A pioneering female bores through the bark and releases an aggregation pheromone. Arriving beetles add their own signals, and the tree is attacked by thousands of individuals at once, faster than it can push resin into every entry point. Once the tree is fully occupied, the beetles switch to an anti-aggregation pheromone that redirects later arrivals to neighbouring trees, spreading the attack outwards.
The beetles also carry blue-stain fungi in specialised structures. The fungi grow into the sapwood, disrupt water transport, and help kill the tree, while providing nutrition for developing larvae. Tree and attacker are therefore in a race between resin flow and fungal colonisation.
Outbreak severity has been linked to winter cold and to stand condition. Larvae are killed by sustained deep cold, and mild winters over recent decades removed much of that check across parts of the range, while extensive even-aged mature lodgepole pine provided abundant susceptible hosts. British Columbia's outbreak from the late 1990s affected an area measured in tens of millions of hectares.