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

Symposium Introduction pubs.acs.org/JAFC

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

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INTRODUCTION Agricultural production systems in countries situated in the tropics (i.e., locations situated between the Tropic of Cancer and Tropic of Capricorn) are generally under a greater pest pressure due to favorable environmental conditions for insect pests and weeds to proliferate. This, coupled with changing farming practices (e.g., reduced tillage, herbicide-tolerant crops), is leading to a greater reliance on pesticides than in temperate regions. Indeed, pesticide usage has greatly increased in tropical regions in recent times with agricultural expansion and changing agricultural practices including the implementation of conservation agriculture.1,2 The types of products being used has also changed with the introduction of transgenic crops such as herbicide-tolerant crops (e.g., maize and soybean),2 and these will change again as weeds develop herbicide resistance.3 It has, however, been well recognized in the literature that the behavior, fate, and effects of pesticides in tropical environments are considerably less understood than for temperate regions and represent a major research gap.1,4−6 The warmer climates, more variable rainfall, different soil types, and distinct biota that characterize tropical locations imply the behavior, fate, effects, and management of pesticides in these locations may prove quite different from those for temperate locations. Knowledge of the behavior, fate, and effects of pesticides in the tropics, which are characterized by intense rain storms, is particularly important when managing runoff losses from agricultural lands into sensitive receiving aquatic ecosystems. Key reviews, such as by those by Racke et al.6 and Kookana et al.,7 specifically called for further research to identify the key dissipation pathway(s) of each particular pesticide so that targeted management strategies can be developed. Indeed, it is the pesticides that are dissipated via volatilization (controlled by temperature and vapor pressure) and/or runoff/leaching (high solubility and low Koc properties) that would be most prone to be influenced by tropical conditions. Since these earlier reviews, much research effort has occurred to examine the fate, sorption potential, and dissipation pathways of several pesticides, and considerable advances have also been made in environmental monitoring techniques, analytical capabilities, modeling, toxicity, and risk assessment of pesticides. New data and technical advancements have undoubtedly led to improvements in pesticide use and regulation (e.g., the phasing out of the highly volatile, toxic, and persistent endosulfan). Never© XXXX American Chemical Society

theless, monitoring programs, including those within high conservation value areas, continue to highlight that certain pesticides are present in rainfall and surface and ground waters, occasionally at concentrations at which negative effects to biota would be expected.8−15 Such cases suggest that gaps in pesticide management still exist, although it is not clear whether an incomplete knowledge of certain pesticide processes or dissipation pathway(s), risk assessments, and modeling performed by the regulators to determine label recommendations or knowledge transfer to farmers is resulting in the problematic concentrations measured in the environment. This special issue presents a collection of research studies designed to increase our knowledge of pesticide behavior across the dry and wet tropical regions in the Great Barrier Reef catchment area in Australia and from other tropical locations in South America. The soils and environmental conditions in tropical environments are clearly articulated by Racke et al.,6 and this material will not be repeated here. In this contribution we aim to provide an overview of the state of knowledge on three aspects: (1) environmental risk of pesticides in tropical environments, (2) the fate and behavior of pesticides in tropical settings, and (3) the management of pesticides in tropical locations. In doing so, we have tried to identify the unique features of tropical systems and the knowledge gaps that are relevant to such systems. A conceptual outline of the key research gaps/data requirements to understand pesticide behavior, fate, and risk and to best manage their use under tropical soil and environmental conditions is presented in Figure 1.

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ENVIRONMENTAL RISK OF PESTICIDES IN TROPICAL ENVIRONMENTS Although monitoring programs in most tropical regions are generally limited both spatially and temporally, notable exceptions include monitoring within internationally significant wetlands and World Heritage Areas such as the Pantanal Wetland, Brazil,8,9 the Bowling Green Bay Ramser wetland, Special Issue: Pesticide Fate and Effects in the Tropics Received: March 22, 2016

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DOI: 10.1021/acs.jafc.6b01320 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry

Symposium Introduction

Figure 1. Conceptual framework of the key research gaps/data requirements to understand pesticide behavior, fate, and risk and to best manage their use under tropical conditions.

