Proceedings of the Sixty-first Annual Meeting of the Northeastern ...

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HOW THE SPATIAL SCALE OF DISPERSAL MODELING HAS INCREASED WITH
GLYPHOSATE-RESISTANT HORSEWEED. J.T. Dauer, D.A. Mortensen, The
Pennsylvania State Univ., University Park, E.C. Luschei, Univ. of Wisconsin, Madison,
M.J. VanGessel, Univ. of Delaware, Georgetown, and E.S. Shields, Cornell Univ.,
Ithaca, NY.
ABSTRACT
Population dynamics modeling of species spread often assume no vector
assisted movement and can be represented using cellular automata models where a
population contributes genes, seeds or pollen, to nearest neighboring cells. In the
simplest sense, these models can be adjusted to include vectored movement by
increasing the probability of seed landing at long distances. This method ignores the
underlying mechanisms of vectored movement and fails to accurately simulate longdistance
dispersal. The importance of glyphosate-resistant horseweed has generated
interest in quantifying the likelihood that seed are being distributed to nearest neighbor
fields versus long-distance dispersal. Instead of a cellular approach, we defined the
landscape by a series of polygons outlining actual fields in a 10 km x 9 km aerial photo
of Pennsylvania cropland. Polygons were assigned initial crop type, corn or soybean,
which were rotated yearly and impacted the survivorship of seed that arrived in the
previous time step. A 2-dimensional 2-parameter dispersal model dependent on
distance from the source and source strength was applied to seed movement in the
landscape. Dispersal was normalized by area to determine seed arrival in every field in
the landscape. Survivorship varied from zero percent (best management with
alternative herbicides, tillage) to 100 percent (glyphosate only). Simulations conducted
thus far have not included a directional wind vector but will be included as the model
advances. Spread from randomly initiated source fields was slow for three years,
increasing source strength but spreading less than 1 km per year. In the fourth and fifth
years after initiation, seed dispersal and establishment reached fields at the extent of
the described landscape (at least 5 km). Continued manipulation of the model will
explore the importance of field size and quantify the necessary efficacy to reduce
spread to less than 1 km per year. The ability of horseweed seed to disperse long
distances has enlarged the scale at which resistance management can have an impact.
Focusing on a single field or a small region will not prevent spread of this biotype to
neighboring farms. As more species develop resistance to glyphosate, questions about
how to reduce the impact will require predictive models created on the correct spatial
scale.
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