The potential benefits of herbicide regulation: a cautionary note for the Great Barrier Reef catchment area

Pesticides in the Great Barrier Reef catchment area: Plausible risks to fish populations

Waterways that drain the Great Barrier Reef catchment area (GBRCA) transport pollutants to marine habitats, provide a critical corridor between freshwater and marine habitats for migratory fish species, and are of high socioecological value. Some of these waterways contain concentrations of pesticide active ingredients (PAIs) that exceed Australian ecotoxicity threshold values (ETVs) for ecosystem protection. In this article, we use a "pathway to harm" model with five key criteria to assess whether the available information supports the hypothesis that PAIs are or could have harmful effects on fish and arthropod populations. Strong evidence of the first three criteria and circumstantial weaker evidence of the fourth and fifth criteria are presented. Specifically, we demonstrate that exceedances of Australian and New Zealand ETVs for ecosystem protection are widespread in the GBRCA, that the PAI contaminated water occurs (spatially and temporally) in important habitats for fisheries, and that there are clear direct and indirect mechanisms by which PAIs could cause harmful effects. The evidence of individuals and populations of fish and arthropods being adversely affected species is more circumstantial but consistent with PAIs causing harmful effects in the freshwater ecosystems of Great Barrier Reef waterways. We advocate strengthening the links between PAI concentrations and fish health because of the cultural values placed on the freshwater ecosystems by relevant stakeholders and Traditional Owners, with the aim that stronger links between elevated PAI concentrations and changes in recreationally and culturally important fish species will inspire improvements in water quality. Integr Environ Assess Manag 2024;20:1256-1279. © 2023 Commonwealth of Australia and The Commonwealth Scientific and Industrial Research Organisation. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Assessing the negative impact of chlorantraniliprole, isoxaflutole, and simazine pesticides on phospholipid membrane models and tilapia gill tissues

The indiscriminate and, very often, incorrect use of pesticides in Brazil, as well as in other countries, results in severe levels of environmental pollution and intoxication of human life. Herein, we studied plasma membrane models (monolayer and bilayer) of the phospholipid Dioleoyl-sn-glycerol-3-phosphocholine (DOPC) using Langmuir films, and large (LUVs) and giant (GUVs) unilamellar vesicles, to determine the effect of the pesticides chlorantraniliprole (CLTP), isoxaflutole (ISF), and simazine (SMZ), used in sugarcane. CLTP affects the lipid organization of the bioinspired models of DOPC π-A isotherms, while ISF and SMZ pesticides significantly affect the LUVs and GUVs. Furthermore, the in vivo study of the gill tissue in fish in the presence of pesticides (2.0 × 10-10 mol/L for CLTP, 8.3 × 10-9 mol/L for ISF, and SMZ at 9.9 × 10-9 mol/L) was performed using optical and fluorescence images. This investigation was motivated by the gill lipid membranes, which are vital for regulating transporter activity through transmembrane proteins, crucial for maintaining ionic balance in fish gills. In this way, the presence of phospholipids in gills offers a model for understanding their effects on fish health. Histological results show that exposure to CLTP, ISF, and SMZ may interfere with vital gill functions, leading to respiratory disorders and osmoregulation dysfunction. The results indicate that exposure to pesticides caused severe morphological alterations in fish, which could be correlated with their impact on the bioinspired membrane models. Moreover, the effect does not depend on the exposure period (24h and 96h), showing that animals exposed to pesticides for a short period suffer irreparable damage to gill tissue. In summary, we can conclude that the harm caused by pesticides, both in membrane models and in fish gills, occurs due to contamination of the aquatic system with pesticides. Therefore, water quality is vital for the preservation of ecosystems.