Australia,13−15 and the Great Barrier Reef, Australia.10−12,16,17 Much of the environmental monitoring and risk assessment of pesticides in tropical locations in the past century (until 2000) largely focused on the organochlorines,18−21 most of which have since been widely banned. Since that time, developments in environmental monitoring techniques such as the use of passive sampler devices,11,15,17,22−24 improvements in laboratory methods to achieve lower detection levels,25 and advances in ecotoxicological approaches (and relevant data for keystone biota in the tropics) to target specific modes of action26−33 have revealed that a new suite of pesticides is of concern across many tropical locations. Indeed, monitoring programs across tropical Australia, Brazil, Chile, Barbados, and Hawaii have detected several relatively polar pesticides in rainwater, surface runoff, and groundwater. These include PSII-inhibiting herbicides (diuron, metribuzin, simazine, atrazine, tebuthiuron, and hexazinone), the chloroacetanilide herbicide metolachlor, the auxin growth regulator 2,4-D, and the neonicotinoid insecticide imidacloprid at concentrations that may pose environmental or human-health risks.8−13,15−17,34−37 Furthermore, the highly volatile endosulfan continues to be an issue in Brazil.8,38 Indeed, lessons from temperate environments highlight that where pre-emergent, soil-applied pesticides (such as those listed here) are used in large amounts, their off-site migration may be an issue, as they are often highly prone to runoff.39 Several recent studies have used various approaches to examine the environmental risk of pesticides in tropical locations14 including the Pesticide Impact Rating Index (PIRI),40 Predict the Ecological Risk of Pesticides in freshwater ecosystems (PERPEST),13 OECD REXTOX modeling,41 the Species At Risk index (SPEAR),42 and fugacity-based modeling to compare risk between tropical and temperate locations.1 Furthermore, improvements in statistical approaches have allowed for more reliable derivation of ecological guideline values and ways to account for additive effects of pesticides that

have similar modes of action.12,43−45 These approaches combined with monitoring data can then be applied to better quantify the spatial and temporal risk of pesticides.15,37,46 However, there is a lack of ecotoxicological data specific to tropical biota that precludes a full appreciation of ecological risk in tropical environments.5,47,48 This issue presents several advances in monitoring and risk assessment of pesticides in tropical locations. These include O’Brien et al.,15 who used a combination of grab and passive sampling techniques to produce a continuous pesticide profile along a stream over two years. The data were then used to quantify the risk of multiple pesticides used in agriculture in an area that hosts an internationally recognized wetland. However, in data-poor regions, pesticide risk assessments mostly rely on modeling approaches. Binder et al.49 apply an integrative model (Be-WetSpa-Pest) that considers farmer behavior, hydrology, and pesticide emission models to assess the environmental and human risk of pesticides in the Colombian Andes. Garcı ́aSantos et al.50 conducted spray drift trials of hand-held knapsack sprayers. These new empirical data were used to compare and optimize existing predictive models of spray drift to better predict the environmental and human health risk of pesticide spray drift of knapsack sprayers. With continued agricultural expansion, changing management regimes, and new pesticide products available, there is an increasing need for tools/models that allow a fast and reliable assessment of the potential for offsite losses and human and ecological risk of pesticides to be performed. Furthermore, sophisticated monitoring programs that can provide continuous pesticide profiles of streams and groundwaters and statistical approaches to quantify the risk of multiple pesticides are required over wider temporal scales to fully appreciate pesticide risk. It is imperative that problematic pesticides are quickly identified and effectively managed before their impacts become evident on human health or the environment. B



