Effects of Dicamba on monarch oviposition and larval growth and development

Abstract

Eastern monarch butterflies (Danaus plexippus L.) are iconic creatures that migrate thousands of kilometers every year to overwinter in Mexico's Oyamel fir forest. The eastern monarch butterfly population has decreased as much as 84% between 1997 and 2015 (Thogmartin et al., 2017), and it comprises a large proportion of the global monarch population (Brower, 2014). Many factors contribute to this decline, including habitat destruction, lack of host plants, agricultural practices, climate change, and infection by the parasite Ophryocystis elektroscirrha (OE). Herbicide use in crop fields across monarch butterflies breeding range may impact host plants' availability and quality within and adjacent to fields. The rise in herbicide-resistant weeds has resulted in changes in herbicide use patterns. One of these changes is an increase in dicamba use following the introduction of dicamba-resistant soybean and cotton varieties. The increase in dicamba use has raised concerns because of its ability to injure plants outside of the treated field. Thus, experiments were conducted during 2019 and 2020 to determine if dicamba injury on common milkweed (Asclepias syriaca) affected monarch (Danaus plexippus) oviposition preference and larval development. Common milkweed plants in a no-till soybean field were sprayed with 5 g a.e. ha-¹ dicamba or left unsprayed as untreated controls. Common milkweed leaves that emerged within two weeks after dicamba application were malformed, with leaf cupping typical of dicamba injury. Plants were examined for monarch eggs twice a week for 11 weeks. In 2019, 35% fewer eggs were found on dicamba-treated plants than on control plants, but no effect on oviposition was observed in 2020. Caterpillars were fed leaves harvested 27 days after dicamba application. Dicamba treatment did not affect monarch larval weight, pupa weight, adult forewing length, or sex ratio. Larvae fed leaves near the apical bud had greater weight gain than larvae fed older leaves lower on stems, likely due to the higher nutritional quality of young leaves compared to older leaves.