It takes some serious cold to kill the larvae. I can’t recall the specific numbers but IIRC -10F for 1-2 weeks solid to chill the trunks enough to freeze the little rascals dead. Snowfall doesn’t help a bit. In fact, high snowpack acts as an insulating blanket.
You can see in infected wood a purple stain. Every pine tree I cut down in Roosevelt National Forest in the late 70s (firewood) was stained purple. The need to develop GM pine trees.