Somewhere in a forest in the mountain West, a female beetle no bigger than a grain of rice releases pheromones, hormone-like chemicals that attract other beetles to mass-attack ponderosa pine trees. If successful, they start chewing their way into the bark and lay their eggs. These beetles commonly carry spores of a blue-staining fungus that will also infect trees and could disrupt their water transportation systems.

If it’s been a rainy, snowy year and some other calamity hasn’t stressed the tree, it fights off the attack by pushing resin out through the beetle-chewed hole, blocking more beetles from entering and preventing the fungus from girdling the tree and cutting off water and nutrients. A subsequent cold winter would kill a large number of beetles, putting the beetle invasion on ice.

In drought, it’s a different story. Stressed by the lack of water and unable to push resin out against the invaders, ponderosas struggle to mount a defense. More and more female beetles lay their eggs in the grooves of the bark, where the larvae feed on the soft, tasty inner bark and the phloem, the veiny material that passes sugars down from the leaves.

Zachary Robbins is a researcher at Los Alamos National Laboratory, graduate student at North Carolina State University and lead author of the recent paper “Warming increased bark beetle-induced tree mortality by 30% during an extreme drought in California.” Chonggang Xu, a senior scientist at Los Alamos, simulates forest-vegetation dynamics in his research and is a coauthor of the paper.

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