Chapter 1
Synthetic Pyrethroid Use Patterns, Properties, and Environmental Effects
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Frank Spurlock and Marshall Lee Environmental Monitoring Branch, California Department of Pesticide Regulation, 10011 Street, Sacramento, C A 95812-4015
In this paper we present a broad overview of the class of insecticides known as synthetic pyrethroids. The discussion includes a summary of agricultural and urban pyrethroid use patterns and trends, pyrethroid chemical structure and properties, the significance of photostability to pyrethroid environmental fate, and hydrophobicity, persistence and relative aquatic toxicity as compared to other pesticides. Finally we provide a brief summary of California's regulatory response to recent detections of pyrethroids in aquatic sediments and a discussion of scientific and regulatory issues associated with ongoing pyrethroid aquatic exposure assessments and mitigation efforts.
Introduction California leads the nation in agricultural production, so it's no surprise that the state accounts for approximately 20% of all agricultural insecticides applied to U.S. crop lands (/). Insecticides are also used extensively in California's urban areas. For example, synthetic pyrethroids are one of the most widely used families of insecticides, and we estimate that approximately 70% of California's total pyrethroid use occurs in urban areas. The importance of synthetic pyrethroids as a pest management tool in California is evidenced by the number © 2008 American Chemical Society
In Synthetic Pyrethroids; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
3
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4 of registered pyrethroid products. In 2006 there were 1255 California registered synthetic pyrethroid products from 128 registrants. These products accounted for more than 40% of all registered insecticide products in the state. Total California synthetic pyrethroid sales in 2004 were approximately 1.4 million lbs active ingredient (AI). Pyrethroids are used in nearly all agricultural crops, nurseries, various urban structural and landscaping sites, construction sites (pre-construction termiticides), the home/garden environment, and many other sites. Several desireable characteristics contribute to the commercial success of pyrethroids, including their efficacy against a broad range of insect pests and mites, low mammalian and avian toxicities, low potential to contaminate ground water, and relatively low application rates. However, there have been numerous recent reports of pyrethroid detections in California aquatic sediments, and toxicity to the sediment dweller Hyallela aiteca has been observed in concomitant bioassays (2-5). Coupled with steadily increasing use of pyrethroids, these observations have led to renewed interest in the environmental fate and transport of these insecticides. This chapter provides an overview of synthetic pyrethroid environmental fate characteristics, summarizes pyrethroid use patterns and trends in California, and summarizes some of the unique issues associated with synthetic pyrethroid aquatic risk assessment.
Background To understand different synthetic pyrethroids and their uses, it is instructive to review the major milestones in synthetic pyrethroid synthesis and development. Only a brief overview is given here. Readers seeking more information should consult the excellent review by Davies (6).
Pyrethrins Pyrethrum is a preparation of dried Chrysanthemum cinerariaefolium and/or Chrysanthemum cineum flower heads that contains the six insecticidally active chemicals known as pyrethrins. Each of the six naturally-occurring pyrethrins is comprised of a cyclopropane-carboxylic acid group and a cyclopentenolone (alcohol) group joined by an ester linkage (Table I). The various synthetic pyrethroid analogues are generally similar in structure to the pyrethrins, although there are some deviations from the basic chryanthemic acid ester structure.
In Synthetic Pyrethroids; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
In Synthetic Pyrethroids; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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jasmolone esters
^
cinerolone esters
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pyrethrolone esters
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CH
alcohol >w moieties
acid moieties
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chrysanthemic acid esters
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pyrethrin II
C H I
C H
\ /
C H
H-JC
3
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ch H C CH
Y
H C CH \/ 3
0
CH,
pyrethric acid esters
c-
Table I. Structures of the six naturally-occurring pyrethrin esters
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2
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6
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First-generation Photolabile Synthetic Pyrethroids Many early attempts at pyrethroid synthesis focused on substitutions to the alcohol portion of the molecule (6, 7). Allethrin was one of the earliest synthetic analogues to eventually achieve commercial success, and allethrin-containing products are still marketed throughout the world today. Numerous other "alcoholsubstituted" chrysanthemic acid esters were synthesized between the 1950s to the early 1970s, and many are still registered for use in the United States today, including resmethrin, tetramethrin, and phenothrin (Table II). These chrysanthemic acid derivatives are often called "first-generation" synthetic pyrethroids. The first-generation synthetic pyrethroids are similar to naturallyoccurring pyrethrins in that they photolyze relatively easily (6). While their photolysis half-lives vary depending on measurement method and experimental conditions, half-lives on surfaces exposed to sunlight or simulated sunlight are generally on the order of hours (8-10).
Photostable Type I and Type II synthetic pyrethroids Modifications to the chrysanthemic acid portion of the pyrethroid molecule improved photostabilities. In particular, esters of chrysanthemic acid dihalovinyl analogues were found to display much improved photostabilities compared to the esters of chrysanthemic acid (11,12). The first commercial photostable synthetic pyrethroid based on this approach was permethrin, synthesized in the early 1970s. Permethrin is still the most widely used synthetic pyrethroid in California today. While various photostable synthetic pyrethroids have since been developed based on different structural modifications to the basic chrysanthemate ester moiety, the halogenated vinylcyclopropylcarboxylates are among the most important in agriculture today, and include the various cypermethrins, cyfluthrins, and cyhalothrins (Table III). Reported aqueous and soil photolysis half-lives are generally on the order of tens to occasionally hundreds of days for the various photostable pyrethroids (13).
