Authors:
- Abbey Bowser, Research Associate I, Colorado State Forest Service
- Dan West, Ph.D., Forest Entomologist, Colorado State Forest Service
- Zach Smith, Research Associate II, Colorado State Forest Service
Editors:
- David Rubin, Research Associate I, Colorado State Forest Service
- Amanda West Fordham, Ph.D., Associate Director, Science and Data Division, Colorado State Forest Service
Key takeaways
Semiochemicals can be used as part of an integrated pest management (IPM) strategy for bark beetles. The Colorado State Forest Service (CSFS) began deploying semiochemicals in 2012 to manage bark beetles in Colorado’s forests.
A combination of a literature review and CSFS field observations indicates the following:
- Semiochemical efficacy is influenced by beetle population pressure, stand structure, host condition and spatial arrangement.
- Semiochemicals are most effective when used early and strategically.
- Performance declines with increasing beetle populations.
- Push–pull and combination treatments outperform standalone approaches.
- Synergistic compounds can significantly enhance outcomes.
- Semiochemicals are best used as part of a broader IMP strategy, especially for high-value areas or localized protection.
Table of Contents
Introduction
In 1966, a short Science publication described an experiment involving aluminum foil, 128 Ips beetles and three newly synthesized compounds isolated from beetle frass. The results showed that when the beetles arrived at the source of these compounds, “they take part in boring, feeding, mating and oviposition” (Silverstein et al., 1966). This publication was one of the first to recognize how pheromones affect bark beetle (Coleoptera: Curculionidae: Scolytinae) behavior. Since then, a range of pheromones associated with these insects have been described. Many of these compounds can also be found in the fragrances of flowers, secretions from microorganisms and even in male Asian elephants (Francke et al., 1995), but a few pheromones are uniquely bark-beetle in chemistry and highly species specific.
The utility of these more characteristic pheromones for bark beetle management has been explored by chemical ecologists, entomologists and forestry professionals. In the United States, it is estimated that bark beetles affect approximately 50 million forested acres annually (Dale et al., 2001), altering not only the forest composition and biodiversity, but creating lasting effects on soil acidity, snowmelt, river ecosystems, property values, carbon budgets, timber supply and potential community conflict (Grégoire et al., 2015).
Colorado’s forests have been impacted by several of these periodic outbreaks over the last few decades. In the early 2000s, mountain pine beetle killed 47% of the lodgepole pine in Rocky Mountain National Park, reducing basal area by 71% and average tree diameter by over 6 inches (Nelson et al., 2014). Pine forests experienced 3.4 million acres of mortality from 1996-2013, representing approximately 82% of the pine resources in Colorado (Negrón & Cain, 2018). Gray Engelmann spruce snags blanket Wolf Creek Pass, and each year, more Douglas-fir trees fade across the Gunnison Valley, where Douglas-fir dominates much of the landscape. Most recently, attention has turned to the faded red ponderosa pines along the high-traffic corridors of Interstate 70 and U.S. Highway 285 along the Front Range (Figure 1). In the past decade, nearly 2 million acres of Colorado’s forests have been damaged by spruce beetle (Dendroctonus rufipennis), Douglas-fir beetle (D. pseudotsugae) and mountain pine beetle (D. ponderosae), according to data from the annual USDA Forest Service Aerial Detection Survey program.
This destruction seemingly violates the ecological rules of optimal foraging behavior, as well as millions of years of coevolution between bark beetles and their tree hosts – so why are trees dying? Under normal environmental conditions, bark beetles provide important ecosystem services, including forest sanitation, canopy opening and nutrient cycling. Outbreaks are symptomatic of a general decline in forest health due to variable precipitation and temperature regimes that weaken tree defenses, leaving them susceptible to beetle attack (Negrón & Cain, 2018).
Changing forest conditions will continue to favor bark beetle outbreaks across Colorado. Traditional management options include silviculture techniques, which alter forest composition and age class through mechanical treatments, and direct control measures, which include spraying preventive chemical insecticides on the tree’s bole, usually to reduce current infestations. There has been growing interest among land managers and private landowners in preventive management strategies that minimize reliance on insecticides while reducing the economic, logistical and operational constraints associated with mechanical treatments. Semiochemical packets offer an effective and relatively low-cost approach that can be implemented efficiently in field settings with minimal equipment.
