Deactivation of Plant Defense: Correspondence Between Insect Behavior and Secretory Canal Architecture

Abstract

To determine if the arrangement of secretory canals in leaves affects foraging by folivorous insects, we examined the behaviors of 33 species found on diverse canal—bearing plants. Insect behaviors were categorized into three principal classes: vein cutting, trenching, and neither behavior. Canal architectures were ascertained by damaging leaves with standardized tests and measuring the secretory response Our observations document a precise correspondence between herbivore behavior and canal morphology. Vein—cutting insects occur on plants with arborescent canals (resin canals in Anacardiaceae and nonarticulated laticifers in Apocynaceae, Asclepiadaceae, and Moraceae). By severing the secretory canals in leaf veins, insects prevent the flow of secretion to distal branches of the canals. Trenching insects, in contrast, are found on plants with net—like canal systems (anastomosing articulated laticifers in Asteraceae and Caricaceae, and exuding phloem in Cucurbitaceae). To eliminate secretion outflow from these plants, the insects must transect all strands of the network by cutting a trench. The secretory canals of the Convolvulaceae (nonanastomosing articulated laticifers) differ from the preceding categories in being restricted primarily to the major leaf veins. The behavior of herbivores on this family is also distinct: all feed between the major veins without prior vein cutting or trenching. Thus, insects employ the same behavior on similar canal architectures, even when their host plants otherwise differ in taxonomy, secondary chemistry, and canal type. The occurrence of vein—cutting and trenching behaviors in multiple lineages of insects that include caterpillars, beetles, and katydids indicates that the behaviors have evolved repeatedly, apparently through convergence. Our comparative analysis supports the view that these preingestive behaviors function specifically to deactivate defensive canal systems.