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Pollen Drift and Refuge-Management Considerations for Transgenic Hybrids

April 17, 2003

The unseasonably warm weather across Illinois has hastened the pace of planting, or at least the preparations for planting. As noted in the first issue of the Bulletin (March 21, 2003), transgenic hybrids (MON 863 event, Cry3Bb1) that are designed to protect root systems from corn rootworm larval injury will be planted for the first time this spring. Despite the lingering concerns among many producers, farmer organizations, and major grain processors regarding grain-channeling issues, YieldGard Rootworm hybrids are being planted. Questions concerning pollen drift from YieldGard Rootworm hybrids into adjacent non-GMO fields of neighbors are being raised. At the 2003 Crop Protection Technology Conference, Martin Bohn, assistant professor of corn breeding in the Department of Crop Sciences, University of Illinois, offered remarks on the following topic: "Pollen Drift and Its Impact on Gene Flow Between GM and Non-GM Cultivars." I'll attempt to summarize his key points on this important issue.

Let's begin with some facts about the physical characteristics and longevity of pollen that Bohn discussed in his article:

  • Corn pollen is 90 to 100 µ in diameter.
  • Corn pollen is spherical.
  • Pollen of many other species that depend on wind pollination is much smaller. For instance, the pollen of ragweed species or timothy is three to four times smaller than corn pollen.
  • Corn pollen is among the largest of particles commonly found in the air.
  • Corn pollen drifts to the earth quickly (0.3 meter per second).
  • Wind speed and direction influence pollen drift.
  • Corn pollen may remain viable for approximately 24 hours.
  • Viability of corn pollen diminishes rapidly with desiccation.
  • At flowering, 60% of pollen fresh weight consists of water.
  • Pollen longevity drops rapidly if the water content falls below 40%.

How far will pollen drift? To answer this question, Bohn conducted a review of the scientific literature. Results from a 1948 study (Jones and Newell, Journal of the American Society of Agronomy, 40: 195-204) of pollen longevity of buffalo-grass and corn revealed a 1% pollen concentration at 427 meters in the atmosphere. Based on these findings, other researchers concluded that, if each corn plant produces 25 million pollen grains, 125,000 pollen grains could be expected at a distance of 500 meters from the original source.

In 1972, G. S. Raynor, C. O. Eugene, and V. H. Janet (Agronomy Journal 64: 420-427) used
wind-impact samplers to estimate pollen concentration at selected distances from the original pollen source. They indicated that, at a distance of 60 meters from the source of pollen (in the main wind direction), the concentration of pollen averaged only 1%, compared with the pollen samples collected only 1 meter from the pollen source.

In 1947, A. J. Bateman (Heredity 1: 235-246) reported the results of some cross-pollination studies. He reported that the number of outcrosses was reduced by half at the relatively short distance of 3.77 meters from the pollen source. At a distance of 12 to 15 meters, the number of outcrosses was reduced by 99%.

In contrast to these results, M. D. Jones and J. S. Brooks reported (Oklahoma Agricultural Experimental Station, Bulletin T-38) in 1950 that outcrosses could occur at a distance of 503 meters from the pollen source. A more recent study published by S. V. Luna and others in 2001 (Crop Science 41: 1551-1557) indicated that cross pollination occurred at a distance of 200 meters. Variation in the distance of pollen movement among these studies is reflective of the different wind speeds that occurred during the times these experiments were conducted. For instance, the Jones and Brooks study (1950) was conducted in Oklahoma, well-known for its windy and stormy weather.

Based on the review of this literature, Bohn offered the following statements: "An adjustment of technical farm procedures can be used to avoid mixing of GM and non-GM seed, e.g., planting and harvesting conventional crops before GM crops. However, a containment of pollen employing normal farming procedures is not possible."

Are there strategies that producers can use to minimize pollen drift from YieldGard Rootworm hybrids into the nearby fields of neighbors? In February, Kevin Steffey and I presented some insect management information at a workshop near Ottawa. A producer in the audience asked us what our opinion was regarding the use of a 20% refuge deployed exclusively as a perimeter (border rows) around an interior block planted with a YieldGard Rootworm hybrid. We indicated that although this strategy may prove a bit more challenging at planting time, it certainly has some merit with respect to reducing the amount of unwanted pollen that could drift into a neighbor's field located nearby.

As a courtesy to their neighbors, I encourage producers to consider this approach. In addition, it's a good idea to let your neighbors know that you intend to plant a YieldGard Rootworm hybrid.--Mike Gray

Author: Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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