What is a semiochemical?
Semiochemicals, also called behavioral or signaling chemicals, are compounds used by plants and animals for communication. Pheromones are semiochemicals used specifically for communication between individuals of the same species.
Bark beetles are excellent communicators – their characteristic clubbed antennae are equipped with specialized cells that are highly sensitive to these signaling chemicals (Figure 2). Bark beetle pheromone scents can be categorized as either aggregates or anti-aggregates. Simply put, aggregation pheromones signal the presence of suitable habitat, food resources and potential mates. When a beetle attacks a susceptible tree, the tree’s monoterpenes are oxidized within the beetle’s digestive system and released as an aggregation pheromone signaling to other beetles of the same species that the tree is a suitable host and its defenses are weakened. The tree is subsequently mass attacked following this chemical communication that goes out to any nearby bark beetles seeking a host tree. However, this tree has limited resources available as the attack progresses, turgor pressure decreases and resin reserves are exhausted. As incoming beetles continue to infest the tree, they simultaneously produce anti-aggregation pheromones to signal that the tree has reached maximum capacity and incoming beetles should find a new host tree to avoid resource competition (Safranyik & Carroll, 2006).
The chemicals composing aggregation and anti-aggregation pheromones have been identified and synthesized in a laboratory setting for Douglas-fir beetle, spruce beetle and mountain pine beetle, amongst others, and have become increasingly important tools to monitor and reduce bark beetle populations in Colorado.
The release devices and application methods of semiochemicals have evolved with time and research: plastic beads dropped from airplanes, flakes sprinkled on the ground by fertilizer spreaders or dropped from the sky via aircraft, or a wax-based matrix applied to tree bark with a caulking gun. These methods have all been tested on expansive geographic scales across different forest types (Ross, 2021). Ultimately, these experiments yielded mixed results in preventing beetle attacks and the inconvenient manufacturing processes, inefficient and ineffective elution rates, and residual plastic waste made such methods a challenge for widespread commercial use. In 1989, a semi-permeable polyethylene film was created. This design slowly releases the product into the environment, creating a treatment “plume” across the forest (Ross, 2021). This release device is the application method primarily used by the Colorado State Forest Service (CSFS) today (Figure 3).
Unlike traditional pesticide treatments that generally target the nervous and reproductive systems of insects, semiochemicals work by affecting beetle behavior, making them an environmentally safer alternative for pest management. In 1979, the EPA recognized that their natural derivation and low toxicity make semiochemical treatments inherently safer than traditional pesticides, classifying them as “biochemical pesticides” (Ross, 2021). Biochemical pesticides are generally encompassed within the EPA’s biopesticide classification, which includes biochemical pesticides (e.g., pheromones), microbial agents and plant-incorporated protectants. The term “biopesticide” is commonly used to describe reduced-risk pest management products that are either naturally derived or structurally similar to naturally occurring compounds and exhibit selective modes of action.
Semiochemical treatments
Anti-aggregates: MCH and Verbenone
MCH (3-methyl-2-cyclo-hexen-1-one) is the anti-aggregation pheromone used to prevent attack from Douglas-fir and spruce beetles. The compound was first isolated in 1971 from female beetles (Kinzer & Fentimen, 1971) and has been in commercial use since 2000. Early field tests applied to fallen Douglas-fir trees resulted in over 90% reduction in beetle colonization, and protecting stands of live Douglas-fir from infestations with MCH has proven consistently effective at all beetle population levels (Ross & Sullivan, 2020). The application and overall success of MCH set a precedent for the development of potential pheromone-based treatments for managing other bark beetle species.
Verbenone (vur-BEN-ohn; trimethyl-bicyclo-heptenone) is an anti-aggregation pheromone produced by the mountain pine beetle and can be applied to all varieties of pine trees. Although the compound was isolated from mountain pine beetle in 1968, its role in the species’ communication system was not described until 1983 (Ryker & Yandell, 1983). Several major field tests using verbenone were conducted in the late 1980s across Canada, Idaho and Colorado (Amman et al., 1988; Lindgren et al., 1989; Bentz et al., 1989), but the reported efficacy of this treatment has been inconsistent among years, locations, host species, outbreak status, temperature, etc., leading to debate within the forestry community about the utility of this treatment.
