Chapter 13
Ecological Risk Characterization for the Synthetic Pyrethroids Mah T. Shamim, Michael D. Hoffmann, Jose Melendez, and Mohammed A. Ruhman U.S. Environmental Protection Agency, Office of Pesticide Programs, Environmental Fate and Effects Division, 1200 Pennsylvania Avenue, NW, Washington, DC 20460
In its reevaluation of pesticides under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), the Environmental Protection Agency (EPA) examines all major routes of exposure. As individual pyrethroids have been reevaluated, a distinct pattern of similarities and differences have emerged among them. The synthetic pyrethroids are characterized not only by similar environmental fate and transport properties, but also by their common toxicity endpoints. This chapter includes a discussion of these trends and provides innovative modeling approaches to estimate the exposure of pyrethroids to aquatic organisms in wastewater and freshwater from use of these pesticides in agricultural and urban environments. Also included is a discussion of potential risks to non-target aquatic and terrestrial organisms.
NOTE: This document has been subjected to review by the Environmental Fate and Effects Division in the Office of Pesticide Programs and approved for publication. Approval does not signify that the contents reflect the views of the Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.
© 2008 American Chemical Society
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Introduction Under the Federal Insecticide, Fungicide, and Rotendicide Act (FIFRA), EPA evaluates the use of a pesticide before it can be sold or distributed in commerce. E P A will register or approve a new pesticide i f scientific data show that, when used according to label directions, it will not cause unreasonable effects on human health and the environment. Under the 1988 Amendments to FIFRA, E P A is also required to review and re-register older pesticides to ensure that they meet current health, safety, and environmental standards. The Environmental Fate and Effects Division (EFED) in EPA's Office of Pesticide Programs (OPP) is responsible for conducting ecological risk assessments, endangered species risk assessments and drinking water exposure assessments (to be used in the dietary human health risk assessments) in support of the registration and reregistration of pesticides. The registered pesticides involved in these two programs cover a wide range of compounds with many belonging to specific classes of compounds. One of the major classes of pesticides that have been introduced over the past three decades for a variety of insecticidal uses is the synthetic pyrethroids. The pyrethroids share similar modes of action and are considered neurotoxins that act on the axons in both the peripheral and central nervous system. The primary biological effects of pyrethroids on insects and vertebrates are caused by an inhibition of the correct firing of neurotransmitters that deliver a signal from one cell to another via voltage gated calcium ( C a and sodium (Na channels) (7). Two different types of pyrethroids are recognized: Type I and Type II. This classification is based on differences in basic structure (presence or absence of a cyano group in the alpha position) and the symptoms of acute poisoning in rodents. In general, the symptoms of synthetic pyrethroids poisoning follow the typical pattern of nerve poisoning: (a) excitation, (b) convulsions or tremor, (c) paralysis, and (d) death. Marked differences exist in the duration of action on sodium channel gate, particularly between Type T and Type II pyrethroids. Pyrethroids with an alpha-cyano group, such as cyfluthrin, esfenvalerate, and cypermethrin, produce a more prolonged transient increase in sodium permeability of the nerve membrane during excitation than do Type I pyrethroids such as bifenthrin. This chapter describes the approaches and methodologies for estimating exposure and characterizing ecological risk from the use of synthetic pyrethroids in both agricultural and non-agricultural environments. It also includes a discussion of the relative toxicity and fate of several synthetic pyrethroids as well as resulting risks from their use in specific settings. ++)
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Regulatory History Registration Since the early to late 1970s, nearly 22 synthetic pyrethroids, including enriched isomers, have been registered and used for controlling various insects in agriculture and non-agriculture settings. In 1971, 1975, and 1977 resmethrin, sumithrin, and permethrin were registered, respectively. In 1990, six synthetic pyrethroids were conditionally registered on cotton as substitutes for organophosphate, carbamate and organochlorine pesticides: bifenthrin, cyfluthrin, cypermethrin, esfenvalerate, lambda-cyhalothrin and. tralomethrin. More pyrethroids were added later, including zeta-cypermethrin, deltamethrin, fenpropathrin, and gamma-cyhalothrin. Most of the synthetic pyrethroids were classified as restricted use pesticides because of their high toxicity to fish and other aquatic organisms. In order to support new registration actions and maintain existing ones, the Environmental Protection Agency (EPA) issued data call-in (DCI) notices in 1985 for chemical specific