With rising temperatures and increasing incidences of drought associated with climate change, spider mites, including the two-spotted spider mite (Tetranychus urticae, TSSM) and the Banks grass mite (Oligonychus pratensis, BGM), are becoming ever more important agricultural pests. An extreme generalist, the TSSM is documented to feed on more than 1,100 plant hosts, while the BGM, a grass specialist, feeds on cereal crops like maize, wheat, sorghum and barley. Historically, studies of plant-herbivore interactions have focused primarily on insects, while far less is known about how plants respond to mite herbivores, especially in grasses. To identify how plant defense pathways respond to both generalist and specialist mites, we compared time course RNA-seq data from barley (Hordeum vulgare L.) infested with TSSMs and BGMs. Strikingly, significant changes in the expression of hundreds of genes were detected at only two hours after mite infestation, and by 24 hours this number had increased to several thousand. As assessed by comparisons of TSSM- and BGM-induced responses at both time points, transcriptional reprogramming was strongly correlated for 3963 genes, including the down-regulation of genes involved in photosynthesis and the up-regulation of genes in the hormone signaling and secondary metabolic pathways. In particular, genes in the jasmonic acid biosynthesis and signaling pathway, as well as grass-specific secondary metabolic pathways, were among those with the strongest transcriptional changes. While many shared transcriptional changes were observed, the magnitude of barley responses to the specialist BGM was markedly stronger than that of the generalist TSSM, an observation that was reflected in enzymatic assays of infested leaves (e.g., peroxidase activity). In addition, 3261 genes exhibited BGM-specific changes in expression compared to only 535 genes unique to TSSM-infested plants. A potential explanation for this result is that the broad host range of generalist mite herbivores involves (at least in part) suppression of plant defenses, a finding that has been reported for some other herbivore species. In contrast, host adaptation for the specialist BGM may have involved the evolution of robust detoxification pathways for secondary compounds that typify members of the grass family upon which BGM is restricted.