Effects of elevated carbon dioxide on milkweed and Monarch Butterfly larva and adults

Background/Question/Methods

Elevated carbon dioxide, CO₂, has been shown to affect plant performance and insect feeding preference. Monarch butterfly larva feed on plants in the milkweed (Asclepiadaceae) family as their sole food source. Milkweed imparts predator defense to the larva through toxic chemicals found in the plant.  If elevated carbon dioxide is affecting plant performance, it could in turn affect monarch butterfly larva. Since milkweed is the sole food source for monarch larva, the monarch butterfly population could be at risk. This study examined the effects of three levels of carbon dioxide (subambient, ambient, and elevated) on the growth of three species of milkweed (Asclepias asperula (antelope horn milkweed), A. syriaca (common milkweed), and A. tuberosa (butterfly weed)) and monarch butterfly larvae (Danaus plexippus) feeding preference. Number of leaves, plant height, number of branches, aboveground dry mass, and belowground dry mass of the plants was measured after exposure to the carbon dioxide treatments for fourteen weeks.  Danaus larvae were offered fresh leaf material from plants from each CO₂ treatment level, and were monitored for feeding preference. 

Results/Conclusions

There was a significant difference in plant growth parameters across the three milkweed species. Significant differences were found in aboveground dry mass among all three species with A. asperula significantly lower than all species, and A. tuberosa significantly higher than all other 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 Asclepias syriaca (common milkweed) mean leaf number and number of branches across carbon dioxide treatments. Mean leaf number was significantly greater in elevated levels of CO₂ (104) than seen in ambient (26), or subambient (35). A. syriaca had a significantly greater number of branches in the elevated treatment (12) compared to the ambient (1) and subambient (3). 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 larval feeding preference to leaf tissue among the three Asclepiadaceae 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.