A new review article looks at 40 years of research and development of a leading method for protecting Douglas-fir trees from infestations by Douglas-fir beetles (Dendroctonus pseudotsugae), using the pests’ antiaggregation pheromone against them—essentially putting up a “no vacancy” sign that sends the beetles elsewhere. (Photo by Lindsey Seastone, USDA APHIS PPQ, Bugwood.org)

By Darrell Ross, Ph.D.

The Douglas-fir beetle (Dendroctonus pseudotsugae) is one of the most important insect pests associated with Douglas-fir throughout western North America. Local or regional scale disturbances such as storms, fire, or defoliator outbreaks provide an abundance of newly dead or highly stressed trees—the preferred breeding site for the beetle. In such cases, D. pseudotsugae populations increase and kill large numbers of nearby healthy, live trees over several years. During these outbreaks, the beetle preferentially colonizes large, mature trees putting any remnant old-growth forests at high risk of infestation.

Darrell Ross, Ph.D.

Historically, these outbreaks were seen as a threat to timber resources, but over the past several decades the potential negative impacts to high-value trees in campgrounds, residential sites, and other special use areas has become apparent. Consequently, by the early 1990s interest increased in developing effective methods for protecting mature high-risk, high-value trees. As a new assistant professor in the College of Forestry at Oregon State University in 1990, this became a focus of my research program that has continued to the present.

A recent article I authored in The Canadian Entomologist recounts over 40 years of research and development on management of the Douglas-fir beetle, from the discovery of its pheromones to present recommendations for operational use of its antiaggregation pheromone to protect live trees. The paper is part of a special collection that resulted from a symposium held at Entomology 2018, the Joint Annual Meeting of the Entomological Society of America, Entomological Society of Canada, and the British Columbia Entomological Society in Vancouver, British Columbia, Canada.

Local or regional scale disturbances such as storms, fire, or defoliator outbreaks provide an abundance of newly dead or highly stressed Douglas-fir trees—the preferred breeding site for the Douglas-fir beetle (Dendroctonus pseudotsugae). In such outbreaks, D. pseudotsugae populations increase and kill large numbers of nearby healthy, live trees over several years. Shown here is a group of mature Douglas-fir trees killed by the Douglas-fir beetle in the Blue Mountains of northeastern Oregon, USA, during an outbreak in the early 1990s (Photo by Darrell Ross, Ph.D.)

hown here is an adult Douglas-fir beetle and its eggs in a gallery. The actual size of the adult is 4.5 to 7 millimeters (about 3/16 to 1/4 inch) long. (Photo courtesy of Oregon Department of Forestry.)

Following the identification of the first bark-beetle pheromone, from the California fivespined ips (Ips paraconfusus), in 1967, a period of rapid research identified the pheromones of other economically important bark-beetle species, including the Douglas-fir beetle. The Douglas-fir beetle pheromone system is now one of the most well-studied of any bark-beetle species.

Bark beetles produce two types of pheromones: aggregation and antiaggregation. Unlike sex pheromones, which only attract the opposite sex for mating, aggregation pheromones of bark beetles attract both sexes. Since bark beetles must kill live trees to successfully breed in them, aggregation pheromones, released by females of the Douglas-fir beetle, serve the purpose of not only attracting mates but also attracting large numbers of beetles to overwhelm host-tree defenses. Antiaggregation pheromones, which are released by male Douglas-fir beetles after mating, serve to prevent over-populating hosts and subsequent larval competition for a limited amount of food—essentially a “no vacancy” sign. Late-arriving beetles receive a signal that a colonized tree is full and continue dispersing to locate another suitable host tree.

In the 1980’s, a system was developed to use the beetles’ “no vacancy” signal against them. The method formulated and applied the Douglas-fir beetle antiaggregation pheromone, 3-methylcylcohex-2-en-1-one (MCH, for short), to trees uprooted by wind to prevent beetles from breeding in that highly suitable host material and reaching outbreak densities. Although the treatment was shown to be highly effective, the formulation was not registered for commercial sale and never used in operational treatments due to concerns about the environmental fate of the plastic beads and administrative obstacles to registration of novel pheromone formulations at the time.

The bubble capsule formulation of the Douglas-fir beetle antiaggregation pheromone, known as MCH, is attached to a tree in a treatment area, and the capsule slowly releases the pheromone over the period of adult beetle dispersal. The development of this application was the first time a bark-beetle antiaggregation pheromone was shown to be effective at preventing the successful colonization of live trees. (Photo by Darrell Ross, Ph.D.)

In the early 1990s, however, MCH was shown to be effective in preventing the colonization of high-risk, live trees. This was made possible by the development of a bubble capsule formulation of the pheromone that allowed it to be easily applied and consistently released over the period of adult beetle dispersal. This was the first time that a bark-beetle antiaggregation pheromone was shown to be effective at preventing the successful colonization of live trees.

Further research led to the development of management recommendations for protecting high-value trees and, following registration with the U.S. Environmental Protection Agency, it became available for commercial sale and operational treatment in the spring of 2000.

The MCH treatment for protecting live Douglas-fir from Douglas-fir beetle infestation is unparalleled for any other bark-beetle species in the world. The treatment is consistently effective and has been used to treat high-value trees and stands throughout western North America for the past 20 years. Despite the successful development of this treatment through empirical studies, a great deal remains that we do not know about the response of beetles to pheromones during the behavioral steps involved in host-tree location and colonization. In the future, a better understanding of bark-beetle behavioral response to pheromones could potentially lead to more effective pheromone-based management treatments for other species.

Darrell Ross, Ph.D., is a professor in the Department of Entomology at North Dakota State University in Fargo, North Dakota. Email: darrell.ross@ndsu.edu.