The Search for Survivors

In the portion of a cornfield planted to rootworm Bt corn there will most likely be surviving CRW.  The survival of CRW larvae when exposed to rootworm Bt corn is a complex issue and not entirely understood.  Both exposure to and the subsequent possibility of repellency from the Cry toxins expressed in root tissue are involved in controlling rootworm larvae.  The dose of Cry proteins that is expressed in root tissue of commercially available rootworm Bt hybrids is relatively low (especially when compared to the dose expressed in above-ground tissue in lepidopteran active Bt hybrids) which allows some larvae to survive.  Also, smaller larvae are most easily controlled with rootworm Bt corn; if larvae survive to second instar they may be more likely to complete development on rootworm Bt corn and emerge as adults because later instars are even less susceptible to the Bt proteins.  Rootworm Bt corn can directly kill CRW because the larvae ingest the Cry toxin found in the roots of the plant. However, rootworm Bt corn may also indirectly contribute to mortality because larvae that survive after taking a bite of rootworm Bt corn roots are often repelled and therefore may die from factors such as starvation, disease or predation. 

Surviving CRW larvae could be the result of the factors just discussed or, perhaps, some individuals are able to survive on Bt plants because these larvae are genetically resistant to the Bt protein in the corn plant.  The genetic resistance to the Bt protein could have already been present in a small number of CRW in the population due to natural genetic mutations.  With the selection pressure being placed on the CRW population from the rootworm Bt corn, the resistant CRW larvae can survive and reproduce.  So in a nutshell, this means that some CRW larvae feed on Bt corn roots, do not die for a variety of possible reasons, and then emerge as beetles.  Researchers like Dr. Meinke continue to collect data that may provide insight into the impact of rootworm Bt corn on rootworm biology as well as other questions about CRW.

The goal of CRW survivors is to mate and reproduce.  Beetles will mate randomly with one another.  So it is possible that two susceptible beetles, two resistant beetles or a resistant and susceptible beetle could mate.  Keep in mind that the genotype of the individual CRW beetle will determine the offspring genotype (Refer to The Kochia and Dicamba Story for a more detailed discussion on genotype and phenotype.).  What is the genetic outcome of the next generation in these scenarios?  Take a look at the diagram below to see the results of various pairings. 

Table 1: The genotypes and phenotypes from possible crosses assuming the resistance allele (R) is dominant (RR, Rr = resistant to Bt and rr = susceptible to Bt).

Even if the farmer applies an insecticide to the refuge, those CRW that survive or escape the insecticide will not be exposed to Bt protein selection pressure.  The goal of using a refuge is to minimize selective pressures and slow down the potential evolution of resistance.  So, by creating an environment where some susceptible CRW survive, farmers are achieving this goal. 

On the other hand, if refuges are not planted there is a much higher selection pressure being applied to the CRW population by the rootworm Bt corn.  Under this scenario, CRW larvae in a specific field are exposed only to corn roots containing the Bt protein with little chance to consume a non-Bt food source.  This means that any surviving CRW that may be resistant have a higher probability of mating with another resistant (R) beetle since the pool of beetles emerging from non-Bt corn would not be present.  The increased selection pressure over time is what will cause change to occur in the CRW population.  Watch this short video presentation to further understand the current and potential resistance development issues with CRW in rootworm Bt corn fields.