National Geographic : 2015 Apr
Death by a Thousand Bites For centuries the relationship was mutually beneficial: Pine beetles culled older, weaker trees, producing new beetles but also a healthier forest. Climate change, with its warmer, drier conditions, has upset that balance, leaving even healthy trees vulnerable to attack. FIRST WEEK Selection and Invasion The cycle begins in summer, when a lone female beetle bores into a tree’s bark and releases a pheromone that attracts hundreds of other beetles. SECOND WEEK Burrowing and Egg Laying Beetles dig galleries under the bark, depositing eggs and blue fungi to feed the next generation. The galleries block nutrient flow in the tree’s phloem layer. THIRD WEEK TO 4 MONTHS Hatching and Feeding Larvae hatch and chew side galleries, feeding on the phloem and the fungi. 5 TO 12 MONTHS Overwintering and Dispersal The beetle larvae lie dormant until spring, when they’ll turn into pupae, then adults. The new brood feeds on fungal spores before dispersing to another tree. 13 TO 24 MONTHS Red Means Dead The beetles are long gone, and the drying tree turns red. Finally it loses most of its needles and becomes gray. Needles turn yellow in the dry heat of summer. The tree remains green for months after beetles have fatally mauled it. Pupal stage Fungi-carrying new adult JOHN TOMANIO, NGM STAFF; SHELLEY SPERRY ART: SAMANTHA WELKER SOURCE: DIANA SIX, UNIVERSITY OF MONTANA Sixty to eighty eggs are laid in each gallery. Phloem layer The larvae develop cold resistance in time for winter. PINE BEETLE, ACTUAL SIZE The tree tries to suffocate the insects by secreting resin into the beetles’ boreholes.