aquatic toxicological data. In 1990, the Pyrethroid Working Group (PWG) was formed to address data needs for the conditional registration of these chemicals. The P W G included representatives from EPA, the National Cotton Council, and the major manufacturers of synthetic pyrethroids. In this process, manufacturers of pyrethroids were initially requested to submit an interim risk reduction proposal to mitigate aquatic risk. The proposal was to include labeling changes to reduce aquatic exposure such as buffer zones; educational programs that targeted growers, applicators, and consultants and publicized label changes; and field studies to monitor the effectiveness of buffer zones. In February, 1999, E P A presented a preliminary assessment entitled "Sediment Toxicity and Fate of Synthetic Pyrethroids" to the FIFRA Scientific Advisory Panel (SAP) (2). The purpose of the meeting was to seek the SAP's advice concerning the adequacy of the studies conducted by the P W G to characterize the partitioning, bioavailability, and toxicity of synthetic pyrethroids. The SAP also considered issues associated with assessment of risks from exposure of non-target aquatic organisms to synthetic pyrethroids in sediments. The P W G has developed additional data to address the conditional registration of the synthetic pyrethroids. This includes new mobility studies for nine synthetic pyrethroids, using the solid phase micro-extraction (SPME) technique. It also includes acute and chronic sediment toxicity studies for four synthetic pyrethroids. The Agency is reviewing the submitted fate and toxicity data and will use these data to address the uncertainties in the risk assessments.
260 Evaluation of New and Existing Uses of Pyrethroids Since the conditional registration of synthetic pyrethroids on cotton, more than 50 new agricultural and non-agricultural uses have been submitted and evaluated by the Agency for registration. Additionally, under the re-registration program, the Agency completed the re-evaluation of all existing food uses of these chemicals by August 2006 as mandated by Congress. More specifically the Agency completed Reregistration Eligibility Decisions (REDs) for cypermethrin, permethrin, and resmethrin and ecological risk assessments for the proposed new uses of bifenthrin, cyfluthrin, lambda-cyhalothrin, cypermethrin, zeta-cypermethrin, and prallethrin. A n outline of EFED's approach to develop ecological risk assessments in support of the registration or re-registration of pesticides can be found in the document entitled, "Overview of the Ecological Risk Assessment Process in the Office of Pesticide Programs, Environmental Protection Agency (J)."
Use Characterization Use characterization is known to include analysis of use and/or usage. Although use is examined in all regulatory actions, usage can only be examined for pesticides that have been in use for some time. Therefore, use involves examination of suggested label use for new products/crops (that is a potential use), while usage involves examination of current/past use for already registered products, namely current/historical use. Use characterization is an important first step in estimating exposure to pesticides. Use analysis relies on information associated with each use pattern and formulation (e.g., liquids, granular, and U L V ) , application dates and rates, application procedures (including equipment and specific parameters such as efficiency, droplet size and release height), timing and number of applications, and re-treatment intervals. In most cases, labels contain a range of values, and the baseline ecological risk assessment depends on the conservative values. Therefore, at this level, exposure is estimated based on labeled maximum application rates, maximum number of applications/year, minimum re-treatment intervals, and only label stated mitigation measure(s) (e.g., drift/runoff buffers, boom height, and wind speed). It is noted, however, that when parameters are not specifically stated on the label, reasonable conservative estimates are used (referred to as default values). Based on the labeled uses, an application window can usually be inferred, and an application date that gives the most conservative exposure is chosen for the assessment. Usage involves analysis of historical use data and is executed for regulatory actions that involve previously registered chemicals (e.g., for new uses and/or re-
261 registration). Although the main exposure estimates are based on active labeled use, other possible exposures are also characterized based on the analysis of current (or typical) application rates, procedures, and timing. Furthermore, effects of some possible mitigation measures on estimated exposure are also considered. In the U.S., use patterns for pyrethroids can be divided into agricultural and non-agricultural or urban. The highest agricultural uses are typically reported on field crops, while urban uses are typically reported for structural and public health pest control. Although no national data are available for pyrethroids urban use, California data suggest that the total pounds used in agriculture constitute only 25% of the total pyrethroid use. Therefore, information and data needed to quantify urban use of pyrethroids exposure are important.