Toxicity of herbicides to the marine microalgae Tisochrysis lutea and Tetraselmis sp

Pesticides are ubiquitous in the catchments of the Great Barrier Reef (GBR) and regularly discharge into the nearshore waters. Effective management of pesticides requires suitable water quality guideline values (WQGVs), and further ecotoxicological data for many pesticides are needed to improve the reliability of environmental risk assessments. To help address this issue, toxicity thresholds were determined to two species of tropical marine microalgae Tisochrysis lutea and Tetraselmis sp. for a suite of herbicides detected in the GBR. Photosystem II (PSII) herbicides significantly reduced growth with no effect concentration (NEC) and 10% effect concentration (EC10) values spanning two orders of magnitude from 0.60 µg L-1 for diuron to 60 µg L-1 for simazine across both species. However, growth was insensitive to the non-PSII herbicides. The NEC/EC10 thresholds for most herbicide-microalgae combinations were greater than recent WQGVs intended to protect 99% of species (PC99); however, metribuzin was toxic to T. lutea at concentrations lower than the current PC99 value, which may have to be revisited. The toxicity thresholds for alternative herbicides derived here further inform the development of national and GBR-specific WQGVs, but more toxicity data is needed to develop WQGVs for the > 50 additional pesticides detected in catchments of the GBR.

A Pesticide Decision Support Tool to guide the selection of less environmentally harmful pesticides for the sugar cane industry

Pesticides applied to agricultural land have been shown to decrease the quality of water entering the Great Barrier Reef lagoon. This issue is addressed by the Reef 2050 Water Quality Improvement Plan which includes a pesticide reduction target. As part of a wider educational strategy, one method that could help meet the target is to provide stakeholders with information that assists in the selection and use of pesticide active ingredients (PAIs) that pose a lower risk to aquatic environments compared to those currently used. This study developed a Pesticide Decision Support Tool (PDST) in collaboration with stakeholders for the sugar cane industry. The PDST covers all PAIs registered and applied to sugar cane in Australia and four additional PAIs registered for use on crops grown in rotation with sugar cane. The PDST incorporates both the measure of mobility and persistence of a PAI and the measure of effect, which is based on the PAI application rate and ecotoxicity threshold value. The aquatic risk, which is the product of the measure of effect and the measure of mobility and persistence, is a measure of the likelihood that a PAI will reach the aquatic environment and cause harmful effects. Insecticide active ingredients (e.g., cadusafos, chlorpyrifos) posed the greatest aquatic risk, followed by herbicide active ingredients (e.g., MSMA, metolachlor), while fungicide AIs typically had a lower aquatic risk. An interactive spreadsheet allows characteristics, including application rate and tank mixes, to be considered when assessing the potential risk. While focusing on sugar cane, the results are equally appropriate to other crops that use the same PAIs provided the application rates are corrected to the new crop. In addition, the approach used in the PDST can be applied internationally and to any PAIs with sufficient toxicity, mobility, and persistence data.

Direct comparison of runoff of residual and knockdown herbicides in sugarcane using a rainfall simulator finds large difference in runoff losses and toxicity relative to diuron

Runoff losses of herbicides have rarely been compared simultaneously under the same conditions. Our aim was to directly compare herbicide runoff losses, normalised for the amount present (relative runoff loads) and in absolute terms. Toxicity and runoff concentrations were combined to provide a risk ranking relative to diuron. Four rainfall simulation trials were conducted in sugarcane in the Great Barrier Reef catchment. Herbicides studied were older PSII residuals (atrazine, ametryn, diuron, hexazinone), alternative residuals (isoxaflutole, imazapic, metribuzin, metolachlor, pendimethalin) and knockdown herbicides (glyphosate, 2,4-D, fluroxypyr) and the tracer bromide (Br). Simulations were conducted two days after spraying, before differences due to half-lives were apparent. Two trials had bare soil and two had sugarcane trash. Herbicide runoff losses and concentrations were closely related to the amount applied, runoff amounts and partitioning coefficients. Relative runoff losses and absolute losses were similar for most older and alternative residual herbicides, 2,4-D and Br. Glyphosate and pendimethalin relative runoff losses were low, due to greater sorption. Isoxaflutole, imazapic, and fluroxypyr are applied at much lower rates and runoff losses were low. Herbicides were lost in the dissolved phase, except pendimethalin. There was a large range in toxicity relative to diuron. There is a range of herbicide choices posing less offsite risk than diuron and ametryn, which have high application rates and high toxicity. Herbicide choice should consider application rate, runoff losses, sorption, and toxicity.

