Herbivores have a defined range of hostplants that they can feed on, which is mediated by underlying detoxification and sensory repertoires. Insects that feed on Brassicaceae represent one of the striking examples of co-evolutionary arms race. Insects specialized on Brassicaceae have evolved specific mechanisms to detoxify mustard oils (glucosinolates), while generalist species use detoxification enzymes that act on a variety of substrates. Understanding the gene evolution of detoxification and sensory repertoire in specialist and generalist Brassicaceae feeders will shed light on the processes involved in mediating hostplant ranges in herbivores. We use a comparative phylogenomic approach in 12 lepidopterans that feed on Brassicaceae, ranging from specialist to pests in their host range to examine the gene family expansion of detoxification and sensory gene families. We found that gene family expansions and contractions were larger in generalist herbivores compared to specialist herbivores. Gene evolutionary rate of detoxification genes reflected hostplant range where generalists had a higher evolutionary rate of detoxification genes that act on wide substrates while specialists had a higher evolutionary rate in genes that conjugate toxic compounds to hydrophilic byproducts. Our analysis on the nitrile specifier gene, a key innovation for feeding on Brassicaceae, indicated pervasive purifying selection with lineage specific differences in selection. Our results add to the growing body of work addressing gene family evolution and its role in hostplant range and specialization in insects.