PARENT SESSION
Oral Session # 59: Mutualism - Parasitism II: Animal and Insect.
Presiding: L Cochran-Stafira
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 203.
The vein-cutting monarch: a bioassay for vein-cutting stimulants in milkweed latex.
Helmus , Matthew *,1, Dussourd, David2, 1 University of Wisconsin-Madison, Madison, WI2 University of Central Arkansas, Conway, AR
ABSTRACT- Larvae of the Monarch butterfly (Danaus plexippus) eat milkweeds (Asclepiadaceae), plants that when punctured exude a sticky latex. Monarchs sequester milkweed cardenolides, which are toxic to most animals and occur at high concentration in milkweed latex. Paradoxically, monarchs have evolved behaviors that circumvent latex and thus diminish their defensive potential. Before eating, final instar larvae cut the latex canals of a leaf and feed distal to the cuts where latex no longer flows. Although vein cutting clearly functions to reduce larval contact with latex, it is not known what characteristic of latex stimulates this behavior. We have developed a novel bioassay to test for vein-cutting stimulants found in milkweed latex. When given pure latex or a solution containing a vein-cutting stimulant, a feeding monarch will turn and rechew its previous vein-cuts. All larvae tested with fresh latex rechewed their vein cuts, while larvae tested with a sticky polyethylene glycol solution or a solution of cardenolides did not. The supernatant and pellet of centrifuged latex were tested, and the supernatant found to be active. Latex supernatant was dried and washed with methylene chloride (a solvent for cardenolides) and methanol, with the residual dissolved in water. The methanol fraction only, elicited rechewing. Presence for cardenolides was tested in the methanol fraction using a 2,2',4,4'-tetranitrodiphenyl base catalyzed reaction in two methanol fractions. One fraction tested negative while the second showed slight traces(∼0.05mg/ml). Activity was also maintained in a methanol fraction inoculated with a general proteinase suggesting the stimulant(s) is not proteinaceous. These initial results suggest a chemical cue(s) for vein cutting in monarch larvae. Furthermore, since over 100 species of Lepidoptera, Coleoptera and Orthoptera also cut veins on host plants with secretory canals; these test methods (i.e., vein cutting, solution application and rechewing) could be used to characterize vein-cutting stimulants for other taxon.
Key words: milkweed, cardenolide, monarch, vein cutting