Toxicity thresholds of nine herbicides to coral symbionts (Symbiodiniaceae)

Over 30 herbicides have been detected in catchments and waters of the Great Barrier Reef (GBR) and their toxicity to key tropical species, including the coral endosymbiotic algae Symbiodiniaceae, is not generally considered in current water quality guideline values (WQGVs). Mutualistic symbionts of the family Symbiodiniaceae are essential for the survival of scleractinian corals. We tested the effects of nine GBR-relevant herbicides on photosynthetic efficiency (ΔF/F_m′) and specific growth rate (SGR) over 14 days of cultured coral endosymbiont Cladocopium goreaui (formerly Symbiodinium clade C1). All seven Photosystem II (PSII) herbicides tested inhibited ΔF/F_m′ and SGR, with toxicity thresholds for SGR ranging between 2.75 and 320 µg L⁻¹ (no effect concentration) and 2.54–257 µg L⁻¹ (EC₁₀). There was a strong correlation between EC₅₀s for ΔF/F_m′ and SGR for all PSII herbicides indicating that inhibition of ΔF/F_m′ can be considered a biologically relevant toxicity endpoint for PSII herbicides to this species. The non-PSII herbicides haloxyfop and imazapic did not affect ΔF/F_m′ or SGR at the highest concentrations tested. The inclusion of this toxicity data for Symbiodiniaceae will contribute to improving WQGVs to adequately inform risk assessments and the management of herbicides in tropical marine ecosystems.

Characterisation of a 4A QTL for Metribuzin Resistance in Wheat by Developing Near-Isogenic Lines

Wheat (Triticum aestivum L.) production is constantly affected by weeds in the farming system. Chemical-based weed management is widely practiced; broad-spectrum herbicides such as metribuzin have been successfully used to control weeds in Australia and elsewhere of the world. Breeding metribuzin-resistant wheat through genetic improvement is needed for effective control of weeds. Quantitative trait loci (QTLs) mapping efforts identified a major QTL on wheat chromosome 4A, explaining up to 20% of the phenotypic variance for metribuzin resistance. The quantitative nature of inheritance of this QTL signifies the importance of near-isogenic lines (NILs), which can convert a quantitative trait into a Mendelian factor for better resolution of the QTL. In the current study, NILs were developed using a heterogeneous inbred family method combined with a fast generation-cycling system in a population of Chuan Mai 25 (resistant) and Ritchie (susceptible). Seven pairs of NILs targeting the 4A QTL for metribuzin resistance were confirmed with a molecular marker and phenotyping. The resistant allele from the resistant parent increased metribuzin resistance by 63-85% (average 69%) compared with the susceptible allele from the susceptible parent. Segregation analysis in the NIL pairs for thousand grain weight (TGW) (g), biomass per plant (kg), tillers per plant, plant height (cm), yield per plant, and powdery mildew visual score (0-9) indicated that these traits were linked with metribuzin resistance. Similarly, TGW was observed to co-segregate with metribuzin resistance in most confirmed NILs, signifying that the two traits are controlled by closely linked genes. The most contrasting NILs can be further characterised by transcriptomic and proteomic analyses to identify the candidate genes responsible for metribuzin resistance.

Toxicity of the herbicides diuron, propazine, tebuthiuron, and haloxyfop to the diatom Chaetoceros muelleri

