Cattle and bison grazing decreases monarch egg and larval abundance in the Central United States

Abstract

Monarch butterflies have experienced massive population declines and have been proposed to be listed as threatened under the U.S. Endangered Species Act. Milkweeds (Asclepias spp.) are the sole host plant for monarch larvae, and milkweed abundance has dramatically declined in the past 25 years due to increased usage of herbicides and the spread of row crop agriculture that covers 21% of the continental United States. Given the importance of milkweeds to monarch larvae, there has been a major conservation effort to increase milkweed abundance. However, conservation efforts have largely ignored rangeland because of the incorrect assumption that grazers do not consume milkweeds and that milkweed abundance may be near maximum in rangeland. This is an important conservation blind spot because rangeland covers 34% of the continental United States. In 2023, we compared milkweed and juvenile monarch abundance (eggs + larvae) between ungrazed, cattle-grazed, and bison-grazed grassland units at six sites in Nebraska, Iowa, Missouri, and Kansas, United States. The sites are owned by conservation organizations and grazed with moderate intensity. Ungrazed and bison-grazed grassland units did not significantly differ in milkweed abundance, but they had 5.4× more milkweeds than cattle-grazed grassland units. We found juvenile monarch abundance did not significantly differ between bison- and cattle-grazed grassland units, but that juvenile monarch abundance was 6.4× higher in ungrazed grassland units. We conclude that rangeland grazed by cattle and bison can support more than the current level of juvenile monarchs. Although grazing reduced juvenile monarch abundance, continued grazing is important for monarch conservation because rangeland that is not grazed in this study region is typically converted to row crop agriculture or invaded by eastern redcedar and other trees. We also discuss changes to grazing timing that could increase milkweed and monarch abundance.