Type 1 vs. Type 11 An additional structural feature common to several commercially successful synthetic pyrethroids is the "ct-cyano" group. These pyrethroids are ot-cyano-3phenoxybenzyl pyrethroid esters and are commonly referred to as "type II pyrethroids". Type II pyrethroids display markedly increased biological activity relative to their type I 3-phenoxybenzyl analogues (cf. type II cypermethrin vs. type I permethrin, Table III) and also demonstrate certain differences in their
In Synthetic Pyrethroids; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
7 mode of toxic action (14). Other type II pyrethroids include cyfluthrin, cyhalothrin and esfenvalerate (Table III).
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Isomeric Enrichment Most synthetic pyrethroids are comprised of several stereoisomers due to the presence of multiple asymmetric carbons, often in the cyclopropane ring as in the case of cypermethrin and cyhalothrin (Table III). In addition, several of the pyrethroids also possess an alkene moiety, giving rise to cis/trans isomerism (e.g. permethrin, Table III). In general the biological activity of different stereoisomers varies substantially (15-18), so that enrichment of the most active isomer(s) yields a product with greatly enhanced insecticidal activity.. In recent years several isomerically enriched pyrethroid active ingredients have been introduced into commercial use. One of the first such pyrethroids registered in California was esfenvalerate in 1988. Esfenvalerate is now widely used and there are no longer any registered products containing the original racemate fenvalerate. Numerous other isomerically enriched synthetic pyrethroids have since been introduced, including lambda cyhalothrin, gamma cyhalothrin, beta cyfluthrin and (S)cypermethrin (zeta-cypermethrin). One consequence is that application rates expressed on an A I basis are lower for the more active enriched products due to their enhanced activity. However, several recent articles have reported differences in persistence also, likely due to differences in biodegradability among different isomers (16-18). Data on stereoselective biodegradation of pyrethroids are relatively sparse, so the practical signficance of stereoselective biodegradation is not well understood.
General Use Patterns and Trends Due to their instability in sunlight, chrysanthemate ester pyrethroids such as allethrin are not used in agriculture. These pyrethroids are formulated primarily as indoor or residential products such as aerosol ant and roach sprays, foggers, pet products, carpet and upholstery sprays, and commercial/institutional uses such as in food preparation or storage facilities. In California, the chrysenthamate esters account for 10 of 24 synthetic pyrethroid active ingredients in registered products, where isomerically-enriched mixtures (e.g. allethrin, d-allethrin, bioallethrin) are considered different active ingredients. These chrysanthemate esters accounted for approximately 8% of total synthetic pyrethroid sales in California in 2004 (19).
In Synthetic Pyrethroids; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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Table II. Chyrysanthemate ester "first-generation" photolabile pyrethroids registered in California as of 2006 (stereochemistry not shown). Chemical allethrin, esbiothrin, d-trans-allethrin, dallethrin H C
CH3
3
h 3
A
H C
C H
3
?
3
Uses/Product Types Products of allethrin and it's various isomers are used indoor/ outdoor in household, industrial, commercial and institutional settings. Many are pressurized aerosols or foggers; a few pet shampoos. Most often coformulated with other pyrethroids and/or synergists.
cyphenothrin H3C
C H
3
no
A.o Jl
o i l
H (T
X H
3
N
3
imiprothrin H C
CH
3
3
II H C 3
CH
0
V
Vv^
X yl^
Aerosol or fogger insecticide; animal husbandry premises or indoor/outdoor household use.
0
0
Mostly pressurized aerosols, used in household, industrial, commercial and institutional settings. A few crack/crevice products. Typically co-formulated with other pyrethroids and/or synergists.
3
In Synthetic Pyrethroids; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
9
Table II. Continued phenothrin H C
CH3
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3
H3C
Two main uses: household indoor/outdoor flying insect control and flea control products (collars, direct application drops). Typically co-formulated with other pyrethroids and/or synergists.
Jk^ C H
0
3
prallethrin H3C
C H
9 3
V
H3C
H
C H
N
3
resmethrin CH-a
H3C
H C 3
C H
Primarily pressurized aerosols used in household, industrial, commercial and institutional settings. Often co-formulated with other pyrethroids and/or synergists.
3
-
3
Q
Liquids or pressurized aerosols used in household, industrial, commercial and institutional settings. Some outdoor garden and ornamental uses. Used in animal husbandry premises. Often coformulated.
tetramethrin CHo
H C 3
H3C
C H
3
Q»
Primarily pressurized aerosols used in household, industrial, commercial and institutional settings. Often co-formulated with other pyrethroids and/or synergists.
0
In Synthetic Pyrethroids; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
10
Table III. Photostable Type I and Type II pyrethroids registered in California as of 2006 and their general use patterns (stereochemistry not shown). Structure bifenthrin
Use Pattern (2004) bifenthrin use: 110,000 lbs 20% agricultural use (cotton, corn); 40% commercial structural and landscape; 40% home and garden.
cyfluthrin, beta-cyfluthrin
cyfluthrin use: 50,000 lbs 30% agricultural (alfalfa, citrus, cotton), 70% commercial structural,
