Intra-annual Spatiotemporal Dynamics of the Monarch Butterfly (Lepidoptera: Danaidae), Regal Fritillary (Lepidoptera: Heliconiinae), and Their Floral Resources in North Dakota, United States

Many butterflies in the United States, along with their nectar resources, are declining. Large-scale conservation planning can be challenging, however, when butterflies and their nectar sources exhibit disparate spatiotemporal dynamics. We initiated a multi-year survey on monarchs (Danaus plexippus), regal fritillaries (Speyeria idalia), and floral resources across North Dakota from 2017 to 2019 to understand resource use through space and time. We surveyed three sites in each county in North Dakota, twice a year for 3 yr, for a total of 954 site visits. Our objectives were to 1) identify important floral resources for monarchs and regal fritillaries, 2) determine intra-annual spatial and temporal distributions of the butterflies and their nectar sources, and 3) quantify how floral resources influenced butterfly abundance and interacted with spatiotemporal factors (latitude, longitude, and flight season). Milkweeds (Asclepias spp.), native thistles (Cirsium spp.), and blazing stars (Liatris spp.) were identified as particularly important nectar resources for monarchs and regal fritillaries; all plant and butterfly species exhibited unique spatiotemporal distributions. Monarch abundance at a site was positively related to the availability of flowering milkweeds, and regal fritillary abundance was positively related to the floral availability of native thistles and coneflowers (Echinacea spp.). These relationships were consistent across the spatial and temporal extent of our study, indicating that the butterfly–flower relationships we detected are maintained and relatively consistent across our study region and years. By accounting for spatiotemporal dynamics, we can help elucidate the persistence of butterfly-flower species interactions at large scales and inform conservation efforts for these threatened species.