Herbicide Toxicity Testing with Non-Target Boreal Plants: The Sensitivity of Achillea millefolium L. and Chamerion angustifolium L. to Triclopyr and Imazapyr

Abstract

Terrestrial plant toxicity tests were conducted to determine the sensitivity of two boreal plants, yarrow (Achillea millefolium L.) and fireweed (Chamerion angustifolium L.), to the herbicides imazapyr and triclopyr. Both plants are common non-target species on northern powerline rights-of-way where the impacts of proposed herbicide applications are of concern. In the vegetative vigour test, triclopyr foliar spray caused extensive damage to A. millefolium at <50% of the maximum field application rate (inhibition concentration (IC) = 1443.8 g a.i. ha) and was lethal to C. angustifolium at the lowest dose tested (1210.9 g a.i. ha). Both species demonstrated extremely high sensitivity to imazapyr foliar spray: IC = 8.29 g a.i. ha and 4.82 g a.i. ha (<1.5% of the maximum field rate). The seedling emergence and seedling growth tests were conducted in the organic horizon of five boreal soils. Few differences in herbicide bioavailability between soils were detected. Triclopyr limited growth of A. millefolium, C. angustifolium and standard test species Calamagrostis canadensis at low levels (most IC estimates between 2-20 µg g). For imazapyr, IC estimates could not be calculated as there was >75% inhibition of endpoints at the lowest doses of ~2 µg g. A foliar application of triclopyr or imazapyr for woody species control would likely cause significant damage to boreal non-target plants. The high sensitivity of both species to herbicide residues in soil indicates long term impacts are dependent on herbicide degradation rates in northern conditions. A. millefolium performed well and is recommended for use in toxicity testing relevant to boreal regions.

Linking Herbicide Dissipation to Soil Ecological Risk along Transmission Rights‐of‐Way in the Yukon Territory, Canada

“…Consideration of indirect effects associated with herbicide application is needed to understand large-scale ecosystem response to herbicide use at northern latitudes. Using all lines of evidence, including the data presented in this study, as well as the vegetation studies conducted by Isbister et al (2017Isbister et al ( , 2018, triclopyr appears to present less potential risk overall. This hypothesis is based on relatively rapid initial dissipation (mobile-phase DT 50 of 1 DAT), low risk to soil invertebrates, and nontarget vegetative recovery within one growing season.…”

“…and <10% damage to nontarget forbs in field plots 365 DAT (Isbister et al, 2017, 2018). For imazapyr, LC 50 estimates were at 1.5% of the maximum application rate for both species, and damage assessments from field plots showed 25 to 35% damage to nontarget forbs 365 DAT (Isbister et al (2017,2018). Given this evidence and the data presented here, triclopyr may be a better choice for implementation along Yukon Territory ROWs.…”

“…In summer 2014 and 2015, five sites (Carmacks [CAR], Dawson [DAW], Haines Junction 1 [HJ1], Haines Junction 2 [HJ2], and Little Salmon [LS]) were selected along transmission ROWs in the Yukon Territory, Canada, to assess the dissipation of two herbicides, triclopyr and imazapyr, in northern soils (Supplemental Table S1). Isbister et al (2017, 2018) provided full site descriptions and herbicide application details and reported on both the initial efficacy of the herbicides for woody species control and the potential for nontarget plant effects. In brief, a complete randomized block design with three blocks consisting of six 6‐m 2 treatment plots were installed at each site.…”

“…Table S1). Isbister et al (2017Isbister et al ( , 2018 provided full site descriptions and herbicide application details and reported on both the initial efficacy of the herbicides for woody species control and the potential for nontarget plant effects. In brief, a complete randomized block design with three blocks consisting of six 6-m 2 treatment plots were installed at each site.…”

“…Although acidic herbicides sorb weakly, soil properties including pH, clay content and soil organic matter can alter the degree to which the herbicides adsorb to soil particles (Johnson et al, 1995a; Gianelli et al, 2014; Douglass et al, 2016). In the Yukon Territory, initial testing of triclopyr and imazapyr for use along ROWs suggested that, while both herbicides are likely to be effective for woody control, persistence of imazapyr in soil may impact non‐target plant species (Isbister et al, 2017; 2018). Thus, it is important not only to assess the dissipation of these herbicides but also the response of soil dwelling invertebrates to herbicide exposure in northern soils (Princz et al, 2012).…”