A tracer test program was developed to study the transport and dispersion of pheromone plumes released from insects in forest canopies. In recent years, the artificial release of insect pheromones has been investigated as a control strategy for agricultural and forest pests. However, the pheromone composition, the insect response and tracking behaviors, as well as atmospheric transport and diffusion of pheromones from natural and artificial sources are not completely understood.

Instantaneous measurement of pheromone plumes has proven difficult and currently no robust method exists. In order to measure dispersion, sulfur hexafluoride (SF6) is used as a surrogate. Tracer releases were conducted during three separate field studies in oak hickory, lodgepole pine, and ponderosa pine forests. During each tracer experiment, SF6 was released continuously at a steady rate from approximately 1 m above the surface within a uniform stand of trees. As many as sixty portable syringe samplers were deployed at 1 m height along sampling rings at 5 m, 10m, and 30 m from the source. At some locations, samplers were also deployed on vertical masts. In all cases, sequential 30 min average samples were collected during each 4.5 hr tracer test. In addition to the time averaged data, a fast-response, real-time continuous SF6 analyzer was deployed on a small cart within the sampler array to obtain a continuous record of SF6 concentration fluctuations. The cart was moved periodically during each test to remain downwind of the source. Meteorological data were collected from several short instrumented stations as well as from a tower extending above the canopy. In most cases, the meteorological measurements included 3-d sonic anemometer velocity data from instruments at several levels on the tower and from an instrument located at the tracer source.

In this paper, results are presented that show SF6 plumes with steep spatial concentration gradients due to plume meander in narrow filaments on the order of single meters or less. Due to the low wind speeds that occur in the canopy, wind directions are highly variable and tracer plumes can meander over 360 degrees of wind direction during a release period. This results in intermittent concentration time series at any specific receptor. The tracer data have been used to determine dilution rate curves as a function of downwind distance and meteorological conditions. These curves can be used to determine conditions where the pheromone concentration falls below the insect's response level, and this can be directly applied to guide pheromone deployments. In addition, the tracer data are used to evaluate a meandering puff model driven by the on-site turbulence data. The objective is to develop tools to predict pheromone diffusion within a canopy that land managers can use to determine placement of artificial pheromones within timber stands and agricultural orchards.