Although it is difficult to generalize about verbenone efficacy as stand and environmental conditions vary from site to site, an overall pattern in the literature has emerged: When used at low levels on an individual-tree basis and in the initial stages of infestations, verbenone can be an effective tool for tree protection. When beetle populations are high, pheromone effectiveness can wane, likely because the chemical signals from the packets are overwhelmed by the pheromones being produced by the beetles in the environment.
Research conducted in Utah and the broader Intermountain West, including work by Barbara J. Bentz and collaborators, demonstrates that verbenone can reduce attack rates of mountain pine beetle in susceptible pine stands. Field studies show that verbenone can lower tree mortality at both individual-tree and small-stand scales, although effectiveness varies with site conditions and year. Across studies, treatments are most effective under low to moderate beetle pressure and in high-value or targeted areas, while efficacy declines during outbreak conditions as mass attack behavior can overwhelm anti-aggregation signals (Bentz et al., 1989; Bentz et al., 2005). These findings support the use of verbenone as a preventive, behavior-based tool within an integrated pest management approach rather than as a stand-alone solution during large-scale outbreaks.
Semiochemical treatments are most effective when applied during early stages of infestation, often interpreted operationally as less than approximately 5-10% of trees infested (commonly cited near approximately 8%). Although no universal threshold has been defined in the literature, efficacy declines as infestation intensity increases, with reduced effectiveness commonly observed beyond approximately 15-20% of trees infested (Gibson, 2009).
The Forest Health Team at the CSFS utilizes verbenone treatments to protect high-value trees as part of an integrated pest management strategy. Depending on the level of community engagement, field surveys to evaluate the efficacy of the treatment often occur following the verbenone application and the mountain pine beetle adult flight period. These surveys are conducted under applied, field-based conditions and are not intended as controlled experimental trials.
For the past three years, the CSFS has collaborated with a property owners association to reduce mountain pine beetle-caused mortality in the ponderosa pine stands comprising the approximately 800-acre community. Adjacent ponderosa pine forests experienced elevated mountain pine beetle activity in 2023, which prompted the initiation of verbenone treatments within the community in 2024 and 2025. Additionally, trapping was conducted adjacent to the POA to monitor beetle presence and flight activity, and the community was surveyed each fall for new beetle activity.
Parcels varied in size but averaged approximately 2.5 acres. Due to forest density and average tree diameter, verbenone was heavily weighted toward using 5.8 gram (85% of total application product) and 0.98 gram (15% of total application product) plastic membrane formulations. Treatments were implemented by both property owners and CSFS staff. While the precise timing and prescription of landowner-applied treatments could not be verified, applications were generally consistent with recommended deployment windows based on available guidance. In 2025, treatment coverage within the POA totaled approximately 90 acres, with an additional 660 acres remaining untreated.
From 2023-2025, a significant reduction in new mountain pine beetle infestations was recorded (Figure 4). A total of 407 infested trees were flagged in the fall of 2023, compared to only 77 active infestations in 2025, indicating that across all areas, treated or untreated, the use of semiochemicals and direct control of infested trees reduced the impact to the POA.
Of the new infestations that occurred in 2025, six were documented on treated parcels. In comparison, surveys of untreated areas identified 71 infested trees. Tree density, size class and landowner participation varied across the POA, which may influence treatment outcomes and comparisons between treated and untreated areas.
A reduction in the severity of individual infestations was also recorded between treatment years. In 2025, the largest cluster of infested trees reported was five (compared to 26 trees in 2024), with the average infestation point affecting 1.3 trees (compared to 2.5 trees in 2024). Notably, infestations near treated parcels occurred predominantly along or near the treatment boundaries, suggesting that the treatment was effective at keeping beetles out of the treated parcels (Figure 5).
Aggregation pheromones
Attractant lures have been developed for various bark beetle species for population management and monitoring. Research into the chemical ecology of bark beetles has demonstrated that while high concentrations of MCH and verbenone deter beetles, low concentrations of these compounds act as attractants (Rudinsky et al., 1974). Lures are generally composed of a combination of aggregate pheromones, low formulations of anti-aggregates and host volatiles.
