The effects of carbon dioxide on three species of milkweed (Asclepiadaceae) and monarch butterfly (Danaus plexippus) larva feeding preference

This research evaluated the effect of three levels of carbon dioxide (280 ppm = subambient, 363 ppm = ambient, and 457 ppm = elevated) on the growth of three species of Asclepiadaceae, which were Asclepias asperula (antelope horn milkweed), A. syriaca (common milkweed), and A. tuberosa (butterfly weed). Number of leaves, plant length, number of branches, color, aboveground dry mass, and belowground dry mass of the plants were measured after exposure to carbon dioxide treatment for fourteen weeks. Danaus plexippus larvae were offered fresh leaf material from plants from each CO2 treatment level, and were monitored for feeding preference. There was a significant difference in plant growth parameters across the three milkweed species. For dry mass, A. asperula had a significantly lower above-, belowground and total dry mass than the other two species. There was no significant difference in most plant growth parameters for the three species across the carbon dioxide treatments. There was a significant difference seen in A. syriaca mean leaf number and number of branches across carbon dioxide treatments. For number of branches, a significant interaction was found between carbon dioxide treatments and species, indicating that the response to CO2 was dependant on the species examined. There was a significant difference in the Danaus plexippus larval feeding preference to leaf tissue from the three Asclepiadaceae species among the species, with the larvae preferring A. asperula and A. syriaca over A. tuberosa; but no significant difference in feeding preference within a species across carbon dioxide treatments.