Combination effects of herbicides on plants and algae: do species and test systems matter?

Abstract

Risk assessment of herbicides towards non-target plants in Europe is currently based solely on tests on algae and floating aquatic plants of Lemna sp. Effects on terrestrial non-target species is not systematically addressed. The purpose of the present study was to compare combination effects of herbicide mixtures across aquatic and terrestrial test systems, and to test whether results obtained in the traditional aquatic test systems can be extrapolated to the terrestrial environment. This was done by evaluating ten binary mixtures of nine herbicides representing the seven most commonly used molecular target sites for controlling broadleaved weeds. Data were evaluated statistically in relation to the concentration addition model, and for selected concentrations to the independent action model. The mixtures were tested on the terrestrial species Tripleurospermum inodorum (L.) Schultz-Bip. (Scentless Mayweed) and Stellaria media (L.) Vill. (Common Chickweed), and on the aquatic species Lemna minor L. (Lesser duckweed) and the alga Pseudokirchneriella subcapitata (Korschikov) Hindak. For the two mixtures of herbicides with the same molecular site of action, the joint effect was additive. For the eight mixtures of herbicides with different sites of action, two of the mixtures were consistently antagonistic across species, while for the remaining six mixtures the joint effect depended on the species tested. This dependence was, however, not systematic, in the sense that none of the species or test systems (terrestrial versus aquatic) had a significantly higher probability of showing synergistic or antagonistic joint effects than others. Synergistic interactions were not observed, but approximately 70% of the mixtures of herbicides with different sites of action showed significant antagonism. Hence, the concentration addition model can be used to estimate worst-case effects of mixtures of herbicides on both terrestrial and aquatic species. Comparing the sensitivity of the species to a 10% spray drift event showed that the terrestrial species were more vulnerable to all herbicides compared with the aquatic species, emphasising the importance of including terrestrial non-target plants in herbicide risk assessment.

Growth rate of Pseudokirchneriella subcapitata exposed to herbicides found in surface waters in the Alqueva reservoir (Portugal): a bottom-up approach using binary mixtures

“…Contrary to our findings, antagonism was the most common type of interaction found in the study of Cedergreen et al (2007b), where effects of herbicide mixtures on aquatic and terrestrial test systems were assessed. In this study the authors found asymmetric isoboles for the mixture of terbuthylazine and diquat on the terrestrial plant Stellaria media, suggesting that only a small amount of terbuthylazine would reduce the effect of diquat dramatically, whereas small amounts of diquat would have only a small effect on the efficacy of terbuthylazine (Cedergreen et al 2007b).…”

Organophosphorous insecticides as herbicide synergists on the green algae Pseudokirchneriella subcapitata and the aquatic plant Lemna minor

“…1a, Table 3). The EC 50 of metsulfuron in this experiment was, however, rather high compared to previous experiments (Table 2) (Cedergreen et al 2007b). A repetition of the experiment could not reproduce the synergy but showed no deviation from either of the reference models (Fig.…”

“…It was therefore possible to make mixture ratio experiments with malathion resulting in dose-response surfaces (Greco et al 1995). This was done according to Cedergreen et al (2007b) using mixture ratios chosen to give a division of effect between the herbicide and the insecticide of 100:0, 83:17, 63:37, 50:50, 37:63, 17:83, 0:100% based on EC 50 values from previous experiments.…”

Macrophyte Bioassay Applications for Monitoring Pesticides in the Aquatic Environment

“…For L. minor, the lethal concentration of 50% (CL50;7d) of atrazine was 5.27 mg L (Cedergreen et al, 2007); to the atrazine, isoproturon and alachlor mixture (50+35+15%), with CL50;96h of 70.0 µg L -1 (Coutris et al, 2011); and than imazapyr, with CL50;7d of 1.06 mg L -1 (Cruz et al, 2015); however, it was more sensitive to glyphosate (CL50 of 19.6 mg L -1 ) and to mecoprop, with 12.2 mg L -1 (Cedergreen et al, 2007 , and epoxiconazole, of 1.14 mg L -1 ; therefore, these authors mention that the magnification factor of the toxicity corresponded to 13.6 times in relation to the individual toxicity of each compound; however, this effect was not characterized for toxicity on L. minor.…”