Direct effects of pesticides and other grassland management practices on the North American Monarch Butterfly (Danaus plexippus plexippus): A systematic review

This review synthesizes recent scientific literature on the effects of grassland management practices and pesticide applications on monarch butterflies (Danaus plexippus), focusing on studies published since the 2020 U.S. Fish and Wildlife Service Species Status Assessment (SSA). While the SSA highlighted habitat conservation efforts to enhance monarch populations, there has been limited empirical research on the direct effects of these management strategies on monarch survival and fitness. Following PRISMA guidelines, we conducted a systematic review to evaluate research published during and after the SSA (2019-2024) on the direct effects of common grassland management practices (mowing, grazing, burning, haying) and pesticides (herbicides, insecticides, fungicides) across monarch life stages. This search identified 29 studies that met our criteria. Of these, only two studies addressed the impacts of grassland management on monarchs, indicating a substantial and important information gap. The other 27 studies reviewed indicate significant risks of pesticide exposure to monarchs, particularly at immature life stages from lethal and sublethal effects. The effects of chronic insecticide exposure, particularly neonicotinoids, are especially critical to consider, as research shows insecticides can result in impairment of growth, development, and reproductive behaviors. Additionally, fungicides pose overlooked risks to monarchs due to their toxicity to non-target organisms. Notably, most studies evaluating pesticide effects were conducted in a laboratory setting and thus it is unclear how these results relate to fieldrealistic exposure, as well as exposure over multiple life history stages and potential synergistic effects of pesticides. Additional relevant information gaps remain, especially on studies of southern migratory paths critical for monarch population persistence. Data gaps at relevant scales for grassland management combined with growing evidence of pesticide persistence and subsequent threats to insects, highlight how more ecologically applicable metrics for monitoring environmental chemicals could help improve understanding of the effects of pesticides on monarchs and characterize contamination of milkweed and nectar resources.