(Posted 9/5/18) Monarch butterflies, with their iconic orange and black-patterned wings resembling stained glass, are arguably one of the most loved and recognized insects in North America. Unfortunately, populations have experienced steep declines in recent years. A major factor is loss of their host plants, milkweeds, along their migration route. Milkweeds, genus Asclepias, are crucial to monarch butterflies because they are the only type of plant that larval monarch butterflies will consume and the preferred plant for egg-laying by adult females. The loss of milkweed has increased drastically since the late 1990s as use of the herbicide glyphosate has increased. Glyphosate is the primary chemical used to prevent milkweed from invading agricultural fields, which historically grew abundantly alongside crops.
Many conservation organizations have called for development and maintenance of native habitats in the United States as a strategy to reverse the decline of monarch butterflies. The Rainwater Basin (RWB) region of Nebraska can play an important role in increasing the area of native habitat for monarch butterflies because of its location along one of the primary migration routes between Canada and Mexico and current landcover.
There may be many opportunities to plant milkweed on conservation lands throughout the RWB. Most of these properties contain both wetland and upland habitats that are managed with cattle grazing. However, the ideal conditions for milkweed survival on RWB conservation lands are unknown. To determine if milkweeds will establish and perpetuate on grazed areas with a mix of upland and wetland habitat, we planted milkweeds on two Natural Resources Conservation Service’s Wetlands Reserve Program easement tracts and monitored their survival.
Seedlings of four milkweed species were provided by the Prairie Plains Resource Institute in Aurora, Nebraska, including common milkweed, Sullivant’s milkweed, swamp milkweed, and whorled milkweed. These species were selected because they have a variety of preferences for dry or wet conditions and wetland soils. In September 2015, 940 seedlings were planted in an ungrazed field; the following September, 585 seedlings were transplanted to a grazed field. We determined the success of transplanting milkweeds by revisiting the seedlings in June 2016, May 2017, and August 2017 and recording the number of surviving plants.
Our analysis indicated that survival was similar for all four species in the ungrazed field. The percentage of plants surviving for one year was estimated to be 67%. Milkweed also survived in the grazed field, although we could not make estimates for all four species, likely due to smaller sample size and shorter survey period. Annual survival rates were 29% for common milkweed and 9% for swamp milkweed in the grazed field. We found that many of the aboveground shoots of the surviving milkweed plants had been partially consumed by cattle, making them more difficult to spot.
It appears that milkweed plants can survive on conservation lands that contain wetland habitats and that are managed with grazing. This is important because it indicates that grazing practices and conservation for the benefit of wetlands and monarch butterflies are not mutually exclusive goals. Based on our results, stakeholders should be able to identify one or more milkweed species that will survive in a variety of wetland conditions. This project was conducted in partnership with the Prairie Plains Resource Institute and was funded with a $5,000 grant from Monsanto-Pioneer through the Sand County Foundation. For more information about this project, please contact Dana_Varner@fws.gov.