Final instar monarch larvae cut a furrow in the midrib or petiole of milkweed leaves before eating distal to the cut. This behavior eliminates latex exudation from the leaf, therefore reducing larval exposure to latex during feeding. If larvae do encounter latex while feeding, they often return to chew on the previously severed midrib or petiole. We used this response as an assay to test if cardenolides or stickiness trigger vein cutting. Final instar monarchs were allowed to sever the midrib or petiole of an Asclepias curassavica leaf and begin feeding. A mixture of cardenolides dissolved in an aqueous solution of 20% TWEEN 80 was placed next to the mouthparts of the larvae. Each larva received a total of 2 ml dispensed as three drops. None of the larvae tested with the cardenolide solution or with a TWEEN 80 control resumed vein-cutting, whereas 100% of the larvae tested with fresh A. curassavica latex did (n=10 larvae tested with each solution). In a second experiment, a solution of polyethylene glycol, used to mimic the sticky nature of milkweed latex, did not elicit vein-cutting. The water control was also inactive, whereas larvae tested with A. curassavica latex returned to the leaf base to extend their previous cut. These data suggest that neither cardenolides nor latex stickiness triggers vein severance; apparently some other chemical or combination of chemicals in milkweed latex elicits vein-cutting. Identifying the active chemical may provide novel insights into the chemical ecology of monarchs and milkweeds.
Species 1: Lepidoptera Nymphalidae Danaus plexippus (monarch butterfly)
Species 2: Gentianales Asclepiadaceae Asclepias curassavica (milkweed)
Keywords: cardenolide, vein cutting
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA