Rootworm control tactics have recently expanded to include transgenic maize, which express insecticidal proteins from Bacillus thuringiensis (Bt) to reduce larval injury and protect yield potential.  Exceptional root protection, increased grower efficiency and improved safety have led to rapid adoption of this technology in the USA.  As a result, there is a recognized need for resistance management programmes aimed at delaying rootworm resistance.  An essential component of resistance management programmes is the development and implementation of effective resistance monitoring techniques.  Five test populations of Diabrotica virgifera virgifera (LeConte) were used to evaluate the sensitivity of two techniques used to describe population susceptibility to the Bt proteins expressed in event DAS-59122-7 maize: a diet bioassay employing purified proteins applied to artificial diet and a novel technique using sub-lethal measures of larval development on seedling maize.  Test populations included Rochelle-US, an unselected susceptible colony, three populations composed of 5%, 25% or 50% Rochelle-S mixed with Rochelle-US, and the Rochelle-S selected colony.  Rochelle-S was derived from the same founding population as Rochelle-US, but selected for survival on DAS-59122-7 maize.  Selections identified a minor trait conferring increased tolerance, and greenhouse plant efficacy evaluations confirmed that after 10 generations of selection with no random mating, Rochelle-S caused significantly more root injury to DAS-59122-7 than Rochelle-US.  Rochelle-S present at 5% of the test population resulted in measurable but not significant increase in injury to DAS-59122-7 maize.  The diet bioassay was relatively insensitive to the susceptibility differences between the Rochelle-US and Rochelle-S populations.  Neither LC50 nor EC50 estimates produced statistically significant differentiation between test populations with 0%, 5%, 25% or 50% Rochelle-S.  The sub-lethal assay clearly identified differences between Rochelle-S and Rochelle-US and an increased rate of larval development was measurable when the test population contained only 5% of Rochelle-S.