Specialized generalists: constraints on host range in some plusiine caterpillars

Insects that feed on plants with secretory canals often cut trenches across leaves, thereby depressurizing the canals and eliminating exudation at their distal feeding site. We compared the trenching ability of three species of polyphagous plusiines (Lepidoptera: Noctuidae) and tested if trenching scores correlate with larval growth. The three plusiines (Trichoplusia ni, Pseudoplusia includens, and Rachiplusia ou) were each tested on prickly lettuce, Lactuca serriola (Asteraceae), which has latex canals, and on Italian parsley, Petroselinum crispum (Apiaceae) with oil ducts. To ascertain how secretory canals affect performance, larvae were tested on intact leaves and on excised leaves with depleted canals. T. ni larvae cut trenches in both plant species, whereas P. includens only trenched prickly lettuce and R. ou only trenched Italian parsley. Intact leaves of Italian parsley were acceptable to all three species. Trenching varied in T. ni and R. ou, but did not correlate significantly with larval growth. In contrast, trenching was required for feeding on intact prickly lettuce. Final-instar T. ni all cut trenches and developed rapidly. P. includens varied in trenching and performance; their trenching scores correlated with growth. R. ou larvae did not trench or feed even though most R. ou on intact Italian parsley cut at least partial trenches. All three plusiine species developed rapidly on excised leaves of both plant species, documenting the suitability of these plants when canals are inactivated. Our results document the efficacy of latex canals as a plant defense and suggest that trenching ability alone does not permit feeding. Larvae must also recognize the need to trench and must tolerate deterrent exudates during the trenching procedure.