Protecting your trees from Mountain Pine Beetle – what works, what doesn’t.

With mountain pine beetle populations on the rise across Colorado and locally in Gilpin County, many homeowners would like to protect their trees from infestation and death. Several products claim to protect trees from mountain pine beetle infestations, but some are backed by more science and field-testing than others.

Pheromones – powerful scents that insects use to communicate with one another – are currently a hot topic. Specific to mountain pine beetles, the antiaggregation pheromone verbenone signals to other beetles that a particular tree is “occupied.” Research has shown that human application of verbenone is effective if pine beetle populations are low. However, the pheromones don’t kill the beetle, they merely divert them to a nearby untreated tree or property where the populations will continue to build. When populations reach epidemic levels, the synthetic material is not able to mask the communication system of the beetle and it loses efficacy. Additionally, research has shown mixed results on the use of these pheromones in a natural forest setting.

The majority of mountain pine beetles fly within 1/4 mile from the source, but high winds and increased populations in Gilpin County widen their reach. Therefore, if there are infected or dead trees near your property, verbenone may be an effective deterrent. It is not a “silver bullet”, but it may temporarily help in an area where the beetle populations are low, and should be used in conjunction with an aggressive plan of searching out and treating infested trees.

Verbenone is commercially available and registered in Colorado. Landowners should have their property assessed by a professional forester before deciding to pursue this option. Should you choose to treat your trees with pheromones, keep in mind the following:

• Target only limber pines, lodgepole pines, ponderosa pines, and bristlecone pines. Mountain pine beetles do not attack other species.
• Focus on large old pine trees, as they are preferred by the mountain pine beetle. For larger acreages and/or to reduce cost, focus on large trees close to homes and major viewsheds.
• Application is most effective in early June, before the beetles fly from one tree to the next.
• Attach pheromone pouches/bubbles on north-facing aspects of the trees (if using two pouches/bubbles, attach on north- and south-facing aspects).
• Staple pouches/bubbles with a staple gun as high on the trunk as you can reach.
• Replace pouches/bubbles yearly; verbenone is a 90 day repellant.

Insecticide treatments are also an option for high value trees. Certain formulations of carbaryl (Sevin and others), permethrin (Astro, Dragnet and others), and bifenthrin (Onyx) are registered for use in Colorado to prevent attacks on individual trees. These sprays are applied to living green trees in early summer to kill or deter attacking beetles. This preventive spray is generally quite effective through one MPB flight (one year). Note that pesticide treatments will also kill other non-target species.

Further Reading

• Efficacy of verbenone and a blend of verbenone and nonhost volatiles for protecting lodgepole pine from mountain pine beetle (Coleoptera: Curculionidae) – Agricultural and Forest Entomology (2020), DOI: 10.1111/afe.12392