Agricultural Use and Usage Patterns Pyrethroids are used as insecticides, miticides, and/or acaricides effective against a broad range of pests. Most pyrethroids are labeled for use on agricultural crops. The list of pyrethroids used in agriculture includes bifenthrin, permethrin, cypermethrin, cyfluthrin, fenpropathrin, lambda-cyhalothrin, esfenvalerate, tralomethrin, and deltamethrin. These chemicals are formulated in common crop use formulations such as emulsifiable concentrates (i?C), granules (G), water-dispersible granules (WDG), capsule suspensions, wettable powders (WP), water soluble bags (WSB), and as ultra low volume sprays (ULV). Pyrethroids are applied through most agrochemical application procedures, which include aerial, ground, airblast, and chemigation. Table I is a summary of important labeled application parameters for various uses in agriculture.
Table I. Pyrethroid use in agriculture: a summary of labeled-application.
" In most labels the total/year is not given and for some crops such as lettuce in California, exposure is estimated assuming more than one crop a year, resulting in significantly higher exposure.
262 The list of labeled agricultural use patterns is extensive and includes two main field crops (corn and cotton); other field crops (soybean, alfalfa, sunflower, peanuts, cereals); vegetables (sweet corn, fruiting vegetables, head/stem and leafy vegetables, tuberous/corm vegetables, bulb vegetables, and cucurbits); fruit and vine trees (grapes, nut trees, peaches, and berries); and ornamental and turf. New uses have also been added to the list or are in the process of being reviewed for registration. Pyrethroid usage on agricultural crops is useful in understanding the extent of use from available national and state data. Potential usage can be easily inferred from yearly crop acreage statistics; however, real usage can only be obtained from historical use data. For pyrethroids, national usage data are available for 1997 (4) and for specific years, crop groups, and program states (5) . Additionally, statewide data are available for California from 1991 to 2005 (6) . The use distributions of various pyrethroid chemicals and crops, which are summarized in Figure l a and lb, respectively, are based on the amount of pyrethroid active ingredients used and on the 1997 national data. Data indicate that permethrin and tefluthrin are the main pyrethroids used in agriculture, followed by cyfluthrin, esfenvalerate, bifenthrin, and cypermethrin (Figure la). The use is apparently concentrated on field crops such as corn and cotton, followed by vegetables and fruits (Figure lb). In 1997, approximately 26 million acres were treated with about 2.4 million lbs of active ingredients, resulting in an average pyrethroid rate of nearly 0.09 lbs a.i./Acre. The total annual minimum and maximum rates, which are listed in Table II, were obtained from the U.S. Department of Agriculture National Agricultural Statistics Service (USDA-NASS) (5) and were compared to labeled seasonal rates for selected pyrethroids. With the exception of cyfluthrin, data suggest that the chemicals were applied at lower rates than those permitted by the label. Although this may be the case, the accuracy of these annual "survey rates" would depend largely on the accuracy of the surveys. Survey rates may be used, with some uncertainty, to obtain other possible exposure estimates. The extent of pyrethroid usage in 1997 is presented in Figure 2. The map shows the national 1997 usage areas for eight pyrethroids (total acres treated with permethrin, tefluthrin, cyfluthrin, esfenvalerate/ fenvalerate, bifenthrin, cypermethrin, tralomethrin, and fenpropathrin). As expected, high usage areas are in the corn and cotton belts in addition to areas where vegetables, fruits, and tree nuts are grown (California, Florida, Georgia, and North and South Carolina). Also, U S D A - N A S S has an agricultural chemical use database, which was examined for pyrethroid usage. Statistics present in this database were based on surveys conducted on farm use for targeted crops in chosen years (20022006) and states (program states) (Figure 3) (7). In comparison to 1997, it appears that more pyrethroid active ingredients (lbs a.i.) were used in 20022006 on vegetables and cotton and less on corn, with no change in fruit trees.