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temperature regimes on pesticide degradation.6 Studies that have examined this influence consistently show faster degradation with higher temperatures including, as examples, the insecticide chlorpyrifos64 and the herbicide atrazine.65 Corrections for the temperature dependence of degradation are widely applied in pesticide transport modeling using the Arrhenius relationship.51,66,67 Uncertainties in the inputs into this relationship for many pesticides (e.g., degradation activation energy) are large; however, it has been concluded that these uncertainties are smaller than the errors of not incorporating a temperature effect on degradation in pesticide risk assessment modeling.67 Studies in this special issue have investigated the leaching, surface runoff, and sorption potential of various pesticides including the insecticides chlorpyrifos and endosulfan38 and the herbicide tebuthiuron.68 Dores et al.38 found that volatilization was likely a key loss pathway for chlorpyrifos and endosulfan, although considerable amounts could be leached through preferential flow sites as well as lost through surface runoff. Soil dissipation rates for these insecticides were found to be much faster than rates reported for temperate locations, although sorption was similar to that measured elsewhere. Thornton and Elledge68 examined the persistence and movement of tebuthiuron in two Australian soils with two different chemical formulations (granular and dry flowable). They found that tebuthiuron was transported largely in the dissolved phase and had the ability to leach in the soil profile and that high concentrations (relative to guideline values) could be lost in surface runoff. Field dissipation rates in soils were shorter than reported in temperate locations but were longer than rates reported for Brazilian soils. Much of the recent research on pesticides has endeavored to understand the variations in sorption affinity of pesticides to soils, which can be highly variable across different locations.1 Extrapolation of sorption data from temperate to tropical soils is fraught with difficulties, owing to major differences in the nature of soil types (highly weathered, variable charge) as well as carbon chemistry in soils in the tropics.69,70 For example, Oxisols comprise 50−60% of the Brazilian landscape and in such soils even the anionic herbicides could be sorbed on iron and aluminum oxides.69 However, in a study on comparative sorption of certain pesticides in 10 tropical and 12 temperate soils, Oliver et al.71 found no significant difference in sorption Koc values of diuron, imidacloprid, and thiacloprid. It is difficult to make a sound comparison on a small set of soils, and there remains a need for a robust comparative assessment of differences in sorption of pesticides across tropical and temperate soils. In this regard, rapid techniques of measuring pesticide sorption on a large number of soils, such as through the use of infrared spectroscopy,72 could be useful. The differences in land uses within tropical regions could also result in substantially different sorption of pesticides. In a national level study on sorption of diuron in Sri Lankan soils, Liyanage et al.73 found a very wide range of Koc values (55.3−962) with higher values in dry soils possibly due to large variations in organomineral interactions in these soils.74 Understanding of pesticide behavior is critical to best manage pesticide use, reduce offsite losses, and model pesticide fate. Often databases on pesticide properties in soils provide only a mean or typical value and do not present the full range of values, whereas data generated from field studies do not always standardize the outputs for temperature and soil moisture. The amount that a pesticide will sorb to soils is governed by a

FATE AND BEHAVIOR OF PESTICIDES IN TROPICAL SETTINGS The behavior of pesticides, the environmental/climate conditions (i.e., soil physical, biological, and geochemical composition, moisture, rainfall, humidity, wind), and management (application timing and rate) all influence the loss pathways of pesticides and their sorption potential, half-life, and field dissipation.7 Soil half-lives reflect the degradation of pesticides, which may include abiotic processes such as hydrolysis and biodegradation under ambient soil and environmental conditions. When measured in the field, dissipation is also affected by processes such as photolysis, volatilization, plant uptake, and runoff/leaching as a result of prevailing soil and environmental conditions. Also, the pesticides are often applied to the surface and are not necessarily mixed into the soil. Clearly, if the offsite environmental risk of pesticides is to be understood and managed, both sorption and half-life or field dissipation data for locally relevant soils are required together with an understanding of soil moisture status, organic carbon content and composition, soil temperature, and previous application history. Such a complex assessment is in the domain of simulation models of the environment, agricultural system, and pesticides, such as RZWQM.51 The amount of pesticide applied and its subsequent dissipation in soils is critical to understanding its potential movement offsite.52 It has long been recognized that dissipation rates can vary greatly for individual pesticides depending on the soil moisture content.53 Indeed, little degradation of pesticides may occur on dry soils due to the inactivity of microbial communities, although degradation will commence once the moisture content is sufficient.54 Enhanced degradation can also occur with repeated application (i.e., adaption of soil microbial communities) for specific compounds.55−57 The resulting changes in rates of degradation can have large effects on the efficacy of pest control and environmental risks. The available data suggest that field dissipation of most pesticides in soils is generally faster in tropical environments that are characterized by warmer and wetter climates, which in turn fosters degradation through hydrolysis and enhanced microbial activity and increased losses through volatilization and runoff/leaching.1,6 Whereas soil dissipation rates are generally faster in tropical environments compared to temperate conditions, the typically lower organic carbon contents in tropical soils (largely due to enhanced microbial activity in tilled/cropped soils) suggest that greater losses of most pesticides can occur in the water phase, presenting a higher environmental risk.1 Biopores in structured soils can lead to preferential flow and facilitate leaching of pesticides to groundwater.38,58,59 Indeed, the surface runoff/leaching loss pathways are particularly important for the relatively polar pesticides (listed previously) that have emerged as posing a risk to nontarget environments. Several studies on these pesticides have demonstrated that they tend to dissipate more quickly in tropical conditions (e.g., metolachlor, atrazine, simazine, and diuron;60,61 imidacloprid;62 atrazine63). Tropical locations are also characterized by relatively uniform (and higher) soil temperatures throughout the year compared to temperate regions, which experience much greater seasonal variability; these uniform conditions enhance soil microbial activity within tropical soils.6 Laboratory studies that examine pesticide degradation commonly standardize temperature (e.g., at 25 °C) and do not always consider the influence of different C




