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 June 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 mountain pine beetle activity. In addition, lodgepole pine forests in Park, Lake and Gunnison counties that largely escaped previous bark beetle outbreaks are now experiencing increased tree mortality as mountain pine beetle populations expand.

In response to concerns about the potential for early adult emergence, CSFS Forest Health staff are monitoring larval development and adult flight activity along the Front Range. At some sites, only a small number of pupae have been observed in early June, which is consistent with typical development timelines. At other sites, however, a higher proportion of pupae relative to larvae has been detected, suggesting that adult emergence may occur earlier than expected. Despite these observations, no adult mountain pine beetles have been captured in monitoring traps as of the first week of June. Several related bark beetle species are already active and may be mistaken for mountain pine beetles by non-experts.

As noted, the CSFS anticipates that mountain pine beetle flight could occur earlier than usual in some portions of the Front Range. While many areas are expected to experience normal flight timing, lower-elevation sites may see adult activity begin up to three weeks earlier than in a typical year; flights typically begin in early July on the Front Range, according to trapping data from prior years. At this time, there is insufficient information to provide a more detailed forecast for this year.

Residents should remain vigilant, recognize that beetle activity may occur slightly earlier than normal and plan management activities accordingly. There is currently no evidence to suggest a second adult flight period will occur or any other significant changes to the mountain pine beetle life cycle.

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.