## 2 Bt Maize and Resistance Management  
Transgenic Bt maize has been used to manage corn rootworm since 2003. The first Bt maize produced a single Bt trait, Cry3 Bb1, and subsequently three addition Bt traits were registered by the US Environmental Protection Agency (EPA): Cry34 Ab1/Cry35 Ab1 (now called Gpp34 Ab1/Tpp35 Ab1 [14]) in 2005, mCry3A in 2006 and eCry3.1 Ab in 2012 [15; 16; 17; 18]. Similar to Cry3 Bb1, both Cry34/35 Ab1 and mCry3A were used initially as single Bt traits targeting corn rootworm, while eCry3.1 Ab was released as a pyramid with mCry3A [19]. Additionally, pyramids of Cry3 Bb1 with Cry34/35 Ab1 and mCry3A with Cry34/35 Ab1 were registered by the US EPA in 2009 and 2012, respectively [20]. Replacement of single Bt traits targeting corn rootworm by pyramids of two Bt traits was a gradual process, and for multiple growing seasons, both single traits and pyramids occurred together in the agricultural landscape [21; 22]. Moreover, resistance to some of these Bt traits, in particular Cry3 Bb1 and mCry3A, was already present in the agricultural landscape prior to the release of pyramided events [3; 23]. In the US, an insect resistance management (IRM) strategy is mandated by the US EPA for the commercial cultivation of any Bt crop, including Bt maize that targets corn rootworm [19]. Currently, EPA-mandated IRM approaches for Bt crops focus on the refuge strategy. Under the refuge strategy, a non-Bt host is provided for an insect pest, with the goal of producing Bt-susceptible individuals that can mate with any Bt-resistant individuals surviving on a Bt crop [24]. The use of refuges to delay resistance can be especially effective when combined with either high-dose Bt events or Bt crops that are pyramided with multiple Bt toxins targeting the same pest [25; 26; 27; 28]. A high-dose Bt crop is defined as one that either produces 25 times more Bt toxin than necessary to kill a Bt-susceptible pest or kills 99.99% of susceptible individuals [29]. When a high dose is achieved, Bt resistance is rendered a functionally recessive trait because the dose of toxin produced is sufficient to kill not only homozygous susceptible individuals, but also heterozygous resistant individuals [28]. Thus, in the high-dose/refuge scenario, susceptible individuals from a refuge mate with resistant individuals surviving on a Bt crop, producing heterozygous progeny that are unable to survive on a high-dose Bt crop, which in turn delays resistance. By contrast, a pyramid delays resistance through redundant killing, with insects that harbor alleles for resistance to one Bt toxin in a pyramid killed by the second toxin and vice versa [30]. In the pyramid/refuge scenario, susceptible individuals from a refuge mate with resistant individuals from a Bt crop, thereby reducing the proportion of individuals that harbor resistance alleles for both Bt toxins in a pyramid, and consequently delaying the evolution of resistance. However, for a pyramid to work effectively, there must be an absence of cross-resistance between the toxins, and alleles for resistance to either toxin must be at a low frequency within the population [31; 32]. If a pest population has evolvedresistance to one toxin in a pyramid, the delay in resistance achieved by combining two toxins in a pyramid will be greatly diminished or lost altogether [30; 32]. A critical factor affecting how quickly a pest will evolve Bt resistance is the inheritance of resistance, and this is especially important when Bt traits are deployed singly or if one of two traits in a pyramid is compromised by resistance [32; 33]. When resistance to a Bt trait is inherited in a non-recessive manner, some proportion of the heterozygous resistant individuals will not be killed by that Bt trait, and resistance will evolve faster than when resistance is recessive [33].