Sawfly larvae were feeding on your bristle cone pine, which is a very cool tree choice by the way. The larvae eat needles down to the fascicle, the sheath that hold the bundled needles together. Later in the year, the winged sawfly lays eggs (and earns its name) by sawing a hole in needles with its ovipositor and deposits its eggs. Eggs hatch in the spring and eat the inside of the needles, and as the larvae mature, they increase in size and eat the needles whole. They feed gregariously and do a lot of damage quickly to many types of pines, spruce, and some deciduous trees too — mountain ash to name one.
Insecticide treatments are very effective in combating this destructive insect. One choice is treating with a systemic insecticide in the fall to kill the larval stage next spring. Another choice is to scout in the spring/summer for their emergence and use a contact insecticide. Some claim to have had had luck spraying them off with just water from a hose.
Bristle cone pine is an underused choice of pines in our part of the world, probably due to our cold winters. They are the longest-lived tree in the world at around five thousand years or so. Since the average low temperatures are increasing in Minnesota, maybe more of these trees can make a home here?
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