Field-evolved resistance to Cry3Bb1 corn by the western corn rootworm: Investigations of resistance, emergence, node-injury ratings, and fitness

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is an economically relevant pest of corn, Zea mays L. In 2003, the United States Environmental Protection Agency (US EPA) approved the use of transgenic Bt (Bacillus thuringiensis Berliner) corn hybrids to manage the larvae of this billion dollar pest. Bt hybrids express various toxic Cry proteins (i.e., Cry3Bb1, Cry34Ab1/Cry35Ab1, mCry3A, eCry3.1Ab) which have proven to be valuable tools for protecting corn roots from western corn rootworm larval feeding. However, the western corn rootworm has repeatedly developed resistance to management techniques such as crop rotation (by ovipositing in non-corn crops) and some conventional insecticides, which supports the expectation that evolution of resistance by the western corn rootworm is the main threat to the continued success of Bt corn. The first documented case of field-evolved resistance to Bt, by western corn rootworms, occurred in 2009 in Iowa to the Cry3Bb1 protein (Gassmann et al. 2011). Field-evolved resistance occurs as a genetically-based decrease in susceptibility of a population to a toxin caused by repeated exposure, and subsequent selection pressure, of the population to the toxin in the field. The potential for resistance has been compounded by several factors including the fact that the Cry3Bb1 toxin is expressed at a low dose rather than a high dose. Low dose expression increases the survival of heterozygous insects, those individuals with resistant and susceptible alleles. A lack of grower compliance with refuge requirements also increases selection pressure for resistance development. The initial resistance allele frequency is also higher than what would be expected for adequate resistance prevention provided by current refuge standards. Furthermore, several researchers have described fitness costs, of western corn rootworms associated with resistance to Cry3Bb1, as being negligible. The central goal of this two-year, multi-site field experiment was to determine if field-evolved resistance to the Cry3Bb1 toxin by the western corn rootworm had occurred in Illinois and then evaluate the suspected resistant population. Plant based bioassays were conducted by Dr. Aaron Gassmann’s laboratory at Iowa State University and confirmed Cry3Bb1 resistance of the western corn rootworms at our study located in LaSalle and Whiteside Counties. Field experiments were established to evaluate node injury ratings, adult emergence, and to test for potential fitness costs associated with Bt resistance. Field studies included treatments with commercially available Bt hybrids and their corresponding near isolines. Experiments were conducted in producers’ fields where Cry3Bb1 performance problems had been observed in previous years. During my research in LaSalle and Whiteside Counties in 2013, I observed similar nodal root injury between corn hybrids expressing either the Cry3Bb1 or mCry3A proteins compared to their respective untreated check, which lacked rootworm Bt proteins. In contrast, I observed reduced nodal root injury to corn hybrids expressing the Cry34/35Ab1 protein compared to their respective untreated checks in 2013. I also observed reduced mean male cumulative emergence from corn pyramided hybrids expressing the Cry34/35Ab1 protein when compared to their respective untreated checks at the LaSalle and Whiteside County sites. Although head capsule widths and adult dry weights fluctuated among treatments at some sites, Bt corn did not have a consistent effect on these fitness measurements which suggests a lack of fitness costs associated with Cry3Bb1 resistance. With the global importance of transgenic crop technology and its use as a tool for successfully managing insect pests, it is important to prolong its usefulness. It is clear that further investigation is required to identify Bt product failures, and improve decisions regarding insect resistance management to ensure the durability and longevity of each Bt product