Conventional photosystem II (PSII) herbicides applied in agriculture can pose significant environmental risks to aquatic environments. In response to the frequent detection of these herbicides in the Great Barrier Reef (GBR) catchment area, transitions towards 'alternative' herbicides are now widely supported. However, water quality guideline values (WQGVs) for alternative herbicides are lacking and their potential ecological impacts on tropical marine species are generally unknown. To improve our understanding of the risks posed by some of these alternative herbicides on marine species under tropical conditions, we tested the effects of four herbicides on the widely distributed diatom Chaetoceros muelleri. The PSII herbicides diuron, propazine, and tebuthiuron induced substantial reductions in both 24 h effective quantum yields (ΔF/Fm') and 3-day specific growth rates (SGR). The effect concentrations, which reduced ΔF/Fm' by 50% (EC50), ranged from 4.25 µg L-1 diuron to 48.6 µg L-1 propazine, while the EC50s for SGR were on average threefold higher, ranging from 12.4 µg L-1 diuron to 187 µg L-1 tebuthiuron. Our results clearly demonstrated that inhibition of ΔF/Fm' in PSII is directly linked to reduced growth (R2 = 0.95) in this species, further supporting application of ΔF/Fm' inhibition as a valid bioindicator of ecological relevance for PSII herbicides that could contribute to deriving future WQGVs. In contrast, SGR and ΔF/Fm' of C. muelleri were nonresponsive to the non-PSII herbicide haloxyfop at the highest concentration tested (4570 µg L-1), suggesting haloxyfop does not pose a risk to C. muelleri. The toxicity thresholds (e.g. no effect concentrations; NECs) identified in this study will contribute to the derivation of high-reliability marine WQGVs for some alternative herbicides detected in GBR waters and support future assessments of the cumulative risks of complex herbicide mixtures commonly detected in coastal waters.

Toxicity thresholds of three insecticides and two fungicides to larvae of the coral Acropora tenuis

Tropical marine ecosystems, such as coral reefs, face several threats to their health and resilience, including poor water quality. Previous studies on the risks posed by pesticides have focused on five priority herbicides; however, as the number of pesticides applied in coastal agriculture increases, a suite of 'alternative' pesticides is being detected in tropical nearshore waters. To improve our understanding of the risks posed by alternative pesticides to tropical marine organisms, the effects of three insecticides (diazinon, fipronil, imidacloprid) and two fungicides (chlorothalonil, propiconazole) were tested on larval metamorphosis of the coral Acropora tenuis. A. tenuis larvae were affected by all five pesticides and the reference toxicant copper. The no effect concentration (NEC) and the 10% and 50% effect concentrations (EC10 and EC50, respectively) for larval metamorphosis were estimated from concentration-response curves after 48 h exposure. The NEC, EC10 and EC50 (in µg L-1), respectively, of each pesticide were as follows: chlorothalonil (2.4, 2.8, 6.0); fipronil (12.3, 13.9, 29.1); diazinon (38.0, 40.8, 54.7); imidacloprid (263, 273, 347); and propiconazole (269, 330, 1008). These toxicity thresholds are higher than reported concentrations in monitoring programs; however, these data will contribute to improving water quality guideline values, which inform the total risk assessments posed by complex contaminant mixtures to which these pesticides contribute.

Combining weed efficacy, economics and environmental considerations for improved herbicide management in the Great Barrier Reef catchment area

The current Australian sugarcane industry transition toward adoption of an 'alternative' herbicide strategy as part of improved environmental stewardship is increasingly complicated by recent farming system, regulatory and herbicidal product changes. This study quantified and compared the efficacy, economic costs and environmental risk profiles of a range of established, emerging, and recently registered pre-emergent herbicides across field trials in the Wet Tropics region of North Queensland. Several herbicides were effective on certain weed species, but lacked broad spectrum control. Better efficacy results from products with multiple active ingredients (i.e., imazapic-hexazinone) demonstrated the benefits of using mixtures of active ingredients to widen the spectrum of weed control efficacy. All tested pre-emergent herbicides behaved quite similarly in terms of their propensity for off-site movement in water (surface runoff losses generally >10% of active applied), with their losses largely driven by their application rate. Herbicides with lower application rates consistently contributed less to the total herbicide loads measured in surface runoff. Results demonstrated alternative choices from the more environmentally problematic herbicides (such as diuron) are available with effective alternative formulations providing between 4 and 29 times less risk than the traditional diuron-hexazinone 'full rate'. However, considerable challenges still face canegrowers in making cost-effective decisions on sustainable herbicide selection. Additional research and effective grower extension are required to address information gaps in issues such as specific weed control efficacy of alternative herbicides and potential blending of some herbicides for more effective broad spectrum weed control, while also minimising environmental risks.

