Old World climbing fern (OWCF) is a highly invasive plant that is spreading rapidly and disrupting natural areas in Florida, with the potential to spread by wind-blown spores into Central and South America. Experiments were performed using gametophytes to determine the dose response of OWCF to metsulfuron methyl (metsulfuron). OWCF gametophytes were highly susceptible to metsulfuron with ≤ 1.4% survival at concentrations ≥ 27 mg ai L⁻¹. Metsulfuron OWCF gametophyte 50 and 95% inhibition (I₅₀ and I₉₅) values were 6.1 and 26.4 mg ai L⁻¹, respectively. Survival of treated gametophytes that developed into sporophytes was ≤ 0.014% at concentrations ≥ 27 mg ai L⁻¹, but no sporophytes developed at concentrations ≥ 432 mg ai L⁻¹. Metsulfuron OWCF sporophyte inhibition I₅₀ and I₉₅ values were 5.6 and 24.1 mg ai L⁻¹, respectively. At 216 mg ai L⁻¹ metsulfuron, half of the standard operational use rate, 1.1 × 10⁻⁸% of the treated gametophytes developed into sporophytes. The results indicate that the potential for tolerant, and potentially resistant, OWCF populations is higher using lower concentrations of metsulfuron as no gametophytes developed into sporophytes at the standard field treatment concentration of 432 mg ai L⁻¹.

Nomenclature: Metsulfuron methyl, Old World climbing fern, Lygodium microphyllum (Cav.) R. Br

Management Implications: Old World climbing fern (Lygodium microphyllum; OWCF) is currently invasive only in Florida outside it native range, but exhibits the potential to spread into areas of high diversity in Central and South America due to microscopic wind-blown spores. No previous research has examined the effects of metsulfuron on gametophytes of this species. OWCF gametophytes were highly susceptible to metsulfuron with ≤ 1.4% survival at concentrations ≥ 27 mg ai L⁻¹ and inhibition concentrations of 6.1 and 26.4 mg ai L⁻¹ for 50 and 95% inhibition rates (I₅₀ and I₉₅), respectively. OWCF sporophytes that developed from treated gametophytes had survival rates of ≤ 0.014% at concentrations ≥ 27 mg ai L⁻¹, with no sporophyte developing at concentrations ≥ 432 mg ai L⁻¹. I₅₀ and I₉₅ concentrations of metsulfuron for OWCF sporophytes that developed from treated gametophytes were 5.6 and 24.1 mg ai L⁻¹, respectively. Our results provide preliminary evidence that the potential for resistance in OWCF populations is higher using lower concentrations of metsulfuron, perhaps through selection for enhanced herbicide metabolism, as no gametophytes developed into sporophytes at the standard field treatment concentration of 432 mg ai L⁻¹.