By Dr. Kelsey E. Fisher, Postdoctoral Research Associate, Department of Entomology
Monarchs are a charismatic species for multiple reasons: 1) they have bright, obvious coloration as caterpillars and as butterflies that warn predators to stay away, 2) their impressive multi-generational migration from Mexico to Canada each year, 3) their roosting behavior along wind breaks in the fall when you can see tons of butterflies clustered together, and 4) their overwintering behavior where all the monarchs that were produced east of the Rocky Mountains during the summer months and successfully migrated to Mexico cluster in the same region of the Oyamel fir forest. It is because of the overwintering behavior that we know the monarch population has been declining since the 1990s. This decline is due, in part, to the loss of milkweed and nectar plants in the Monarchs’ breeding range in the agricultural landscape of the Midwest US.
Breeding-season females are vagile and not patch residents, meaning that they travel among habitat patches and distribute eggs across the landscape. Monarch egg abundance and distribution on milkweed across fragmented landscapes is a result of the monarchs’ movement characteristics, such as perceptual distance, flight directionality, and flight step lengths. Improved understandings of female movement with tracking studies in complex agricultural settings are needed to support the development of conservation plans that create functional habitat connectivity and facilitate increased egg-laying behavior across the landscape.
Since 2016, our research team has been attaching radio transmitters to monarch females in hopes of finding how they navigate to habitat patches. Previous studies indicate butterfly movements are influenced by habitat quality, with short step lengths and torturous flight patterns in suitable habitat patches, indicative of foraging behavior, and directed movements while traversing the matrix or low-quality habitat. In support of this notion, we found that monarchs perform torturous movement in a small, 4-ha restored prairie, and we found that monarchs perform directed movement in a 32-ha resource-devoid sod field. In 2019 and 2020, we were determined to expand our observation area to include a complex configuration of habitats with varying densities of milkweed and nectar plants across ~ 64-ha. Contrary to previous findings, we found no difference in flight patterns in response to habitat class (crop field, prairie, or grass-dominated field). In our landscape-scale study, regardless of habitat class (prairie, grass-dominated, roadside, or crop field), most flight steps were below 50 m, and monarchs traveled with directed steps, even though monarchs were foraging in the prairies and resting in the crop fields. This suggests that not only habitat class influences movement patterns, and further studies are necessary to evaluate these relationships.
tower site (left) monarch butterfly with tracker (right)
By using radio telemetry, we had the opportunity to observe behaviors that would not be possible to observe with the naked eye. Monarchs in our studies were successful at locating resources and often performed up-wind flight in hopes of identifying the scent of a milkweed or nectar resource. Some monarchs flew over 500 m across crop fields to find a prairie habitat. Uniquely, in all of our studies, we observed monarchs take large displacement steps (~ 50 – 2,000 m) that appeared to be unguided. Large displacement steps occurred in approximately half of the monarchs we observed and may indicate initiation of a long-range search for suitable habitats, consistent with their vagile behavior. Therefore, to facilitate efficient movement of individuals and increase the chance of monarchs flying in an unguided direction, we suggest establishing habitat patches 50 m apart in agricultural landscapes.
Our studies on monarch butterfly movement ecology provide the most robust dataset of directly tracked movement of individual breeding-season monarch butterflies to assess utilization of agricultural landscapes. This work advances knowledge at a landscape-scale and would not have been possible without the aid of radio telemetry to relocate individuals. Results provide the means to create realistic models that represent natural population dynamics to support conservation planning at a landscape scale.