263 Table II. Maximum annual "survey rates" compared to maximum labeled rates a
a
Labels for most pyrethroids include buffer zones that vary in type/width with appli cation type.
Additionally, the distribution of usage between various pyrethroids was affected with marked reductions in permethrin and tefluthrin balanced by increases in lambda-cyhalothrin and cypermethrin. Trends in use patterns for pyrethroids in California were obtained for 1991 to 2005 from the P A N N A database and plotted in Figure 4. The data suggest that acreage treated with pyrethroids increased from nearly 2 million acres in 1991 to nearly 3.5 million acres in 1995. This increase was followed by a general decrease to approximately 2.3 million acres in the year 2001. Finally a slight increase occurred from the 2.3 million acres in 2001 to nearly 2.9 million acres in the year 2005. In contrast to the erratic changes in total pyrethroids treated acreage, there was a steady increase in total pounds applied through two periods: the first from 1991 to 1998 (increase from 165,000 to 283,000 lbs) and the second from 1999 to 2005 (increase from 205,000 to 224,000 lbs). It is also noted that pyrethroid use rates for California were near the low side of the reported national rate, which could be a reflection of differences in the dominant California crop use patterns compared to those nationally.
Non-agricultural Use and Usage Patterns The list for non-agricultural use of pyrethroids includes many categories such as structural; household indoor/outdoor; landscaping; public health; animal husbandry; food processing; and commercial, storage, recreational, and uncultivated non-agricultural areas. Table III summarizes data on important pyrethroid chemicals with non-agricultural use patterns along with information on end use products and use purpose (s). In general, non-agricultural pesticides are applied by individuals for many indoor/outdoor household uses; by municipalities or public health authorities for vector control pesticides; and by
264
(a) By chemical (based on lbs a.i. used in the treatment)
a
All Others inlude: Tralomethrin + deltamethrin (3%), and fenpropathrin (90% of the total usage.
pyrethroids are not expected to move to sub-surfaces and leach to ground water. They may reach aquatic environments via erosion during rain events. Their high K Q C ' S indicate that they will partition with the soil, be transported offsite, and reach adjacent bodies of water carried on the solid phase. The chemicals may also reach aquatic systems by spray drift. As the chemicals reach these systems, they are expected to remain adsorbed to suspended solids or organic matter in
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N o t e s : X - a x i s = U s a g e y e a r ; o t h e r s are: Deltamethrin, Fenpropathrin, Resmethrin, Tetramethrin,
and Tralomethrin
Figure 5. Pyrethroids: Agricultural usage in California by chemical (lbs a.i used).
the water column, or partition into the sediment phase. Since it was observed that for seven o f the twelve synthetic pyrethroids examined, the anaerobic metabolism is slower than the aerobic metabolism, and there is a potential for repeated applications, these chemicals may remain in the sediments and the benthic layer for prolonged periods of time. The sediments, which are often anaerobic, may become repositories of the chemicals. Pyrethroid pesticides are insecticides or insecticides/acaricides that represent a challenge in selecting and quantifying possible exposure scenarios. Chemicals in this class are associated with varied and unique fate and transport properties, a multitude of use patterns, formulations, and application types in varied environmental settings. The following section gives a brief description of the methods that are employed, by the Environmental Fate and Effects Division (EFED), to estimate exposure to aquatic and terrestrial compartments of the environment.
