Technical Abstract: The study of the genetic basis of ecological adaptation remains in its infancy, and most studies have focused on phenotypically simple traits. Host plant use by herbivorous insects is phenotypically complex. While research has illuminated the evolutionary determinants of host use, knowledge of its genetic basis is lacking. We used quantitative trait loci (QTL) mapping to examine the genetic architecture of behavioral and physiological differences in host plant use between the specialist HELIOTHIS SUBFLEXA and its close relative, the broad generalist HELIOTHIS VIRESCENS. In large, replicated experiments, we analyzed the performance of interspecific hybrid insects on PHYSALIS ANGULATA, the host of H. SUBFLEXA. The effect of H. SUBFLEXA-origin chromosomes on physiological and behavioral traits associated with host use was measured as the percent variation explained (PVE) in backcross populations and as the percent of the phenotypic gap between H. SUBFLEXA and H. VIRESCENS. Nineteen H. SUBFLEXA-origin chromosomes affected variation in the traits measured. Effect sizes were small when measured as PVE, but were larger when compared to the interspecific difference. Most explanatory chromosomes had effects on more than one trait, and their effects varied between years, sexes, and genetic backgrounds--even within a single lineage, chromosomes associated with a trait varied. Major, saltational changes do not appear to be required for the use of PHYSALIS by a non-adapted species like H. VIRESCENS, and we suggest that behavioral changes probably paved the way for the generalist ancestor of both species to adapt to PHYSALIS.