Toxicity of ten herbicides to the tropical marine microalgae Rhodomonas salina

Herbicide contamination of nearshore tropical marine ecosystems is widespread and persistent; however, risks posed by most ‘alternative’ herbicides to tropical marine microalgae remain poorly understood. Experimental exposures of the important but understudied microalgae Rhodomonas salina to seven individual Photosystem II (PSII) inhibitor herbicides (diuron, metribuzin, hexazinone, tebuthiuron, bromacil, simazine, propazine) led to inhibition of effective quantum yield (ΔF/F_m′) and subsequent reductions in specific growth rates (SGR). The concentrations which reduced ΔF/F_m′ by 50% (EC₅₀) ranged from 1.71-59.2 µg L⁻¹, while the EC₅₀s for SGR were 4-times higher, ranging from 6.27-188 µg L⁻¹. Inhibition of ΔF/F_m′ indicated reduced photosynthetic capacity, and this correlated linearly with reduced SGR (R² = 0.89), supporting the application of ∆F/F_m’ inhibition as a robust and sensitive indicator of sub-lethal toxicity of PSII inhibitors for this microalga. The three non-PSII inhibitor herbicides (imazapic, haloxyfop and 2,4-Dichlorophenoxyacetic acid (2,4-D)) caused low or no toxic responses to the function of the PSII or growth at the highest concentrations tested suggesting these herbicides pose little risk to R. salina. This study highlights the suitability of including R. salina in future species sensitivity distributions (SSDs) to support water quality guideline development for the management of herbicide contamination in tropical marine ecosystems.

Quantifying Water Quality Improvements through Use of Precision Herbicide Application Technologies in a Dry-Tropical, Furrow-Irrigated Cropping System

This study compared water quality effects of using precision herbicide application technologies and traditional spraying approaches across several regulated ‘priority’ and alternative pre- and post-emergent herbicides in a northern Australian cane farming system. Use of herbicide banding spray technologies resulted in pre-emergent herbicide load reductions, extending substantially beyond simple proportionate decreases in the amount of herbicide ingredient applied to paddocks. Aquatic risk assessment from resultant chemical mixtures leaving paddocks, and upscaled to local catchment concentrations, highlighted that precision application technologies could markedly reduce the ecological risk of pre-emergent herbicides. These risk reductions were, however, often complicated by the additional toxicity of post-emergent herbicides in mixtures, some associated with the adoption of band-spraying weed treatments. While the currently regulated priority herbicide, diuron, posed the greatest risk to the environment, alternative herbicides could still pose significant environmental risks, although these relative risks were lower at more ecologically relevant concentrations, typically found in the local freshwater ecosystems. Results underline the need for a carefully considered approach to integrating alternative herbicides and precision application technologies into improved weed management by irrigating cane farmers. Recent government changes to the appraisal of water quality improvement progress, from load-based to ecosystem-based targets, involving a much broader suite of herbicides, also appear likely to complicate assessment of the environmental impacts of practice change adoption for the industry.

Degradation of Herbicides in the Tropical Marine Environment: Influence of Light and Sediment

Widespread contamination of nearshore marine systems, including the Great Barrier Reef (GBR) lagoon, with agricultural herbicides has long been recognised. The fate of these contaminants in the marine environment is poorly understood but the detection of photosystem II (PSII) herbicides in the GBR year-round suggests very slow degradation rates. Here, we evaluated the persistence of a range of commonly detected herbicides in marine water under field-relevant concentrations and conditions. Twelve-month degradation experiments were conducted in large open tanks, under different light scenarios and in the presence and absence of natural sediments. All PSII herbicides were persistent under control conditions (dark, no sediments) with half-lives of 300 d for atrazine, 499 d diuron, 1994 d hexazinone, 1766 d tebuthiuron, while the non-PSII herbicides were less persistent at 147 d for metolachlor and 59 d for 2,4-D. The degradation of herbicides was 2–10 fold more rapid in the presence of a diurnal light cycle and coastal sediments; apart from 2,4-D which degraded more slowly in the presence of light. Despite the more rapid degradation observed for most herbicides in the presence of light and sediments, the half-lives remained > 100 d for the PS II herbicides. The effects of light and sediments on herbicide persistence were likely due to their influence on microbial community composition and its ability to utilise the herbicides as a carbon source. These results help explain the year-round presence of PSII herbicides in marine systems, including the GBR, but more research on the transport, degradation and toxicity on a wider range of pesticides and their transformation products is needed to improve their regulation in sensitive environments.