Fate and Transport Characterization A large number o f variables affect the fate and transport of synthetic pyrethroids in the environment. Some of these variables are chemical dependent such as solubility, vapor pressure, sorption strength, hydrolytic/photolytic characteristics, and susceptibility to biodegradation. Other types of variables are
272 associated with application practices such as timing and place of application (region of the U.S.) and agricultural practices such as cropping and irrigation methods. A summary of the environmental fate properties of the synthetic pyrethroids can be found in Table V below. Except for prallethrin, the synthetic pyrethroids have low solubility. (Prallethrin is 8.03 ppm, while all others range from 0.014 ppb for bifenthrin through 2.32 ppb for cyfluthrin to 84 ppb for tralomethrin.) The chemicals also have moderate to low vapor pressure. (The moderate range is from 1.8xl0* mm Hg for bifenthrin and fenpropathrin to 6.0x10' mm Hg for tefluthrin, while the low range is 1.8xl0' mm Hg for tralomethrin.) In general, Henry's Law Constants indicate a moderate to low potential for these chemicals to volatilize from moist surfaces. The moderate range is from 1.80xl0" Atm-m mol" for bifenthrin, fenpropathrin, and tefluthrin to 3.7xl0" Atm-m mol" for cyfluthrin, while the low range is 1.9xl0' Atm-m mol" for tralomethrin. Synthetic pyrethroids have a high tendency to bind to organic matter in the sediments and in the particulate suspended in the water, resulting in a substantial reduction of volatilization even for those chemicals with moderate Henry's Law Constants. All synthetic pyrethroids are highly non-polar as indicated by their high log K w The Log octanol/water coefficients ( K w ) are 4.5-5.0 for prallethrin and deltamethrin, 5.1-6.0 for resmethrin, cyfluthrin, fenvalerate, tralomethrin, and fenpropathrin and 6.1-7.0 for permethrin, bifenthrin, cypermethrin, lambdacyhalothrin, and tefluthrin. 7
5
n
4
6
10
3
3
3
1
1
1
0
0
The fate of pyrethroids in various compartments of the environment can be deduced from their laboratory abiotic and biotic reactions. A l l synthetic pyrethroids are relatively stable to abiotic hydrolysis, except in alkaline environments. In alkaline media, resmethrin, permethrin, bifenthrin, fenvalerate, and tralomethrin show the highest stability, while fenpropathrin, lambdacyhalothrin, and tefluthrin show moderate reactions (half-life "t. " range=13->30 days at pH 9). Others such as prallethrin, cyfluthrin, cypermethrin, and deltamethrin are highly susceptible to alkaline hydrolysis (t. range = 2-5 days at pH 9). This shows that most pyrethroids are highly stable in water bodies under normal environmental conditions (pH 7). In contrast to abiotic hydrolysis, first generation pyrethroids such as prallethrin and resmethrin show high reactivity towards photolytic reaction (t. = hours to 0.5 day). Others such as cyfluthrin and fenvalerate show moderate reactivity (t. = 4.5 and 6 days). However, all the other pyrethroids are more resistant to photolysis. In comparison to aqueous photolysis, photodegradation on soil does not appear to be a major dissipation route for synthetic pyrethroids (t. = 26 days to stable). A noticeable exception is cyfluthrin, which is reactive to photodegradation on soil, with a half-life of 5 days. This indicates that with a few exceptions (prallethrin, resmethrin, cyfluthrin and fenvalerate) most pyrethroids are very stable under the sun light in both terrestrial and aquatic environments. Aerobic soil metabolism is an important dissipation mechanism for many chemicals, but it is a major route of dissipation for only one pyrethroid, /2
/2
/2
/2
/2
273 prallethrin (t. = 3 days). For other pyrethroids, resistance to aerobic soil biotransformation varies widely and can be categorized into relatively low persistence (t. = 10,000 ppm for permethrin) to slightly toxic ( L C = 1280 ppm for bifenthrin) to avian species (Table VIII). Although acute oral data show that synthetic pyrethroids range from practically non-toxic 5 0
50
50
