Synthetic Pyrethroids. Occurrence and Behavior in Aquatic Environments

470 Pages·2008·41.52 MB·English

by  Jay Gan

ACS SYMPOSIUM SERIES 991
Synthetic Pyrethroids
Occurrence and Behavior in Aquatic
Environments
Jay Gan, Editor
University of California
Frank Spurlock, Editor
California Department of Pesticide Regulation
Paul Hendley, Editor
Syngenta Crop Protection, Inc.
Donald P. Weston, Editor
University of California
Sponsored by the
ACS Division of Agrochemicals
American Chemical Society, Washington, DC
ISBN: 978-0-8412-7433-4
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Chapter 1
Synthetic Pyrethroid Use Patterns, Properties,
and Environmental Effects
Frank Spurlock and Marshall Lee
Environmental Monitoring Branch, California Department of Pesticide
Regulation, 10011 Street, Sacramento, CA 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 3
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.
CH2 2 ^2 CH3
= P —-
s / /
r
e
pyrethrin esters HC CH33 c- CH, V 0 0 pyrethric acid estHC CH33 \/ Y ch 3 IT ch \\ 3HC CH33 a CHH 0 pyrethrin II Y CH ^ 3 8 cinerin II H-JC CHI \/ 0^ 0—/ 1I113 Yv CH 0 jasmolin II
g
n
ri
r
u
c
c
o *
ally- ers
natur " d est ^
e I. Structures of the six AHC CH33 HC CH33 chrysanthemic aciHC CH33 A^ HC CH >\ 330 A^ pyrethrin I CHHC3 3 HC^ ^CH \\ 330 cinerin I AxfH3C CH3 hijC^ CH \^ 30 jasmolin I
bl
a
T
acid moieties alcohol >w moieties CH /=CH32 0 pyrethrolone esters CH CH33 0 cinerolone esters CH /—CH3 3O^^ ^ / 0 jasmolone esters
H
6
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 "alcohol-
substituted" 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 naturally-
occurring 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-3-
phenoxybenzyl 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
7
mode of toxic action (14). Other type II pyrethroids include cyfluthrin,
cyhalothrin and esfenvalerate (Table III).
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 AI 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).
8
Table II. Chyrysanthemate ester "first-generation" photolabile pyrethroids
registered in California as of 2006 (stereochemistry not shown).
Chemical Uses/Product Types
allethrin, esbiothrin, d-trans-allethrin, d- Products of allethrin and it's
allethrin various isomers are used indoor/
outdoor in household, industrial,
HC CH3 commercial and institutional
3
A h3? settings. Many are pressurized
aerosols or foggers; a few pet
shampoos. Most often co-
formulated with other pyrethroids
HC CH
3 3 and/or synergists.
cyphenothrin
H3C CH
3 no Aerosol or fogger insecticide;
A .o animal husbandry premises or
indoor/outdoor household use.
Jl o il
H(T X H N
3 3
imiprothrin Mostly pressurized aerosols, used
in household, industrial,
H3C CH3 0V commercial and institutional
X V v^
settings. A few crack/crevice
products. Typically co-formulated
II
0 with other pyrethroids and/or
yl^ 0 synergists.
HC CH
3 3
9
Table II. Continued
phenothrin
Two main uses: household
indoor/outdoor flying insect
HC CH3
3 control and flea control products
(collars, direct application drops).
Typically co-formulated with
Jk^ other pyrethroids and/or
0 synergists.
H3C CH
3
prallethrin
Primarily pressurized aerosols
H3C CH used in household, industrial,
3
V 9H3 commercial and institutional
settings. Often co-formulated with
other pyrethroids and/or
synergists.
H3C CH3 NQ
resmethrin
Liquids or pressurized aerosols
used in household, industrial,
H3C CH-a -
commercial and institutional
settings. Some outdoor garden and
ornamental uses. Used in animal
husbandry premises. Often co-
formulated.
HC CH
3 3
tetramethrin
Primarily pressurized aerosols
HC CHo Q» used in household, industrial,
3
commercial and institutional
settings. Often co-formulated with
other pyrethroids and/or
synergists.
H3C CH 0
3