The rootworm’s opinion of reduced refuge
Illinois Natural History Survey
University of Illinois
Department of Entomology
University of Illinois
Growers have planted over 2 billion acres of biotech crops worldwide since 1996, making agricultural biotechnology the most rapidly adopted new crop technology. In the Corn Belt, insect-resistant Bt (Bacillus thuringiensis) transgenic corn hybrids have transformed pest management and yielded better grower safety and cost savings, improved crop quality, and protection of non-target species. While transgenic crops are very effective against target pests, their use imposes selection on pest populations for resistance.
To delay development of resistance to the insect-killing Bt-proteins expressed in transgenic crops, the EPA mandates that growers must set aside a small portion of each field as a non-transgenic refuge. The refuge is an area where susceptible pests can develop without exposure to the Bt-proteins expressed elsewhere in the field. The 5% or 20% refuges planted by Corn Belt growers who use corn rootworm-protected transgenic corn hybrids, will yield large populations of Bt-susceptible western corn rootworm (WCR) and northern corn rootworm (NCR) adults. Mate-seeking rootworm males are expected to move into adjacent transgenic areas and find mates among the sparse populations of potentially-resistant WCR and NCR adults. Because refuge males will vastly out-number any potentially-resistant transgenic field males, they will secure most matings with potentially-resistant females. The abundance of susceptible beetles entering transgenic corn reduces the likelihood that any two resistant rootworms will mate; this slows the rate of resistance development.
Mating WCR pair
The minimum required refuge size is changing as hybrids with multiple unique modes of action against rootworm are commercialized. In 2010, some hybrids require only a 5% refuge block or strips within or adjacent to the transgenic field; others still require a 20% refuge. In the future, blends of refuge and transgenic seed (‘refuge in the bag’) will be commercialized; these will simplify planting and assure compliance with refuge requirements. The importance of refuge compliance is growing with adoption of transgenic hybrids and disappearance of ‘natural’ refuge fields of non-transgenic corn. In 2009, 84% of Illinois maize was a stacked or non-stacked transgenic hybrid, a percentage that has more than doubled since 2005.
Despite the Corn Belt management renaissance, it is curious that the science of protecting transgenic crop technology from insect resistance depends on rootworm biology data that were often generated long before commercialization of any transgenic crops. The goal of this study is to measure the behaviors of WCR beetles as they move and mate in different configurations of refuge and rootworm-resistant transgenic corn. We go beyond the assumptions and measure what rootworms do in field conditions—what better way to test how well refuges perform, than to observe pests using them. We present data on WCR emergence, movement and mating from four refuge configurations: 1) 20% block refuge, 2) 5% block refuge, 3) 5% seed blend, and 4) 0% refuge.
Surprising preliminary mating patterns indicate that refuge beetles don’t always go where we assume and that male mating capability may be a significant factor limiting refuge efficacy in the field. Besides learning whether WCR behave as we expect in refuges, our data on patterns of WCR behavior are contributing to decisions about refuge-size reductions and the feasibility of seed blends.
(This project is supported by USDA-National Institute of Food and Agriculture (NIFA) Grant #2009-65104-05976 to J.L.S.)