About Mountain Pine Beetles

Mountain pine beetles complete a generation within one year from egg to adult. Adults typically fly to seek new host trees in July through August, though a small proportion of adults emerge earlier and some later through the summer months. Attacking females initially prefer large-diameter trees until they are depleted from the forest. However, they may attack smaller-diameter trees during an outbreak as populations explode and fewer host trees are available. 

Mountain pine beetles predominantly infest ponderosa pine (Pinus ponderosa), lodgepole pine (P. contorta) and limber pine (P. flexilis). However, numerous species of pines – and all pines found in Colorado except for piñon pine – are susceptible to attack when mountain pine beetle populations explode, including ornamental pines. 

Outbreaks in Colorado 

The Colorado State Forest Service monitors the spread of mountain pine beetle outbreaks in Colorado through aerial surveys and on-the-ground reports. 

Updated April 2026: Ponderosa pine forests in portions of Larimer, Gilpin, Clear Creek, Boulder, Jefferson, Park, Douglas, El Paso and Teller counties are experiencing elevated levels of infestation from mountain pine beetles. In addition, lodgepole pine forests in Park and Gunnison counties that evaded prior bark beetle outbreaks are experiencing elevated mortality of trees as populations of mountain pine beetles expand.

Given concerns about early emergence of adult mountain pine beetles, the CSFS Forest Health staff has set up several pheromone traps for mountain pine beetles on the Front Range and is monitoring larval development. They have not yet seen any pupae under the bark and have not caught any adults in their traps as of early April. The CSFS is not expecting mountain pine beetle adult emergence to occur earlier than normal or differ significantly from last year. 

Explore where aerial observers detected mountain pine beetle activity in Colorado’s forests over the past four years. This map shows data from an annual aerial survey conducted by the Colorado State Forest Service and U.S. Forest Service. The pink area depicts acreage surveyed from the air in 2025.