Enzyme activity and microorganisms diversity in soil contaminated with the Boreal 58 WG herbicide

Next-generation herbicides are relatively safe when used properly, but the recommended rates are relatively low, which can lead to overdosing. This study evaluated the responses of soil-dwelling microorganisms and soil enzymes to contamination with the Boreal 58 WG herbicide. The analyzed product contains active ingredients flufenacet and isoxaflutole. All tests were performed under laboratory conditions. The analyzed material was sandy clay. Boreal 58 WG was introduced to soil in four doses. Soil without the addition of the herbicide served as the control. The soil was mixed with the tested herbicide, and its moisture content was maintained at 50% of capillary water capacity. Biochemical and microbiological analyses were performed on experimental days 0, 20, 40, 80 and 160. Accidental contamination of soil with the Boreal 58 WG herbicide led to a relatively minor imbalance in the soil microbiological and biochemical profile. The herbicide dose influenced dehydrogenase activity in only 0.84%, urease activity in 2.04%, β-glucosidase activity in 8.26%, catalase activity in 12.40%, arylsulfatase activity in 12.54%, acid phosphatase activity in 42.11%, numbers of organotrophic bacteria in 18.29%, actinomyces counts in 1.31% and fungi counts in 6.86%.

Effect of sugarcane cropping systems on herbicide losses in surface runoff

Herbicide runoff from cropping fields has been identified as a threat to the Great Barrier Reef ecosystem. A field investigation was carried out to monitor the changes in runoff water quality resulting from four different sugarcane cropping systems that included different herbicides and contrasting tillage and trash management practices. These include (i) Conventional - Tillage (beds and inter-rows) with residual herbicides used; (ii) Improved - only the beds were tilled (zonal) with reduced residual herbicides used; (iii) Aspirational - minimum tillage (one pass of a single tine ripper before planting) with trash mulch, no residual herbicides and a legume intercrop after cane establishment; and (iv) New Farming System (NFS) - minimum tillage as in Aspirational practice with a grain legume rotation and a combination of residual and knockdown herbicides. Results suggest soil and trash management had a larger effect on the herbicide losses in runoff than the physico-chemical properties of herbicides. Improved practices with 30% lower atrazine application rates than used in conventional systems produced reduced runoff volumes by 40% and atrazine loss by 62%. There were a 2-fold variation in atrazine and >10-fold variation in metribuzin loads in runoff water between reduced tillage systems differing in soil disturbance and surface residue cover from the previous rotation crops, despite the same herbicide application rates. The elevated risk of offsite losses from herbicides was illustrated by the high concentrations of diuron (14μgL(-1)) recorded in runoff that occurred >2.5months after herbicide application in a 1(st) ratoon crop. A cropping system employing less persistent non-selective herbicides and an inter-row soybean mulch resulted in no residual herbicide contamination in runoff water, but recorded 12.3% lower yield compared to Conventional practice. These findings reveal a trade-off between achieving good water quality with minimal herbicide contamination and maintaining farm profitability with good weed control.

Pesticide Behavior, Fate, and Effects in the Tropics: An Overview of the Current State of Knowledge

This special issue presents a collection of papers covering the environmental fate, effects, and risk of pesticides in tropical environments, which is expected to facilitate improved management of pesticides. Environmental monitoring programs of surface and ground waters in the tropics, including areas of high ecological value, have detected several relatively polar pesticides at concentrations that are of ecological concern. Novel monitoring techniques have the capacity to reveal the spatial and temporal extent of such risks. To best manage these pesticides, their sorption, dissipation rates, leaching, and runoff potential need to be better understood. On these aspects, important insights have been provided by several studies within this issue. Improved understanding of the environmental fate, effects, and risks through studies presented in this special issue is crucial for minimizing the nontarget impacts of pesticides on biodiversity-rich tropical regions.

