Does secretory canal architecture determine the sabotage behaviors of insect folivores?

Abstract

Insect folivores on plants protected by secretory canals commonly sever leaf veins or cut trenches before feeding beyond the cuts. Previous studies reported that vein cutting occurs when canals have an arborescent arrangement, whereas trenching is found when canals have a net-like arrangement. However, some danaine species, such as the monarch caterpillar, Danaus plexippus, show both behaviors on the same milkweed plant; early instars cut circular trenches and later instars chew furrows in the leaf midrib. This study tests the hypothesis that milkweed canals differ in arrangement at different scales, thus requiring different behaviors from early and late instars. I compared common milkweed, Asclepias syriaca (Apocynaceae) with prickly lettuce, Lactuca serriola (Asteraceae). Leaves were damaged with standard wounds and the response of the laticifers was compared by measuring latex exudate. With L. serriola, severing either the primary or secondary veins failed to reduce latex emission beyond the cuts. The veins and associated laticifers form an interconnected network; plusiine caterpillars on L. serriola disarm the network with a trench. With A. syriaca, transecting the midrib virtually eliminated distal exudation. However, severing a secondary vein caused only a partial reduction. To decrease exudation beyond secondary veins, milkweed insects need either to sever multiple adjacent veins (as shown by Labidomera clivicollis beetles) or to cut a trench (as in early instar danaine larvae). Thus, in both A. syriaca and L. serriola, herbivore behaviors match the laticifer systems as predicted by the hypothesis that canal architecture has a central role in determining behavior.