Properties and Uses of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 21

of this insecticide, and thus, the toxicity of CPYO is implicitly considered when the
toxicity of CPY is studied. As CPYO is also formed in the atmosphere (Mackay
et al. 2014 ), it is considered in the risk assessments.

3.2 Interactions with Other Pesticides

Because conversion of CPY to CPYO is essential to the mode of action, compounds
that induce multifunction oxidase activity in animals can influence the toxicity of
CPY by increasing the rate of formation of CPYO. Atrazine, a herbicide with lesser
toxicity than CPY and no activity on AChE, has been reported to synergize (increase
or result in supra-additivity) the toxicity of CPY and some other organophosphorus
pesticides in aquatic animals such as the midge, Chironomus dilutus ( formerly ten-
tans ) (Belden and Lydy 2001 ). The mechanism of this synergism was via induction
of multifunction oxidases by atrazine and the resulting increase in the formation of
CPYO (Belden and Lydy 2000 ). Similar synergism has either not been observed or
was observed only at small synergistic ratios (<2) in other invertebrates (Trimble
and Lydy 2006 ) and vertebrates (Tyler Mehler et al. 2008 ; Wacksman et al. 2006 ).
In addition, synergism was only observed at greater concentrations of atrazine and
CPY, which rarely co-occur (Rodney et al. 2013 ). For this reason, synergistic inter-
actions between CPY and other chemicals were not included in the assessment of
the risks of CPY to aquatic organisms (Giddings et al. 2014 ).
Synergism of CPY by the sterol-inhibiting fungicide prochloraz was reported to
occur in the red-legged partridge (Johnston et al. 1994 ), but this was only observed
in birds pretreated at a large dose of 180 mg prochloraz kg −1 (bwt), an extremely
unlikely exposure in birds. The synergism was attributed to induction of multifunc-
tion oxidases and an increase in the formation of CPYO. As for aquatic organisms,
interactions of this type were judged to be very unlikely to occur in terrestrial organ-
isms and were not included in the risk assessment.

4 Use of Chlorpyrifos and Its Formulations

CPY is a widely used organophosphate pesticide with broad spectrum insecticidal
activity. It is used against a broad array of insects and mites, primarily as a contact
insecticide, although it does have some efficacy through ingestion. It provides con-
trol for many adult and larval forms of insects. Foliar pests for which CPY provides
control include: aphids, beetles, caterpillars, leafhoppers, mites, and scale. CPY is
also effective against many soil insects, including rootworms, cutworms, wire-
worms, and other grubs. Although it does not translocate readily, CPY can effec-
tively control boring insects in corn, fruit, and other crops through contact exposure.
It can also provide contact control of such insects as case-bearers, orange-worms,
and other flies that damage fruits and nuts. The diversity of arthropod pests subject
to control with CPY has made it one of most widely used insecticides.