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

Abstract

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.

Cardenolides from Gomphocarpus sinaicus and Pergularia tomentosa (Apocynaceae: Asclepiadoideae) deter the feeding of Spodoptera littoralis

“…Insects feeding on cardenolidecontaining plants are either behaviourally (Helmus and Dussourd 2004) or physiologically adapted (Vaughan and Jungreis 1977;Moore and Scudder 1985;Scudder et al 1986) to deal with cardenolides and often accumulate them in their tissues as an acquired chemical defence against vertebrate predators (for example, Rothschild 1972;Martel and Malcolm 2004).…”

Reward Tracking and Memory Decay in the Monarch Butterfly, Danaus plexippus L. (Lepidoptera: Nymphalidae)

“…The monarch butterfly ( Danaus plexippus ) is a long‐lived nymphalid that has been extensively examined with respect to its interaction with milkweeds (Malcolm ; Zalucki et al. ; Helmus & Dussourd ), role as a Batesian model (Brower ; Ritland & Brower ), and remarkable long‐distance, multi‐generational migration (Calvert & Brower ; Mouritsen & Frost ). Because monarchs are likely to encounter a large variety of nectar sources over the course of yearly migrations that can extend from southern Canada to central Mexico (Ackery & Vane‐Wright ), an ability to associate floral features with nectar quantity and/or quality is likely to be of great importance.…”

A natural history of conspecific aggregations in terrestrial arthropods, with emphasis on cycloalexy in leaf beetles (Coleoptera: Chrysomelidae)

“…Excellent examples of behavioral ecology of insect trench-cutting to overcome plant toxins have been reported by Becerra et al (2009), Dussourd (2009), Helmus and Dussord (2005), and Oppel et al (2009), among others. Sadly, sometimes chemical defenses are loosely referred to by the almost chemically meaningless term, "latex".…”