continued feeding. The rotenone and pyrethrin plant derivatives have very brief residual properties.

Insecticides with the active ingredients acephate, malathion, carbaryl, esfenvalerate, rotenone and pyrethrin have longer residual properties, providing some extended control against European pine sawfly larvae.

Really all that has to be done is get the spray on the insect bodies and it is usually killed. If you want to stay closer to a "natural" insecticide they will work as well as the chemical insecticides for sawflies. insecticide