Biological Aspects of Mountain Pine Beetle in Lodgepole Pine Stands of Different Densities in Colorado, USA

Abstract

Research Highlights: The biology of mountain pine beetle (MPB), Dendroctonus ponderosae Hopkins, in Colorado’s lodgepole pine forests exhibits similarities and differences to other parts of its range. Brood emergence was not influenced by stand density nor related to tree diameter. The probability of individual tree attack is influenced by stocking and tree size. Findings have implications for understanding MPB as a disturbance agent and for developing management strategies. Background and Objectives: MPB causes extensive tree mortality of lodgepole pine, Pinus contorta Douglas ex Loudon, across the western US and Canada and is probably the most studied bark beetle in North America. However, most of the current knowledge on the biology and ecology of MPB in lodgepole pine comes from the Intermountain Region of the US and western Canada. Little information is available from Colorado. This is the first study addressing effects of stand stocking levels on the biology of MPB and quantifying phloem consumption. In addition, although data are available on the conditions that foster stand infestation, this is the first study estimating the probability of individual tree attack among stands of known different stocking. Materials and Methods: Studies were conducted in managed lodgepole pine stands in Colorado. Unbaited traps were used to monitor MPB flight across stands of different densities. Cages were used to monitor emergence and bark samples to determine attack densities, and phloem consumption in trees growing under different stocking. Beetle collections were used to determine emergence across the growing season. Tree mortality data from plots of different densities were used to examine the probability of individual tree infestation. Results: More beetles were caught flying through higher density stands. More attacks were observed in lower stocking stands but there were no differences in the number of insects emerging nor phloem consumption. There was no relationship between tree size and beetle emergence. Peak flight occurred in early to mid-August and only one peak of beetle emergence occurred. The probability of tree attack was influenced by stand stocking and tree diameter. Conclusions: In general, aspects of the biology of MPB in Colorado exhibit similarities and differences with other regions. The data suggest the need to more closely examine how MPB functions in stands of different stocking and how the distribution of tree sizes influence the probability of infestation and extent of mortality in stands. Biological characteristics of MPB in Colorado need further examination, particularly as climate change continues to manifest. Baseline information will be critical to refine management approaches, and extend the understanding of how MPB contributes to shape forest composition and structure in Colorado.

Bayesian Predictions of Bark Beetle Attack and Mortality of Three Conifer Species During Epidemic and Endemic Population Stages

“…Our results suggest that largediameter trees were more likely to be attacked and killed by bark beetles than small-diameter trees during epidemic years for all species. Similar preferences for large-diameter trees have been reported for mountain pine beetles (D. ponderosae) by Negrón (2019) and spruce beetles (D. rufipennis) by Bakaj et al (2016) in Colorado. However, there was no sustained shift toward preferentially attacking small-diameter trees in post-epidemic years (Figures 4-7).…”

Neighborhood context mediates probability of host tree mortality in a severe bark beetle outbreak

“…At the neighborhood scale, we found that the probability of individual lodgepole pine mortality increased with neighborhood density, supporting our expectation. At the stand scale, dense stands can have microclimate characteristics (e.g., lower temperature and wind speeds) that are more favorable for, and therefore preferred by, MPB (Bartos and Amman 1989, Negrón 2019). The increase in predicted mortality probability with neighborhood density in our model suggests neighborhood density may mediate tree scale mortality rates during MPB outbreak.…”

“…At the tree scale, growth‐defense trade‐offs may result in a positive (de la Mata et al 2017) or negligible (Boone et al 2011) relationship between pre‐outbreak growth and host tree mortality. At the tree neighborhood scale, we expected mortality probability would be greater in high‐density neighborhoods due to more favorable microclimate for MPB (e.g., temperature and wind speed; Bartos and Amman 1989) or preference of MPB for denser stands (Negrón 2019). We also expected mortality would be greater in tree neighborhoods dominated by lodgepole pine due to high beetle pressure resulting from the emergence of adult MPB from neighboring colonized trees.…”

Factors influencing dispersal by flight in bark beetles (Coleoptera: Curculionidae: Scolytinae): from genes to landscapes

“…Based on capture of beetles in pheromonebaited traps, there is a positive relationship between pine tree host density and capture of H. ligniperda, but most H. ater are captured at intermediate host density (Chase et al 2017). Similarly, more D. ponderosae were caught using passive traps in dense stands compared with thin stands, but trees with the highest attack density were those in thin stands (Negrón 2019). Beetle diversity is highest at intermediate levels of canopy cover in trapping experiments conducted in Norway spruce stands (Zach et al 2010).…”