Spot Spraying Reduces Herbicide Concentrations in Runoff

Rainfall simulator trials were conducted on sugar cane paddocks across dry-tropical and subtropical Queensland, Australia, to examine the potential for spot spraying to reduce herbicide losses in runoff. Recommended rates of the herbicides glyphosate, 2,4-D, fluoroxypyr, atrazine, and diuron were sprayed onto 0, 20, 40, 50, 70, or 100% of the area of runoff plots. Simulated rainfall was applied 2 days after spraying to induce runoff at one plant cane and three ratoon crop sites. Over 50% of all herbicides were transported in the dissolved phase of runoff, regardless of the herbicide's sediment-water partition coefficient. For most sites and herbicides, runoff herbicide concentrations decreased with decreasing spray coverage and with decreasing herbicide load in the soil and cane residues. Importantly, sites with higher infiltration prior to runoff and lower total runoff had lower runoff herbicide concentrations.

Precision Herbicide Application Technologies To Decrease Herbicide Losses in Furrow Irrigation Outflows in a Northeastern Australian Cropping System

This study compared water quality benefits of using precision herbicide application technologies in relation to traditional spraying approaches across several pre- and postemergent herbicides in furrow-irrigated canefarming systems. The use of shielded sprayers (herbicide banding) provided herbicide load reductions extending substantially beyond simple proportionate decreases in amount of active herbicide ingredient applied to paddocks. These reductions were due largely to the extra management control available to irrigating growers in relation to where both herbicides and irrigation water can be applied to paddocks, coupled with knowledge of herbicide toxicological and physicochemical properties. Despite more complex herbicide mixtures being applied in banded practices, banding provided capacity for greatly reduced environmental toxicity in off-paddock losses. Similar toxicological and loss profiles of alternative herbicides relative to recently regulated pre-emergent herbicides highlight the need for a carefully considered approach to integrating alternative herbicides into improved pest management.

Spatial and Temporal Variability in Pesticide Exposure Downstream of a Heavily Irrigated Cropping Area: Application of Different Monitoring Techniques

Pesticide exposure threatens many freshwater and estuarine ecosystems around the world. This study examined the temporal and spatial trends of pesticide concentrations in a waterway within an agriculturally developed dry-tropics catchment using a combination of grab and passive sampling methods over a continuous two-year monitoring program. A total of 43 pesticide residues were detected with 7 pesticides exceeding ecologically relevant water quality guidelines/trigger values during the study period and 4 (ametryn, atrazine, diuron, and metolachlor) of these exceeding guidelines for several months. The presence and concentration of the pesticides in the stream coincided with seasonal variability in rainfall, harvest timing/cropping cycle, and management changes. The sampling approach used demonstrates that the application of these complementary sampling techniques (both grab and passive sampling methods) was effective in establishing pesticide usage patterns in upstream locations where application data are unavailable.

Acute and additive toxicity of ten photosystem-II herbicides to seagrass

Photosystem II herbicides are transported to inshore marine waters, including those of the Great Barrier Reef, and are usually detected in complex mixtures. These herbicides inhibit photosynthesis, which can deplete energy reserves and reduce growth in seagrass, but the toxicity of some of these herbicides to seagrass is unknown and combined effects of multiple herbicides on seagrass has not been tested. Here we assessed the acute phytotoxicity of 10 PSII herbicides to the seagrass Halophila ovalis over 24 and/or 48 h. Individual herbicides exhibited a broad range of toxicities with inhibition of photosynthetic activity (∆F/F_m′) by 50% at concentrations ranging from 3.5 μg l⁻¹ (ametryn) to 132 μg l⁻¹ (fluometuron). We assessed potential additivity using the Concentration Addition model of joint action for binary mixtures of diuron and atrazine as well as complex mixtures of all 10 herbicides. The effects of both mixture types were largely additive, validating the application of additive effects models for calculating the risk posed by multiple PSII herbicides to seagrasses. This study extends seagrass ecotoxicological data to ametryn, metribuzin, bromacil, prometryn and fluometuron and demonstrates that low concentrations of PSII herbicide mixtures have the potential to impact ecologically relevant endpoints in seagrass, including ∆F/F_m′.