percentage (80% compared to conventional broadcast application in furrow-irrigated systems; banded application was particularly effective because the herbicides were applied to the raised bed while the water flowed in the unsprayed furrow, supporting previous findings for a variety of pesticides.88,89,93 Melland et al.92 trialled more targeted weed management techniques (i.e., weed recognition) and showed that, for herbicides that were largely transported in the dissolved phase, rainfall-runoff losses of herbicides were reduced in proportion to the lower area/amount of product applied, consistent with previous studies.88,89 In principle, the reduced application of herbicides using precision application

number of complex interactions relating to the chemical properties of the pesticide (and surfactant/additive) and the composition of the soil including particle size, cation-exchange capacity, soil organic carbon (composition and content), clay mineralogy, and moisture content. Although it is not clear how pesticide sorption varies between temperate and tropical locations, the molecular composition of the soil organic carbon and organomineral interactions have been found to be among key factors that determine pesticide sorption potential in soils.74−78 Many of the papers within this special issue examine the drivers of pesticide sorption in tropical soils and highlight the need for very detailed systematic experiments in this field of research. For example, Bonfleur et al.78 examined the influence of various types of soil organic matter/carbon (measured by 13 C nuclear magnetic resonance), different particle size fractions/aggregates, and iron and aluminum oxide contents on the sorption potential of the herbicides alachlor, bentazon, and imazethapyr. They showed that the test pesticides preferentially sorbed onto the 2−53 μm aggregates. They also noted that soil aggregation (likely from sesquioxides) facilitated by organomineral interactions could mask pesticide sorption sites (i.e., the weathered clays effectively coat the organic matter surfaces and prevent pesticide sorption). Regitano et al.77 also investigated the influence of different organomineral aggregate sizes and soil water contents on the retention of diuron (measured following 42 days of incubation) in soils. They observed that the retention increased with higher water-holding content, indicating that sufficient water was required to diffuse diuron to the sorption sites. The study highlighted the potential issues in terms of relevance of sorption measured by short-term batch equilibration in the laboratory to the field behavior of pesticides. Toniêto et al.79 showed that the herbicides tebuthiuron and hexazinone were more highly sorbed in soils that contained higher clay contents as those soils had higher amounts of organic carbon. The authors found that little of the herbicides was sorbed to green sugar cane trash/straw that was applied to paddocks as a blanket. Likewise, Dang et al.80 also showed that little of the herbicides atrazine, ametryn, diuron, hexazinone, tebuthiuron, and metolachlor remained in sugar cane trash/ straw following washoff by rainfall, which also implies negligible sorption potential. Sorption, dissipation, and washoff from crop residues (straw, stubble, or trash) are highly relevant to the behavior and fate of pesticides in tropical agriculture because a large proportion of cultivated land has crop residues retained on the surface (for moisture conservation and control of soil erosion), and this is the initial substrate for many of the applied herbicides and some of the other pesticides used.

