Some populations of mountain pine beetles now produce two generations of tree-killing offspring annually, dramatically increasing the potential for bugs to kill lodgepole and ponderosa pine trees, CU-Boulder researchers have found. Because of the extra annual generation of beetles, there could be up to 60 times as many beetles attacking trees in any given year, the study found. And in response to warmer temperatures at high elevations, pine beetles also are better able to survive and attack trees that haven't previously developed defenses.