“Rootworms Use Refuges, Do You?”
|Dr. Joseph L. Spencer
Center for Ecological Entomology
Illinois Natural History Survey
Commercialization of corn rootworm-resistant Bt-transgenic corn hybrids in 2003, provided producers with a management tool that was as effective as a soil insecticide. When a grower plants a transgenic corn hybrid, the U.S. EPA mandates that a non-transgenic corn refuge also be planted. The refuge must have an area >20% of that planted with the transgenic hybrid and be located within or adjacent to the transgenic hybrid field(s). Refuges provide habitat where pest species (e.g., larvae of the western corn rootworm beetle (WCR)) may develop free of exposure to the insecticidal Cry protein expressed in the transgenic crop; refuges are areas where susceptible pest genotypes are preserved. Modest Bt-susceptible pest populations, generated in refuges, are expected to spread into the transgenic crop where mate-seeking refuge males will vastly out-number and out-compete potentially-resistant transgenic field males in the ‘scramble’ to mate with potentially-resistant transgenic field females. Male movement from refuges into transgenic corn during the period of adult emergence and mating is critical to the success of the refuge strategy. Factors that limit refuge male dispersal or reduce their mating success in transgenic fields may distort expected patterns of behavior and increase the risk of rootworm resistance to transgenic hybrids.
The threat of resistance to transgenic corn is real. The WCR has a history of resistance dating to the late 1950s and early 1960s when WCR became resistant to aldrin, chlordane, and heptachlor; other insecticides have since followed. By the mid-1990s, 20 years of near universal adoption of annual crop rotation across the eastern Corn Belt lead to behavioral resistance to crop rotation. Rotation resistance is manifest as egg-laying outside of cornfields in rotated crops like soybean; it can cause serious root injury and yield loss in rotated corn. The great efficacy of both insecticides and crop rotation not only made them popular among producers, but also sowed the seeds of resistance once these options were adopted over large areas. Wide adoption of a highly effective management option, like a rootworm-resistant transgenic corn hybrids, without adherence to refuge requirements, imposes selection for rootworm resistance to the Cry protein expressed in a transgenic hybrid. Refuge requirements are ultimately intended to preserve the long-term sustainability of transgenic hybrid technology, thus avoiding devastating losses that would likely accompany WCR resistance to transgenic corn.
Since 2005, I have evaluated movement of WCR adults between fields of a rootworm-resistant Bt-transgenic corn hybrid (YieldGard® Rootworm) and adjacent block refuges without transgenic rootworm resistance (YieldGard® Cornborer). I measured movement rates of mate-seeking males traveling from the refuge to as far as 60 rows into a transgenic corn plot, characterized seasonal patterns of beetle abundance in refuge and transgenic corn, and observed mating beetles.
Refuge males move rapidly into transgenic corn; the mean movement rate for refuge males moving into transgenic corn was 16.17±0.8 m/d (most females from transgenic corn move no more than a few rows before they are rapidly discovered and mated by a mate-seeking male). Greatest male movement from refuge into transgenic corn occurs during the vegetative period of corn development; at this time, nearly 40% of males collected in transgenic corn had moved into that field from an adjacent refuge within the previous 24 hours. Almost half of the population of moving males travel >12 rows into transgenic corn; 14% moved 60 rows into the transgenic cornfield. The vegetative period also coincided with the greatest mating activity in the field. Substantial intrafield movement continues throughout the period of pollination and post-pollination, but the proportion of males in transgenic corn that have recently entered from the refuge is only one half and one quarter, respectively, of the proportion that moved during the vegetative period. These data suggest that current refuge designs promote desirable patterns of mate-seeking male movement; the longterm sustainability of transgenic technology may hinge on current refuge compliance.