Attractants are frequently deployed in combination with Lindgren funnel traps. This trap design was developed by B.S. Lindgren in the 1980s for collecting bark and ambrosia beetles (Lindgren, 1983). The ingenuity of his design was inspired by a self-proclaimed “aversion to sticky material” and utilizes a series of collapsible black funnels to mimic the silhouette of a tree. Traps are easily deployed and beetles are collected in a cup attached below a series of funnels (Figure 6). Funnel traps are now regularly used by land managers and government agencies across North America to monitor populations of bark beetles in forests, wood processing facilities and ports of entry. In 2025, the CSFS deployed 96 funnel traps across the state, targeting six different species to record emergence timing and to inform decision-making regarding population management. The 2025 trapping effort processed approximately 395,000 target specimens collected as part of this trapping effort.
Anti-aggregation and aggregation pheromones can be used in tandem in a “push-pull” management practice: MCH or verbenone is applied within forested stands to disrupt host-finding and reduce local bark beetle attack pressure, redistributing beetle activity and pressure away from high-value stands. Concurrently, traps baited with species-specific aggregation lures can be deployed in non-host areas (e.g., meadows or stands dominated by non-host tree species, such as aspen) to attract and concentrate beetles away from susceptible host trees. Field studies with mountain pine beetle and related Dendroctonus spp. demonstrate that push-pull treatments generally outperform push-only approaches by improving spatial control of beetle activity and reducing attack rates within treated areas (Borden et al., 2006; Gillette et al., 2012). Reported outcomes vary, but reductions in beetle activity and tree attack have been substantial under favorable conditions, with some syntheses indicating decreases approaching approximately 70% in Dendroctonus populations (Afzal et al., 2023).
The effectiveness of push–pull systems is strongly influenced by stand conditions, beetle population pressure and treatment design. Semiochemical treatments are most effective at low to moderate infestation levels and at relatively small spatial scales, where repellent “push” signals and attractant “pull” traps can be properly spaced and maintained. Under high-outbreak conditions, mass attack behavior can overwhelm semiochemical signals, reducing their efficacy. Additionally, improper trap placement may inadvertently concentrate beetles near susceptible hosts, resulting in unwanted spillover effects. Therefore, push-pull strategies are typically recommended as part of an integrated pest management approach rather than as a standalone solution (Borden et al., 2006; Gillette et al., 2012; Afzal et al., 2023).
Synergistic compounds
Although most of a bark beetle’s life occurs in the inner layer of tree bark, the defining moment in each beetle’s life is dispersal. Most bark beetle species are associated with one or a few specific tree species (hosts), thereby making host selection critical to their reproductive success. To reduce predation risk, lessen exposure to inclement weather and reserve energy, bark beetles have evolved the ability to recognize volatile compounds associated with both tree hosts and non-hosts. When exposed to the scents of an undesirable tree species, dispersal is disrupted and the beetles are more likely to keep moving in search of a suitable host. Non-host volatiles (NHVs) interfere with host location by masking or altering the olfactory cues emitted by suitable host trees and are often used in combination with pheromone-based treatments to enhance efficacy (Seybold et al., 2018).
Laboratory studies indicate that many bark beetle species elicit a reaction to compounds associated with a wide range of non-host species, including cottonwood, aspen, birch and maple (Huber et al., 2000). Two specific NHVs deployed by the CSFS in recent years are Acer Kairomone blend (AKB) and Green Leaf Volatiles (GLV).
AKB is derived from maple tree volatiles and has been synthesized as a semiochemical repellent to be deployed alongside MCH, especially in spruce forests, where MCH has proven less effective at providing beetle protection.
GLVs are commonly emitted by green plants when they are under stress – the distinctive scent of these compounds is familiar to anyone who has walked across a freshly mowed lawn. Studies have shown that GLVs disrupt mountain pine beetle attraction and may direct beetles away from host trees (Wilson et al., 1996).
