To test if secretory canals limit the host ranges of polyphagous moth larvae, we measured the growth of Trichoplusia ni (cabbage looper) and Spodoptera ornithogalli (yellow—striped armyworm) on plants in nine families, including species with and without secretory canals. Larvae were reared to the final instar on excised leaves of the test plant, weighed, then enclosed in the field on either detached leaves with depressurized canals or intact leaves with undamaged canals. After 24 h, the larvae were reweighed and the leaves were examined for evidence of trenching. For comparison, nine additional species of generalist caterpillars were tested on Lactuca serriola (with latex canals) using the same procedure; five of these species were also tested with Petroselinum crispum, which has resin canals. Trichoplusia larvae gained mass on detached and intact leaves of all plant species tested. Larvae cut trenches in plants with canal—borne secretions (Apiaceae–resin, Asteraceae: Cichorieae–latex, Cucurbitaceae–phloem sap), and thereby deactivated the canal systems. Plant species lacking exudates were not trenched. Spodoptera larva failed to cut trenches and performed poorly (with one exception, Daucus carota) on intact leaves of plants with exudates, the same species that T. ni trenched. Detached leaves supported rapid growth by S. ornithogalli larvae, documenting the suitability of these plants in the absence of functional canals. On the latex—bearing Lactuca serriola. S. ornithogalli also grew rapidly on leaves distal to trenches cut by T. ni. This laboratory experiment confirmed that armyworms could feed on plants bearing secretory canals if the larvae were capable of trenching. Performance of the nine other generalist species was likewise correlated with behavior; only species adept at trenching grew rapidly on intact L. serriola and P. crispum. Our results, corroborated across both plant and insect taxa, indicate that the effectiveness of canal—borne exudates as a defense is dependent upon the behavioral capabilities of the herbivore. Generalists lacking trenching behavior are unable to compensate through biochemical or physiological mechanisms. Their host ranges are indeed limited by canalicular defenses due to their inability to deactivate the canals by trenching.