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MANAGEMENT OF PESTICIDES IN TROPICAL SETTINGS The management of pesticides in tropical settings presents a considerable challenge for farmers. A balance needs to be established so that they may achieve desired pesticide efficacy without any adverse environmental footprint. On the one hand, some level of chemical persistence is desirable to kill/suppress pests, but longer than necessary persistence may result in losses from sites of application through leaching, drift/volatilization, or surface runoff, especially under the tropical warm, humid, and heavy rainfall conditions. Several studies, including those in this special issue, have shown that although only a small D



Symposium Introduction †

(compared with blanket application) also reduces the potential losses in leaching. Importantly, several studies presented in this special issue have highlighted that there is a need to develop management tools that are specific to the conditions and agricultural methods employed in tropical regions. Garcı ́a-Santos et al.50 showed that existing guidelines for calculating spray drift, which had been derived from boom spray situations, were insufficient to predict hand-held spray drift for tropical mountainous environments. Existing equations generally underestimated the spray drift, and hence pesticide risk to both the operator and the environment would be underestimated. Furthermore, the work of Binder et al.49 highlights the need for policy interventions to be targeted to the needs of individual communities, with the development of farmer cooperatives predicted to result in more favorable environmental outcomes than increasing the training offered by pesticide producing companies. The warm tropical conditions coupled with socioeconomic and cultural aspects contribute to insufficient personal protective equipment being used in the application of pesticides, which increases risks to human health and exposure.94 These examples highlight the need for predictive tools and management solutions to be tailored to the unique conditions experienced in tropical environments. In summary, several relatively polar pesticides including diuron, metribuzin, simazine, atrazine, tebuthiuron, hexazinone, metolachlor, 2,4-D, and imidacloprid have been detected in environmental monitoring programs across many tropical locations at concentrations that may cause environmental harm. Novel monitoring and modeling techniques highlight the spatial and temporal risks of these problematic pesticides, whereas detailed investigations of their behavior in soils reveal complex organomineral interactions that influence sorption potential and their ability to leach or be lost via surface runoff. The management of pesticide losses to reduce their offsite risk is dependent on understanding these properties as well as implementing a whole-of-farm approach that considers product selection, timing of application, precision application, and the use of a trash/straw blanket if possible. However, understanding the dominant loss pathway of the pesticide is critical so that management can be adequately targeted. In the case of relatively polar and mobile pesticides, the timing of application with respect to intense rainfall events and precision application are the two likely approaches that will best reduce the offsite risk. There is a need to avoid direct transfer of knowledge and results from temperate environments to the tropical locations because the usage patterns, behavior, and fate of pesticides as well as the downstream aquatic ecosystems are unique in many ways. However, studies on tropical systems (such as those presented here and those published elsewhere) can help us adapt the successful management options developed on the basis of experience in temperate regions to the tropical environments. Currently the knowledge base on the fate, effect, and risks of pesticides in tropical environments is inadequate, and therefore the characterization and mitigation of pesticide pollution in tropical environments should be a priority in future research.

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Catchment to Reef Research Group, TropWATER, James Cook University, Townsville, QLD 4811, Australia § Department of Natural Resources and Mines, Toowoomba, QLD 4350, Australia # National Centre for Engineering in Agriculture, University of Southern Queensland, Toowoomba, QLD 4350, Australia ⊥ CSIRO Land and Water/University of Adelaide, Waite Campus, Glen Osmond, SA 5064, Australia

AUTHOR INFORMATION

Corresponding Author

*(S.E.L.) E-mail: [email protected]. Phone: +61 7 4781 6629. Fax: +61 7 4781 5589. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank all of the authors who have contributed their work to make this special issue possible. We are extremely grateful to the handling editor Professor Takayuki Shibamoto for ensuring all manuscripts were thoroughly reviewed and processed in a timely manner and to Ivonne Hofmann-Sellier and Eileen Richardson for their organizational role in the special issue. Comments from two anonymous reviewers and Professor Takayuki Shibamoto improved this paper.

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Stephen E. Lewis*,† D. Mark Silburn§,# Rai S. Kookana⊥ Melanie Shaw§

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