The CSFS employs species-specific semiochemical treatments to improve management efficacy across bark beetle systems. MCH-AKB was used to target spruce beetle avoidance in high-elevation Engelmann spruce (Picea engelmannii) forests, whereas verbenone in combination with GLVs was applied in mixed-conifer pine systems to reduce mountain pine beetle pressure. Applications have been concentrated in central Colorado, where spruce beetle outbreak severity peaked in 2022, resulting in estimated stand-level mortality of 14-22% (Colorado State Forest Service; unpublished data, West).
GLVs in combination with verbenone have been deployed on a private parcel in Park County since 2017 to mitigate impacts from mountain pine beetle in high-value limber pine (Pinus flexilis) and bristlecone pine (Pinus aristata) stands. Prior to treatment, consistent annual MPB-caused mortality occurred with several trees lost each year across approximately 76 acres. Following treatment implementation, only a single limber pine has been successfully attacked and killed over a nine-year period. Beetle pressure has remained substantial, as push-pull trapping efforts documented 1,037 mountain pine beetles captured within the treatment area in 2025, suggesting treatments have reduced successful attacks despite sustained beetle activity.
CSFS semiochemical program
The CSFS has utilized semiochemical treatments as part of an integrated pest management strategy since 2012. The CSFS coordinates management efforts regarding the use of MCH and verbenone with local partners, communities and private landowners.
In 2025, a total of 65,291 pheromone packets were distributed across the state with a monetary value of approximately $320,000 (Figure 7). In that same year, 18,880 forested acres in 27 Colorado counties were treated with this product in coordination with the CSFS Forest Health Team. The highest number of semiochemical packets were deployed in Gunnison, Jefferson and Chaffee counties due to high activity by Douglas-fir and mountain pine beetles (Figure 8).
From 2024-2025, the number of participating landowners quadrupled, a direct outcome of increased mountain pine beetle activity in Colorado. Every year, trained aerial observers from state and national agencies, including the CSFS, USDA Forest Service and U.S. Fish and Wildlife Service, fly in small aircraft to record the forest pest activity across the 24 million forested acres in Colorado. Beginning in 2022, Aerial Detection Survey data recorded increased mortality from mountain pine beetle in the ponderosa pine stands along the Front Range (see map). Most of this new activity has been documented in ponderosa pine forests under private ownership (46%) where many Coloradans live, with the remaining susceptible forest type occurring within federal (43%) and state and local (10%) ownership. These conditions are ideal for the implementation of pheromone-based treatments to protect large, high-value trees. In 2022, the CSFS distributed 220 verbenone packets to a single landowner. By 2025, 41,550 packets were distributed by CSFS staff to landowners, homeowners associations and state partners across Colorado (Figure 9). Projected values indicate that by 2027, 92,000 total packets (approximately 70,000 verbenone pouches and bubbles) would be distributed by the state’s semiochemical program, an estimated monetary value just under $450,000.
Conclusion
The ecology of forested ecosystems is complex and often unpredictable. Tree health and bark beetle survival are directly affected by abiotic factors, including temperature and precipitation. These variables influence bark beetle behavior, development and emergence (Negrón & Cain, 2018). How these beetles interact with the forest – and one another – is further determined by a myriad of chemical signals in the environment. Studying these ecological interactions has given us valuable insight and the ability to exploit these communication systems for bark beetle management. The use of MCH and verbenone packets has successfully prevented living trees from infestation, and we advocate for the use of these tools when conditions are appropriate under small scales and when bark beetle populations are low.
Multiple factors need to be considered when evaluating the viability of semiochemical-based management strategies: local conditions, beetle populations, stand type and density, the scale at which the treatment may be warranted and forest management goals, among others. More research is needed to understand why semiochemical treatments fail in certain scenarios, particularly those involving verbenone. A greater understanding of forest ecology enables the development of additional semiochemical tools.
Colorado recently experienced one of the warmest winters on record from 2025-2026, and at the time of publication of this Science & Data Byte, snowpack is at 19% of the median across all river basins. The dearth of precipitation combined with elevated temperatures in early 2026 will unequivocally exacerbate the risk of disturbance and the need for forest management. Semiochemicals are only one piece of the management suite but should not be overlooked when considering a treatment path to reduce mortality.
In Colorado, drought conditions in early 2026 will continue to favor bark beetle development, and semiochemicals will continue to be increasingly important tools to monitor and control bark beetle populations.
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