Survival and oviposition of monarch butterflies (Danaus plexippus) on invasive pale swallow-wort (Cynanchum rossicum) in Massachusetts

Background/Question/Methods

Monarch butterflies (Danaus plexippus) are facing increasing threats to their population, and there is concern about habitat loss both in their summer habitat in North America and in their overwintering sites in Mexico. In addition to these problems, many monarchs are now encountering pale swallow-wort (Cynanchum rossicum), an invasive plant in the same family as their native host plant, at increasing in density in their summer habitat. There have been several reports of monarchs laying eggs on this novel host, but previous studies have found low levels of oviposition. One found no oviposition, one found less than one percent, and one found 10.5–21.7% percent. The experiments took place in New York, Ontario, and Rhode Island, respectively. These studies also found either low or no survival for larvae raised on swallow-wort. We studied monarch oviposition on swallow-wort using organisms collected in Massachusetts in order to determine if their preference was more similar to those in the New York and Ontario or the Rhode Island studies. We were also interested in whether the monarchs were responding to an oviposition stimulant in the swallow-wort or if they would have a similar response to any substrate presented to them.

Results/Conclusions

We found in a series of choice tests that, while monarchs laid significantly more eggs on milkweed than swallow-wort, they chose swallow-wort over species from other families (e.g., apple) . In addition, we observed monarchs tapping their forelegs to test the quality of the host plant prior to oviposition on the swallow-wort, but not on the apple. Monarch performance on swallow-wort was very poor with no individuals surviving to pupation. A significant number of larvae died during molting when they failed to lose their head capsule and often did not completely shed their cuticle. This interaction not only presents a complex conservation problem, but also raises intriguing questions about the potential evolutionary effects this invasive plant might have on the monarchs over time. We suggest that the foreleg tapping behavior and the preference for swallow-wort over apple indicates the potential for monarchs to incorporate it into their host preference hierarchy.