286 (LD o = >9869 mg/kg-bw for permethrin) to moderately toxic ( L D = 75 mg/kgbw for resmethrin) to avian species, only two of the twelve pyrethroids considered in this chapter were moderately toxic (resmethrin and esfenvalerate; L D = 75 and 381 mg/kg-bw, respectively). O f the remaining ten, three were slightly toxic (prallethrin, fenpropathrin, and bifenthrin; L D = >1000, 1089, and 1800 mg/kg-bw, respectively), and the rest were classified as practically non-toxic. Although the acute toxicity classification of the synthetic pyrethroids to mammals ranges from highly toxic ( L D = 16.2 mg/kg-bw for cyfluthrin) to practically non-toxic ( L D = 8900 mg/kg-bw for permethrin), only three of the twelve pyrethroids classified in this chapter were considered as highly toxic to mammals (cyfluthrin, tefluthrin, and fenpropathrin; L D = 16.2, 21.8, and 48.5 mg/kg-bw, respectively; Table VIII). O f the remaining nine, seven were classified as moderately toxic (bifenthrin, lambda-cyhalothrin, deltamethrin, tralomethrin, esfenvalerate, cypermethrin, and prallethrin; L D = 53.8, 56, 66.7, 84.9, 87.2, 247, and 460 mg/kg-bw, respectively), and the remaining two were classified as practically non-toxic (resmethrin and permethrin; L D = 4639 and 8900 mg/kg-bw, respectively). 5
50
5 0
5 0
50
50
5 0
5 0
5 0
In terms of reproductive toxicity, the range in the magnitude of effects exhibited by avian species is bound at the lower end by the N O A E C and L O A E C values of 12 ppm and 60 ppm, respectively, for resmethrin, and at the upper end by the N O A E C and L O A E C values of 500 ppm and >500 ppm, respectively, for permethrin. Some of the observed effects at the reported L O A E C levels for all pyrethroids considered here (Table VIII) include reductions in eggshell thickness, percent of normal hatchlings from live 3-week embryos, number of eggs laid, embryo viability and hatchability, and 14-day survivor weight, and increased number of cracked eggs and early embryonic deaths. However, it should be made clear that although reproductive toxicity studies with avian species indicate that there is potential for reproductive effects from chronic exposure to this class of compounds, there is considerable uncertainty surrounding the characterization of effects because of the limitations of the data set available to the Agency. O f the twelve synthetic pyrethroids considered here, half were unable to elicit toxic effects up to the highest concentrations tested (bifenthrin, lambda-cyhalothrin, cypermethrin, deltamethrin, permethrin, and tefluthrin); however, all of these studies were conducted with test concentrations < 500 ppm, and four of these were conducted with test concentrations < 100 ppm (bifenthrin, lambda-cyhalothrin, cypermethrin, and tefluthrin). The N O A E C and L O A E C values for mammalian reproductive toxicity range from 3 (no observed effects) for prallethrin, respectively. Some effects observed at the listed L O A E C s for freshwater fish include reduced reproduction, egg production, number of fry, hatchability, growth (adult and larval), and survival (adult and larval). Estimated N O A E C and L O A E C values for freshwater invertebrates range from 0.0013 and 0.0029 ppb for bifenthrin, respectively, to 0.65 and 1.3 for prallethrin, respectively. Some effects observed at the listed L O A E C s for freshwater invertebrates include reduced survival, growth (length), and reproduction (number of young per female per reproductive day and total offspring). However, it should be noted that chronic data are lacking for cypermethrin and resmethrin. Because the most sensitive acute toxicity value for freshwater invertebrates is based on data for cypermethrin ( L C = 0.0036 ppb), it appears reasonable to conclude that the lower bound for the range of chronic toxicity exhibited by the synthetic pyrethroids may be underestimated by the range estimated in this chapter based on actual data. 50
288 Table VIII. Most sensitive terrestrial toxicity data available to the Agency for the synthetic pyrethroids.
a
Units are ppm. * Units are mg/kg-body weight. LD o values for non-target insects are based on contact toxicity studies and the units are ug/bee. A mammalian reproduction study is not available for lambda-cyhalothrin. This data point was based on a study with lambda-cyhalothrin. c
5
d
289
Table IX. Most sensitive aquatic toxicity data available to the Agency for the synthetic pyrethroids . a
a
All data are in ppb and are based on toxicity testing with the technical grade active ingredient
(TGAI).
290
Figure 6. Summary of the number ofpyrethroids for which the most sensitive acute and chronic toxicity endpoints fall within the specified ranges.
291 For estuarine/marine fish, estimated N O A E C and L O A E C levels range from 0.025 and 0.084 ppb for cyfluthrin, respectively, to
