A synoptic climatology for the emergence and flight of the mountain pine beetle (Dendroctonus Ponderosae Hopkins)

This project is the first in a series of investigations aimed at assessing the role of the atmosphere, and its interaction with complex topography, in describing landscape level movements of mountain pine beetle (Dendroctonus ponderosae Hopkins) in British Columbia. The atmosphere can act to transport, disperse and concentrate aerobiota such as the mountain pine beetle. An understanding of the spatial and temporal scales of atmospheric motions and biological processes is a necessary framework for coupling biological and atmospheric events and processes that influence the redistribution of mountain pine beetle populations. The movements of mountain pine beetle (and other scolytids) associated with pheromone seeking activities within the forest canopy has received considerable attention (Chapman, J.A. 1962; Gray et al. 1972; Byers, J.A. 1988; Safranyik et al. 1989; Safranyik et al. 1992; Turchin and Thoeny 1993; Byers 2000). Movements above the canopy, while being recognized as a potentially important component to landscape level patterns of infestation, especially during epidemics, has been largely ignored. This project therefore focuses on atmospheric scales of motion contributing to movements above the forest canopy.

Mountain pine beetle is a univoltine species with a relatively well-defined period during which maturation of new adults, emergence and flight occur. The onset of the emergence and flight period is generally preceded by warm, dry weather (Safranyik et al. 1999). Emergence in a region generally occurs over a 7 to 10 day period, and field observations have shown that the greatest catches typically occur when the daily average temperature is greater than 20 °C for at least three consecutive days (Safranyik and Linton. 1993). It is believed that convection, which predominates during fair weather conditions typical of the emergence and flight period, may carry some beetles above the forest canopy to be passively transported over long distances (Safranyik et al. 1989; Furniss and Furniss, 1972). The purpose of the current presentation is to identify and describe the large-scale (synoptic) weather pattern, or patterns, that exist during the emergence and flight period. Using the principles of synoptic climatology, a synoptic composite is first constructed to characterize all possible "fair weather" flight days. Variation in the meteorological conditions that exists during the flight window is assessed, and the statistical significance of the composite is tested at the 95% level using a Student's t-test. Using manual classification techniques, map-subtypes are identified to explain the observed variability in surface meteorological variables during the flight period, such as wind speed and direction. The analysis then focuses on identifying and describing the atmospheric conditions that exist during the period of peak emergence. In the absence of sufficient historical emergence data, we use consecutive heating days as an environmental criteria for building a synoptic composite for optimal emergence. Our compositing approach is validated against observations of emergence described in literature. An interesting consequence of the consecutive heating day criteria is that the evidence seems to suggest the period of peak emergence coincides with a large scale drop in atmospheric pressure on the order of 5 hPa/day and a relative maxima in atmospheric instability, which are associated with the intensification of the continental heat low and the movement of weak troughs of low pressure from the north.

Future work will examine in more detail, the fundamental relationships between mountain pine beetle movement patterns and topography, first through a series of idealistic numerical simulations under the prevailing synoptic conditions, and then by the development of probabilistic pathways (trajectories) of mountain pine beetle movements across the landscape. This atmospheric modelling exercise will allow for a better understanding of the between stand spread component of mountain pine beetle infestation, and thereby provide a more complete picture of the historical and future redistribution of the mountain pine beetle population on the landscape level.