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Dr. Joseph L. Spencer
Assistant Professional Scientist
Center for Ecological Entomology
Illinois Natural History Survey
Rootworm Movement In Transgenic And Refuge Corn: Getting Beetles From Here To There.
It's late June in east-central Illinois. The sun is just beginning to burn through a humid morning haze that hugs the eastern horizon. For days, thousands of hungry male western corn rootworm (WCR) beetles have pushed their way up through the moist soil each morning and emerged around the corn plants that nourished them as larvae. However, this morning there is something new in the cornfield: female WCR are emerging. Like the males, newlyemerged females are a pale ghostly yellow in color, both will rest at the base of a corn plant until their exoskeleton hardens before they begin to feed. As the females rest, they expose a gland at the tip of their abdomen and release a sex attractant pheromone into the air. Week-old males that detect the windborne pheromone recognize it as the signal - an unmated female is nearby...
Transgenic corn for rootworm control offers growers protection from economic rootworm injury at a cost comparable to soil insecticide application. However, like insecticides, failure to deploy transgenic corn appropriately may speed the development of resistance to the Bt proteins expressed in the corn. Planting a 20% non-transgenic refuge within or adjacent to transgenic corn will yield susceptible WCR that may leave the refuge and mate with the few potentially-resistant WCR emerging from transgenic corn. The refuge strategy depends on thorough mixing between beetle populations during the mating period. By saturating the transgenic field with Bt-susceptible beetles from refuges, few pairings between resistant beetles will occur. Because almost all females are mated as soon as they emerge, desirable population mixing depends on movement of refuge males into transgenic areas of a field. WCR males move ca. 7-11 m/d, some travel at nearly 16 m/d during the mating period. If refuges are improperly situated, males may not reach all areas of transgenic fields resulting in many matings between potentially resistant males and females, leading to the possibility of resistant offspring.
The males fly off excitedly in search of the unmated females who broadcast their sexual receptivity on the wind. Because females are still scarce in the cornfield, some males will fly many rows to zero in on a female. One lucky male will find the pale female first and mate with her. This morning, most of the males arrive too late; however, more females are emerging, and the air of the field is soon saturated with sexual invitations from thousands of other females. In coming days, females become common and males will easily find willing mates nearby...
It has been suggested that mixing 80% rootworm transgenic seed with 20% non-transgenic seed in the planter boxes, rather than planting separate block refuges, could improve WCR population mixing. With a seed blend, WCR adults emerging from non-transgenic plants would be very close to any beetles that emerge from transgenic plants nearby. WCR movement and mating in an 80:20 seed blend vs. a standard 20% refuge have not been compared. I present data on WCR movement rates in an 80:20 seed blend with YieldGard® Rootworm vs. a 20% structured refuge (80% YieldGard® Rootworm; 20% non-transgenic corn). Though marketing and performance monitoring issues remain, understanding how refuge placement influences WCR movement may simplify use of transgenic corn for rootworm management and reveal new vulnerabilities in the WCR mating and egg-laying process.
...In two weeks, the frenzy is nearly finished, most females have emerged, mated, and dispersed from the field that nurtured them to settle around distant cornfields to begin laying eggs in corn and rotated crops. Males stay behind in their cornfield, which is steadily invaded by mated females from elsewhere. The males will move small distances within and between nearby fields and compete for a dwindling supply of unmated females. By late August, there is little to eat and the WCR are dying. The final eggs are deposited among drying cornstalks...