Beetles are devastating forests across the continent from Colorado to the Yukon, killing millions of acres of trees. In most places, the infestation is spurred by trees weakened by drought, and beetles that thrive in warmer weather. The dead trees increase the risk of forest fires, exacerbating the problem.
Bark beetles have long been present in the reserve monarch reserve, usually attacking a few trees in the driest months of early spring, before heavy seasonal rains that normally start in May.
But this year, little rain had fallen by July, and the trees were weakened. The beetles took advantage, burrowing in and robbing the trees of nutrients until they turned orange and die.
The infestation so far has affected 100 of the 33,482 acres in the reserve's core mountaintop wintering grounds.
But experts are concerned because the outbreak is occurring in patches, indicating that the infestation is spreading.
And a Mexican government report on climate change predicts more late or delayed summer rains, with a 15 percent decline in overall rainfall between now and 2080.
If the bark beetle attacks become a regular occurrence and more trees are felled, monarch butterfly expert Lincoln Brower worries there could be more "holes in the blanket" of the tree canopy that protects the butterflies.
Diana Six, a professor of forest entomology and pathology at the University of Montana, says the best way to protect trees is to spray their bases with the pesticide carbaryl (Sevin), but "you can't use it if you've got monarchs coming in, because it's a general pesticide; it kills everything as far as insects."