MOUNTAIN PINE BEETLE INFESTATION 2026

The Mountain pine beetle (Dendroctonus ponderosae) is back in Jefferson County. This little bug, this native to the western North American pine forests, is responsible for the periodic loss of millions of pine trees – most normally Ponderosa, Lodgepole, Scotch and Limber pines.

Mountain pine beetles (MPB) are always with us, having evolved with the western pine forests. Under ideal conditions, their attacks are low-level, limited to trees that have been stressed due to injuries, drought, over-crowding, fire damage, root disease or old age. However, every 10-30 years, whether due to beetle population explosions, enhanced drought, or warmer temperatures (or all three), major outbreaks occur.  The last major MPB outbreak in Jeffco occurred from 1996 to 2004. Since 2023 there have been signs of a new resurgence of MPB in our area.

MPBs and their larvae live underneath pine tree bark. The adult MPBs tunnel under the bark, leaving behind “boring dust” in crevices and on the ground at the base of the tree. Once inside, they tunnel, creating galleries where they lay their eggs within a few days of taking occupancy. The larvae, when they hatch, join the adults, tunneling under the bark, disrupting the layer that supports the tree’s vascular system, until the tree can no longer move water from its roots into the upper story, and it dies. In doing so, the needles turn rust-colored, hence the old saying “If it’s red, it’s dead”.  The tree will try to mount a resistance to the MPB infestation, producing popcorn-shaped bits of resin, to push the MPBs out, but due to the recent drought, the tree may not have enough extra moisture to create excess resin. Woodpeckers may strip portions of bark from the trees as they search for the larvae.

Timing is critical when attempting to control MPBs. MPBs emerge from their winter homes under the bark in spring, fly to a new tree, bore in and lay their eggs. The eggs hatch, and the beetles and larvae tunnel under the bark through the warmer weather, while the larvae develop into new adults. During the cold season they over-winter under the bark, going dormant, surviving cold temperatures by “supercooling” themselves.

Deep cold during the fall, winter and early spring can kill MPBs before they emerge, but given our persistent droughts and warmer temperatures, where there used to be a single MPB generation each year, currently there are two, even at elevations above 10,000 feet. The first generation can take flight in early May, while the second generation (if there is one) will take flight in August.

How far will an MPB fly when it emerges from its winter hibernation? According to the South Dakota Department of Agriculture, most of the time beetles fly short distances, often less than 300 feet, but during an invasion research has indicated that the MPBs can fly up to 33 miles, searching for new hosts.

here are multiple ways to manage MPB infestations. If you have a few high-value pine trees that you want to protect, then pheromone packets may be your go-to solution. Advice from Alicia Doran, Jeffco’s Invasive Species Coordinator: “[T]he anti-aggregate pheromone is called Verbenone. It is usually recommended that it be put out by mid-June, before the beetles start to fly. The scientific literature has shown mixed results, largely due to varying environmental conditions. The State Entomologist is suggesting that people try them.”

In laymen’s terms, the flying beetles communicate by smell. After emerging in the spring, they fly to a live tree, their new host. If the live tree is protected by a Verbenone packet, the smell produced by the packet indicates that this tree has no vacancy, it’s already inhabited, and the beetle moves on.

In early 2025, the Colorado State Forest Service bulk-ordered these packets and offered them to the public at a reduced price. A recent conversation with our CSFS Golden Field Office Forester indicated that “Previously, the Colorado State Forest Service has placed a bulk order for pheromone packets (MCH and Verbenone) and shipped them out to landowners at a reduced cost. Unfortunately, due to increased demand and staffing shortages, our program is no longer able to ship products out for you to hang them. MCH and Verbenone packets will only be available at the CSFS pricing for those landowners & communities that would like our staff to hang the packets in the following counties: Boulder, Clear Creek, Douglas, El Paso, Gilpin, Jefferson, Larimer, and Park.” For pricing information, please contact the CSFS Forest Health Team, CS***************@*******te.edu.

Individuals may be able to order the packets themselves from retail distributors or arrange for commercial foresters to apply the packets or the equivalent pheromone sprays to their trees. The packets/sprays, coupled with the CSFS FAQ brochure[1], should help individual landowners protect their pine trees.

For larger swathes of pine, where individual packet protection is not practical, there are alternative methods of managing MPB infestations, such as thinning crowded stands of pine trees, physically removing infested trees and slash, solar treatments, and preventative insecticide sprays. This author advises against insecticide sprays, since they can seriously impact the lives and breeding cycle of our wild birds, which are already under enormous stress from bird flu.

For more information go to https://www.jeffco.us/4561/MPB—Treatment

‘Tis a pleasure to live in the mountains, but with that pleasure comes significant responsibility. As a landowner, what must you do about MPBs? Jeffco has designated the MPB as a regulated pest, aka akin to a List A invasive species. Jeffco landowners must inspect their forests and control MPBs wherever they are found. If you find logs that contain live MPB, the logs cannot be moved until they are debarked or solar treated. Advice from Jeffco’s IS Coordinator: “We recommend that owners inspect their property and remove/treat their infested trees during the fall (after Oct 1) through early June.”

For more information on the life cycle and management of the Mountain Pine Beetle:

https://gilpin.extension.colostate.edu/wp-content/uploads/sites/29/2025/05/VerbenoneFAQ2025-CSFS.pdf

https://csfs.colostate.edu/wp-content/uploads/2024/01/Mountain-Pine-Beetle-Fact-Sheet-5.528.pdf

https://csfs.colostate.edu/forest-management/common-forest-insects-diseases/mountain-pine-beetle/

https://www.colorado.edu/mrs/current-events/mountain-pine-beetle-attacks

https://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/formain15814/$file/MPBColdTemperaturesFacts-Jan2010.pdf?OpenElement

Miss Mountain Manners says “For the love of the land, please inspect your trees, even if you don’t live in the mountains or in the rural parts of the County. MPBs don’t recognize city boundaries. When there’s an infestation, they’ll attack almost any conifer.”