Oviposition preference and larval performance of monarch butterflies (Danaus plexippus) on two invasive swallow‐wort species

The potential of two invasive herbaceous vinesVincetoxicum nigrum(L.) Moench andVincetoxicum rossicum(Kleopow) Barbar. (Asclepiadaceae) to reduce monarch butterfly (Danaus plexippusL.) (Lepidoptera: Nymphalidae, Danainae) populations was investigated by evaluating oviposition selection in adult monarch butterflies and larval feeding preference in choice tests comparing the native host plant of monarch butterflies,Asclepias syriacaL. (Asclepiadaceae) and the two non‐indigenousVincetoxicumspecies. In both choice and no‐choice tests, no eggs were oviposited on either of the twoVincetoxicumspecies whereas over 66 eggs per female were oviposited onA. syriacaplants. All first instar larvae allowed to feed onA. syriacafor 48 h survived while a significantly lower proportion survived onV. rossicum(44%) andV. nigrum(14%). Mean weight of larvae that did survive on theVincetoxicumspecies was significantly lower than the mean weight of larvae that fed onA. syriaca. The mean weight of surviving larvae, however, did not differ between the twoVincetoxicumspecies. The mean proportion of leaves consumed by larvae feeding onA. syriacawas significantly greater than the mean proportion of leaves consumed by larvae feeding on eitherVincetoxicumspecies. Findings from this research indicate thatV. rossicumandV. nigrumare not viable hosts of monarch butterflies and are likely to pose little direct threat to their populations as oviposition sinks. The ability of these highly aggressive plants, however, to out‐compete and displace the native host of monarchs,A. syriaca, may pose a more serious threat. The potential of monarch populations to adapt to the twoVincetoxicumspecies as host plants over the long‐term is discussed.