Glues or poisons: which triggers vein cutting by monarch caterpillars?

Summary.

Late instar larvae of the monarch butterfly (Danaus plexippus L., Lepidoptera: Nymphalidae) deactivate the latex defense of milkweeds by chewing a furrow in the leaf midrib or petiole. The larva then feeds beyond the cut where latex outflow is minimal. If a larva does encounter latex exudate during feeding, it often returns to its initial cut to damage the midrib or petiole more extensively before resuming feeding. We used this response to latex as an assay for testing what cue triggers vein cutting. A sticky solution of polyethylene glycol and a mixture of cardenolides both were ineffective; drops applied to the mouthparts of feeding monarchs failed to elicit renewed vein cutting. Activity resided instead within a methanol extract of the supernatant obtained from centrifuged latex of the milkweed, Asclepias curassavica L. (Asclepiadaceae). Treatment with proteinase K did not eliminate activity documenting that the active compound is unlikely to be proteinaceous. Our results indicate that latex adhesiveness and low polarity cardenolides are relatively unimportant releasers of vein cutting. We propose that milkweeds contain noncardenolide stimulants of vein cutting; these compounds presumably serve a defensive role for milkweeds. Over 50 species of insects are known to employ vein-cutting before feeding on plants with canal-borne exudates; most of these species are dietary specialists like the monarch. Our behavioral assay provides a novel approach for identifying ecologically-significant compounds in the exudates of their diverse host plants.