Histology of ecology: How do caterpillars of Theroa zethus (Notodontidae) circumvent host defenses?

Most notodontid caterpillars eat tree leaves; however, an Arizona species, Theroa zethus, feeds on herbaceous euphorbs protected by latex canals, a most unusual host shift.  Many of the tree-feeding notodontids girdle their host plant by chewing a circular groove around stems or petioles and then feed distal or proximal to the girdle. Theroa larvae exhibit behaviors similar to girdling, except that their girdles wither rapidly and the larvae inevitably feed distal to the girdles.  We use histological techniques to examine cross-sections of natural girdles and of girdles produced by caterpillars with cauterized salivary glands or blocked ventral eversible glands.  Our work provides insight into how Theroa deactivates the latex defense of its euphorb hosts and identifies the exact plant tissues impacted by girdling.