Comparison of Expiring CP-25 and 3-Year Old CP-42 Plantings for Monarch Habitat Quality

Monarch butterflies have experienced a population decline of more than 80% in the past two decades, driven by the emergence of Roundup Ready beans and corn varieties. This development resulted in landowners spraying herbicides and killing all other plants in their field, including milkweeds, which is the genus of plants (Asclepias) that monarch caterpillars can only feed upon. Along with this development, an initiative to reintroduce native prairie ecosystems as part of the Conservation Reserve Program (CRP). In Iowa, over 80% of the land once was home to this prairie, now less than 0.1% of this rare ecosystem remains. Reintroducing this prairie to the environment is a necessary step to improving monarch populations. The University of Northern Iowa’s Tallgrass Prairie Center (UNI TPC) is investigating the quality and availability of tallgrass prairie habitat for monarch butterflies. Two CRP programs of interest are the CP-25 and CP-42 plantings. CP-25 are 10-15-year old fields with no milkweed planted and a mixture of grasses and forbs while CP-42 is planted for pollinator use and includes many milkweed and nectar producing plants. There are 218,482 acres of CP-42 planted in Iowa with over 400,000 acres planted countrywide. In 2021, 17 expiring CP-25 research sites were chosen and sampled for plant abundance and ground cover. Nested quadrats provided frequency data to procure 100 m2 of sampling area that was then collated with 3-year old CP-42 site frequency data. This planting comparison was used to evaluate the milkweed density, nectar plant frequency, and frequency by flowering period by changing the time and seed mix. We hypothesized that the younger planting would produce more nectar-producing species and habitat for the monarch butterflies to feed and lay eggs in. Results indicate that CP-42 fields were significantly higher in nectar-producing species abundance in the early flowering period relative to the CP-25 fields (p-value: 0.005); however, overall frequency of nectar plants in the middle and late flowering periods were comparable across seed mixes (p-values of 0.356 and 0.163, respectively). Milkweed density was slightly greater on CP-25 sites, 0.263 stems/m2 as opposed to 0.247 for CP-42, but more data is needed to evaluate the effects of nectar plant flowering time and resource availability on monarch habitat quality. Our results suggest that implementation of fewer milkweed plantings and greater abundance of early season nectar availability is needed to improve monarch larval habitat. CRP sites of differing mixes and ages showed similarities in nectar availability, suggesting that floral resources for monarch butterflies are not a problem