A method is provided for evaluating a protected plant having a protection mechanism. A population of immature insects (i.e., larvae or nymphs) belonging to an insect species having a plurality of instars is first exposed to the protected plant such that the protected plant is at least contacted thereby. The insect exposure extends for a selected time period corresponding to a sublethal exposure of the insect population to the protected plant. The exposure of the insect population to the protected plant is then halted following the selected time period and insects collected. A physical characteristic of at least some collected insects is measured and the immature insects are then sorted into instars. The measured physical characteristics are then statistically analyzed across the instars so as to determine an efficacy of the protection mechanism of the protected plant with respect to the insect population. Associated methods are also provided.