Rootworms and Refuge: Is One Strategy Better?

Corn Root Worm Beetles

Since their initial commercialization in 2003, rootworm-protected Bt (Bacillus thuringiensis) transgenic corn hybrids have changed the way corn insect pests are managed in the Corn Belt. Compared to soil insecticides, Bt hybrids offer improved grower safety, protection of nontarget species and equivalent efficacy.

Unfortunately, the efficacy of Bt crops against pests may select for insect resistance to Bt. To slow the development of pest resistance, the US EPA requires that all growers of Bt crops set aside a portion of each field as a nontransgenic refuge. In the Corn Belt, the EPA requires a 5% to 20% refuge within or adjacent to a rootworm-protected Bt cornfield.

Refuges provide areas for some pest insects to develop without exposure to the Bt toxin expressed in the Bt crop, leaving them susceptible to the effects of Bt technology. Western corn rootworm (WCR) larvae surviving on Bt plants are potentially resistant and may (but don't necessarily) possess genes for resistance they could transmit to their offspring. According to the expectations of the refuge strategy for corn rootworm, Bt-susceptible male WCR beetles emerging from refuges will move into the Bt portion of fields and mate with potentially resistant females emerging there. Because many more WCR are produced in refuge than in Bt-corn areas, nearly every potentially resistant WCR will be mated by one of the abundant susceptible refuge males. These pairings should dramatically dilute the potential for producing resistant offspring.

As more hybrids with multiple unique modes of action against corn rootworms are commercialized, minimum refuge size is shrinking, and options for refuge configurations are changing. Depending on the specific hybrid, the correct refuge size ranges from 5% to 20% and can be planted as blocks or strips within or adjacent to Bt cornfields. Some Bt hybrids and refuges are now offered as seed blends, with the proper percentage of refuge seed premixed in the bag. Refuge seed blends (aka "refuge in a bag") simplify the planting process and guarantee 100% compliance with refuge requirements. Compliance has taken on paramount importance as the high rates of Bt-corn adoption mean that there is little "natural" refuge of non-Bt corn to make up for fields that are noncompliant.

Ironically, much of the rootworm pest biology that informs current refuge requirements for Bt corn has its origins in research conducted before the commercialization of any Bt-transgenic crops. Our study was conducted to update information regarding the movement and mating behavior of WCR beetles within refuge and Bt corn. We present data on WCR emergence, movement, and mating within four refuge configurations: 20% block refuge, 5% block refuge, 5% seed blend, and 0% refuge.

We have learned that WCR beetles do not always behave in ways that meet our long-held assumptions. In the past, we have shown that peaks of WCR emergence and abundance varied with the configuration of refuge. We now also know that WCR movement patterns in block refuges may not always provide adequate mixing between mate-seeking WCR from refuge and Bt corn. In stark contrast, blended refuges provided very uniform distributions of WCR adults that did not vary significantly during the growing season. The configuration of refuge can dramatically influence WCR movement and mating in Bt cornfields. We expect that considerations of pest behavior, rates of refuge compliance, and the threat of pest resistance will play an important role in shaping grower options for deploying refuge corn well into the future.

This project is supported by USDA National Institute of Food and Agriculture (NIFA) Grant #2009-65104-05976 to Joseph Spencer.

Joseph Spencer

Joseph Spencer
Illinois Natural History Survey

Sarah Hughson
Department of Entomology