The Effect of Arbuscular Mycorrhizae on Milkweed Growth and its Implications for Western Monarchs

Western monarch butterfly (Danaus plexippus plexippus) populations in the Pacific Northwest have decreased by approximately 99% in recent decades due in part to the destruction of milkweed (Asclepias spp.) habitat. Milkweeds are one of the only plant genera upon which monarchs will lay eggs and the larvae will feed, so milkweed habitat restoration is critical for the future survival and rehabilitation of western monarch butterfly populations. This research aimed to investigate the effect of the root symbiont arbuscular mycorrhizal fungus (AMF) on growth for two species of milkweed native to Oregon’s Willamette Valley. In showy milkweed (A. speciosa), four of the five growth parameters were significantly affected (two increased and two decreased), while in narrow-leaf milkweed (A. fascicularis) only one was affected (decreased). Our results suggest further testing of different milkweed species, as well as different AMF species, would be valuable in evaluating AMF’s relationship with milkweeds to help restore habitat for the highly imperiled western monarch